User Guide for NVPTX Back-end¶

Introduction ¶

To support GPU programming, the NVPTX back-end supports a subset of LLVM IR along with a defined set of conventions used to represent GPU programming concepts. This document provides an overview of the general usage of the back- end, including a description of the conventions used and the set of accepted LLVM IR.

Note

This document assumes a basic familiarity with CUDA and the PTX assembly language. Information about the CUDA Driver API and the PTX assembly language can be found in the CUDA documentation.

Conventions ¶

Marking Functions as Kernels ¶

In PTX, there are two types of functions: device functions, which are only callable by device code, and kernel functions, which are callable by host code. By default, the back-end will emit device functions. Metadata is used to declare a function as a kernel function. This metadata is attached to the nvvm.annotations named metadata object, and has the following format:

!0 = !{<function-ref>, metadata !"kernel", i32 1}

The first parameter is a reference to the kernel function. The following example shows a kernel function calling a device function in LLVM IR. The function @my_kernel is callable from host code, but @my_fmad is not.

define float @my_fmad(float %x, float %y, float %z) {
  %mul = fmul float %x, %y
  %add = fadd float %mul, %z
  ret float %add
}

define void @my_kernel(ptr %ptr) {
  %val = load float, ptr %ptr
  %ret = call float @my_fmad(float %val, float %val, float %val)
  store float %ret, ptr %ptr
  ret void
}

!nvvm.annotations = !{!1}
!1 = !{ptr @my_kernel, !"kernel", i32 1}

When compiled, the PTX kernel functions are callable by host-side code.

Address Spaces ¶

The NVPTX back-end uses the following address space mapping:

Address Space

Memory Space

0

Generic

1

Global

2

Internal Use

3

Shared

4

Constant

5

Local

Every global variable and pointer type is assigned to one of these address spaces, with 0 being the default address space. Intrinsics are provided which can be used to convert pointers between the generic and non-generic address spaces.

As an example, the following IR will define an array @g that resides in global device memory.

@g = internal addrspace(1) global [4 x i32] [ i32 0, i32 1, i32 2, i32 3 ]

LLVM IR functions can read and write to this array, and host-side code can copy data to it by name with the CUDA Driver API.

Note that since address space 0 is the generic space, it is illegal to have global variables in address space 0. Address space 0 is the default address space in LLVM, so the addrspace(N) annotation is required for global variables.

Triples ¶

The NVPTX target uses the module triple to select between 32/64-bit code generation and the driver-compiler interface to use. The triple architecture can be one of nvptx (32-bit PTX) or nvptx64 (64-bit PTX). The operating system should be one of cuda or nvcl, which determines the interface used by the generated code to communicate with the driver. Most users will want to use cuda as the operating system, which makes the generated PTX compatible with the CUDA Driver API.

Example: 32-bit PTX for CUDA Driver API: nvptx-nvidia-cuda

Example: 64-bit PTX for CUDA Driver API: nvptx64-nvidia-cuda

NVPTX Intrinsics ¶

Reading PTX Special Registers ¶

‘`llvm.nvvm.read.ptx.sreg.*`’¶

Syntax:¶

declare i32 @llvm.nvvm.read.ptx.sreg.tid.x()
declare i32 @llvm.nvvm.read.ptx.sreg.tid.y()
declare i32 @llvm.nvvm.read.ptx.sreg.tid.z()
declare i32 @llvm.nvvm.read.ptx.sreg.ntid.x()
declare i32 @llvm.nvvm.read.ptx.sreg.ntid.y()
declare i32 @llvm.nvvm.read.ptx.sreg.ntid.z()
declare i32 @llvm.nvvm.read.ptx.sreg.ctaid.x()
declare i32 @llvm.nvvm.read.ptx.sreg.ctaid.y()
declare i32 @llvm.nvvm.read.ptx.sreg.ctaid.z()
declare i32 @llvm.nvvm.read.ptx.sreg.nctaid.x()
declare i32 @llvm.nvvm.read.ptx.sreg.nctaid.y()
declare i32 @llvm.nvvm.read.ptx.sreg.nctaid.z()
declare i32 @llvm.nvvm.read.ptx.sreg.warpsize()

Overview:¶

The ‘@llvm.nvvm.read.ptx.sreg.*’ intrinsics provide access to the PTX special registers, in particular the kernel launch bounds. These registers map in the following way to CUDA builtins:

CUDA Builtin

PTX Special Register Intrinsic

threadId

@llvm.nvvm.read.ptx.sreg.tid.*

blockIdx

@llvm.nvvm.read.ptx.sreg.ctaid.*

blockDim

@llvm.nvvm.read.ptx.sreg.ntid.*

gridDim

@llvm.nvvm.read.ptx.sreg.nctaid.*

Barriers ¶

‘`llvm.nvvm.barrier0`’¶

Syntax:¶

declare void @llvm.nvvm.barrier0()

Overview:¶

The ‘@llvm.nvvm.barrier0()’ intrinsic emits a PTX bar.sync 0 instruction, equivalent to the __syncthreads() call in CUDA.

Electing a thread ¶

‘`llvm.nvvm.elect.sync`’¶

Syntax:¶

declare {i32, i1} @llvm.nvvm.elect.sync(i32 %membermask)

Overview:¶

The ‘@llvm.nvvm.elect.sync’ intrinsic generates the elect.sync PTX instruction, which elects one predicated active leader thread from a set of threads specified by membermask. The behavior is undefined if the executing thread is not in membermask. The laneid of the elected thread is captured in the i32 return value. The i1 return value is set to True for the leader thread and False for all the other threads. Election of a leader thread happens deterministically, i.e. the same leader thread is elected for the same membermask every time. For more information, refer PTX ISA https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#parallel-synchronization-and-communication-instructions-elect-sync.

Membar/Fences ¶

‘`llvm.nvvm.fence.proxy.tensormap_generic.*`’¶

Syntax:¶

declare void @llvm.nvvm.fence.proxy.tensormap_generic.release.cta()
declare void @llvm.nvvm.fence.proxy.tensormap_generic.release.cluster()
declare void @llvm.nvvm.fence.proxy.tensormap_generic.release.gpu()
declare void @llvm.nvvm.fence.proxy.tensormap_generic.release.sys()

declare void @llvm.nvvm.fence.proxy.tensormap_generic.acquire.cta(ptr %addr, i32 %size)
declare void @llvm.nvvm.fence.proxy.tensormap_generic.acquire.cluster(ptr %addr, i32 %size)
declare void @llvm.nvvm.fence.proxy.tensormap_generic.acquire.gpu(ptr %addr, i32 %size)
declare void @llvm.nvvm.fence.proxy.tensormap_generic.acquire.sys(ptr %addr, i32 %size)

Overview:¶

The @llvm.nvvm.fence.proxy.tensormap_generic.* is a uni-directional fence used to establish ordering between a prior memory access performed via the generic proxy<https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#proxies>_ and a subsequent memory access performed via the tensormap proxy. nvvm.fence.proxy.tensormap_generic.release can form a release sequence that synchronizes with an acquire sequence that contains the nvvm.fence.proxy.tensormap_generic.acquire proxy fence. The following table describes the mapping between LLVM Intrinsic and the PTX instruction:

NVVM Intrinsic

PTX Instruction

@llvm.nvvm.fence.proxy.tensormap_generic.release.*

fence.proxy.tensormap::generic.release.*

@llvm.nvvm.fence.proxy.tensormap_generic.acquire.*

fence.proxy.tensormap::generic.acquire.* [addr], size

The address operand addr and the operand size together specify the memory range [addr, addr+size) on which the ordering guarantees on the memory accesses across the proxies is to be provided. The only supported value for the size operand is 128 and must be an immediate. Generic Addressing is used unconditionally, and the address specified by the operand addr must fall within the .global state space. Otherwise, the behavior is undefined. For more information, see PTX ISA.

Address Space Intrinsics ¶

‘`llvm.nvvm.isspacep.*`’ Intrinsics ¶

Syntax:¶

declare i1 @llvm.nvvm.isspacep.const(ptr %p)
declare i1 @llvm.nvvm.isspacep.global(ptr %p)
declare i1 @llvm.nvvm.isspacep.local(ptr %p)
declare i1 @llvm.nvvm.isspacep.shared(ptr %p)
declare i1 @llvm.nvvm.isspacep.shared.cluster(ptr %p)

Overview:¶

The ‘llvm.nvvm.isspacep.*’ intrinsics determine whether the provided generic pointer references memory which falls within a particular address space.

Semantics:¶

If the given pointer in the generic address space refers to memory which falls within the state space of the intrinsic (and therefore could be safely address space casted to this space), 1 is returned, otherwise 0 is returned.

Arithmetic Intrinsics ¶

‘`llvm.nvvm.idp2a.[us].[us]`’ Intrinsics ¶

Syntax:¶

declare i32 @llvm.nvvm.idp2a.s.s(i32 %a, i32 %b, i1 immarg %is.hi, i32 %c)
declare i32 @llvm.nvvm.idp2a.s.u(i32 %a, i32 %b, i1 immarg %is.hi, i32 %c)
declare i32 @llvm.nvvm.idp2a.u.s(i32 %a, i32 %b, i1 immarg %is.hi, i32 %c)
declare i32 @llvm.nvvm.idp2a.u.u(i32 %a, i32 %b, i1 immarg %is.hi, i32 %c)

Overview:¶

The ‘llvm.nvvm.idp2a.[us].[us]’ intrinsics performs a 2-element vector dot product followed by addition. They corresponds directly to the dp2a PTX instruction.

Semantics:¶

The 32-bit value in %a is broken into 2 16-bit values which are extended to 32 bits. For the ‘llvm.nvvm.idp2a.u.[us]’ variants zero-extension is used, while for the ‘llvm.nvvm.idp2a.s.[us]’ sign-extension is used. Two bytes are selected from %b, if %is.hi is true, the most significant bytes are selected, otherwise the least significant bytes are selected. These bytes are then extended to 32-bits. For the ‘llvm.nvvm.idp2a.[us].u’ variants zero-extension is used, while for the ‘llvm.nvvm.idp2a.[us].s’ sign-extension is used. The dot product of these 2-element vectors is added to %c to produce the return.

‘`llvm.nvvm.idp4a.[us].[us]`’ Intrinsics ¶

Syntax:¶

declare i32 @llvm.nvvm.idp4a.s.s(i32 %a, i32 %b, i32 %c)
declare i32 @llvm.nvvm.idp4a.s.u(i32 %a, i32 %b, i32 %c)
declare i32 @llvm.nvvm.idp4a.u.s(i32 %a, i32 %b, i32 %c)
declare i32 @llvm.nvvm.idp4a.u.u(i32 %a, i32 %b, i32 %c)

Overview:¶

The ‘llvm.nvvm.idp4a.[us].[us]’ intrinsics perform a 4-element vector dot product followed by addition. They corresponds directly to the dp4a PTX instruction.

Semantics:¶

Each of the 4 bytes in both %a and %b are extended to 32-bit integers forming 2 <4 x i32>. For %a, zero-extension is used in the ‘llvm.nvvm.idp4a.u.[us]’ variants, while sign-extension is used with ‘llvm.nvvm.idp4a.s.[us]’ variants. Similarly, for %b, zero-extension is used in the ‘llvm.nvvm.idp4a.[us].u’ variants, while sign-extension is used with ‘llvm.nvvm.idp4a.[us].s’ variants. The dot product of these 4-element vectors is added to %c to produce the return.

Bit Manipulation Intrinsics ¶

‘`llvm.nvvm.fshl.clamp.*`’ Intrinsic ¶

Syntax:¶

declare i32 @llvm.nvvm.fshl.clamp.i32(i32 %hi, i32 %lo, i32 %n)

Overview:¶

The ‘llvm.nvvm.fshl.clamp’ family of intrinsics performs a clamped funnel shift left. These intrinsics are very similar to ‘llvm.fshl’, except the shift ammont is clamped at the integer width (instead of modulo it). Currently, only i32 is supported.

Semantics:¶

The ‘llvm.nvvm.fshl.clamp’ family of intrinsic functions performs a clamped funnel shift left: the first two values are concatenated as { %hi : %lo } (%hi is the most significant bits of the wide value), the combined value is shifted left, and the most significant bits are extracted to produce a result that is the same size as the original arguments. The shift amount is the minimum of the value of %n and the bit width of the integer type.

‘`llvm.nvvm.fshr.clamp.*`’ Intrinsic ¶

Syntax:¶

declare i32 @llvm.nvvm.fshr.clamp.i32(i32 %hi, i32 %lo, i32 %n)

Overview:¶

The ‘llvm.nvvm.fshr.clamp’ family of intrinsics perform a clamped funnel shift right. These intrinsics are very similar to ‘llvm.fshr’, except the shift ammont is clamped at the integer width (instead of modulo it). Currently, only i32 is supported.

Semantics:¶

The ‘llvm.nvvm.fshr.clamp’ family of intrinsic functions performs a clamped funnel shift right: the first two values are concatenated as { %hi : %lo } (%hi is the most significant bits of the wide value), the combined value is shifted right, and the least significant bits are extracted to produce a result that is the same size as the original arguments. The shift amount is the minimum of the value of %n and the bit width of the integer type.

‘`llvm.nvvm.flo.u.*`’ Intrinsic ¶

Syntax:¶

declare i32 @llvm.nvvm.flo.u.i32(i32 %a, i1 %shiftamt)
declare i32 @llvm.nvvm.flo.u.i64(i64 %a, i1 %shiftamt)

Overview:¶

The ‘llvm.nvvm.flo.u’ family of intrinsics identifies the bit position of the leading one, returning either it’s offset from the most or least significant bit.

Semantics:¶

The ‘llvm.nvvm.flo.u’ family of intrinsics returns the bit position of the most significant 1. If %shiftamt is true, The result is the shift amount needed to left-shift the found bit into the most-significant bit position, otherwise the result is the shift amount needed to right-shift the found bit into the least-significant bit position. 0xffffffff is returned if no 1 bit is found.

‘`llvm.nvvm.flo.s.*`’ Intrinsic ¶

Syntax:¶

declare i32 @llvm.nvvm.flo.s.i32(i32 %a, i1 %shiftamt)
declare i32 @llvm.nvvm.flo.s.i64(i64 %a, i1 %shiftamt)

Overview:¶

The ‘llvm.nvvm.flo.s’ family of intrinsics identifies the bit position of the leading non-sign bit, returning either it’s offset from the most or least significant bit.

Semantics:¶

The ‘llvm.nvvm.flo.s’ family of intrinsics returns the bit position of the most significant 0 for negative inputs and the most significant 1 for non-negative inputs. If %shiftamt is true, The result is the shift amount needed to left-shift the found bit into the most-significant bit position, otherwise the result is the shift amount needed to right-shift the found bit into the least-significant bit position. 0xffffffff is returned if no 1 bit is found.

TMA family of Intrinsics ¶

‘`llvm.nvvm.cp.async.bulk.tensor.g2s.tile.[1-5]d`’¶

Syntax:¶

declare void @llvm.nvvm.cp.async.bulk.tensor.g2s.tile.1d(ptr addrspace(3) %dst, ptr addrspace(3) %bar, ptr %tensor_map, i32 %d0, i16 %mc, i64 %ch, i1 %flag_mc, i1 %flag_ch)
declare void @llvm.nvvm.cp.async.bulk.tensor.g2s.tile.2d(..., i32 %d0, i32 %d1, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.g2s.tile.3d(..., i32 %d0, i32 %d1, i32 %d2, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.g2s.tile.4d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.g2s.tile.5d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, i32 %d4, ...)

Overview:¶

The ‘@llvm.nvvm.cp.async.bulk.tensor.g2s.tile.[1-5]d’ intrinsics correspond to the cp.async.bulk.tensor.[1-5]d.* set of PTX instructions. These instructions initiate an asynchronous copy of tensor data from global memory to shared::cluster memory (indicated by the g2s prefix) in tile mode. In tile mode, the multi-dimensional layout of the source tensor is preserved at the destination. The dimension of the tensor data ranges from 1d to 5d with the coordinates specified by the i32 %d0 ... i32 %d4 arguments.

The last two arguments to these intrinsics are boolean flags indicating support for cache_hint and/or multicast modifiers. These flag arguments must be compile-time constants. The backend looks through these flags and lowers the intrinsics appropriately.
The Nth argument (denoted by i1 flag_ch) when set, indicates a valid cache_hint (i64 %ch) and generates the .L2::cache_hint variant of the PTX instruction.
The [N-1]th argument (denoted by i1 flag_mc) when set, indicates the presence of a multicast mask (i16 %mc) and generates the PTX instruction with the .multicast::cluster modifier.

For more information, refer PTX ISA https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#data-movement-and-conversion-instructions-cp-async-bulk-tensor.

‘`llvm.nvvm.cp.async.bulk.tensor.g2s.im2col.[3-5]d`’¶

Syntax:¶

declare void @llvm.nvvm.cp.async.bulk.tensor.g2s.im2col.3d(ptr addrspace(3) %dst, ptr addrspace(3) %bar, ptr %tensor_map, i32 %d0, i32 %d1, i32 %d2, i16 %im2col0, i16 %mc, i64 %ch, i1 %flag_mc, i1 %flag_ch)
declare void @llvm.nvvm.cp.async.bulk.tensor.g2s.im2col.4d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, i16 %im2col0, i16 %im2col1, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.g2s.im2col.5d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, i32 %d4, i16 %im2col0, i16 %im2col1, i16 %im2col2, ...)

Overview:¶

The ‘@llvm.nvvm.cp.async.bulk.tensor.g2s.im2col.[3-5]d’ intrinsics correspond to the cp.async.bulk.tensor.[1-5]d.* set of PTX instructions. These instructions initiate an asynchronous copy of tensor data from global memory to shared::cluster memory (indicated by the g2s prefix) in im2col mode. In im2col mode, some dimensions of the source tensor are unrolled into a single dimensional column at the destination. In this mode, the tensor has to be at least three-dimensional. Along with the tensor coordinates, im2col offsets are also specified (denoted by i16 im2col0...i16 %im2col2). The number of im2col offsets is two less than the number of dimensions of the tensor operation. The last two arguments to these intrinsics are boolean flags, with the same functionality as described in the tile mode intrinsics above.

For more information, refer PTX ISA https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#data-movement-and-conversion-instructions-cp-async-bulk-tensor.

‘`llvm.nvvm.cp.async.bulk.tensor.s2g.tile.[1-5]d`’¶

Syntax:¶

declare void @llvm.nvvm.cp.async.bulk.tensor.s2g.tile.1d(ptr addrspace(3) %src, ptr %tensor_map, i32 %d0, i64 %ch, i1 %flag_ch)
declare void @llvm.nvvm.cp.async.bulk.tensor.s2g.tile.2d(..., i32 %d0, i32 %d1, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.s2g.tile.3d(..., i32 %d0, i32 %d1, i32 %d2, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.s2g.tile.4d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.s2g.tile.5d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, i32 %d4, ...)

Overview:¶

The ‘@llvm.nvvm.cp.async.bulk.tensor.s2g.tile.[1-5]d’ intrinsics correspond to the cp.async.bulk.tensor.[1-5]d.* set of PTX instructions. These instructions initiate an asynchronous copy of tensor data from shared::cta to global memory (indicated by the s2g prefix) in tile mode. The dimension of the tensor data ranges from 1d to 5d with the coordinates specified by the i32 %d0 ... i32 %d4 arguments.

The last argument to these intrinsics is a boolean flag indicating support for cache_hint. This flag argument must be a compile-time constant. When set, it indicates a valid cache_hint (i64 %ch) and generates the .L2::cache_hint variant of the PTX instruction.

For more information, refer PTX ISA https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#data-movement-and-conversion-instructions-cp-async-bulk-tensor.

‘`llvm.nvvm.cp.async.bulk.tensor.s2g.im2col.[3-5]d`’¶

Syntax:¶

declare void @llvm.nvvm.cp.async.bulk.tensor.s2g.im2col.3d(ptr addrspace(3) %src, ptr %tensor_map, i32 %d0, i32 %d1, i32 %d2, i64 %ch, i1 %flag_ch)
declare void @llvm.nvvm.cp.async.bulk.tensor.s2g.im2col.4d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.s2g.im2col.5d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, i32 %d4, ...)

Overview:¶

The ‘@llvm.nvvm.cp.async.bulk.tensor.s2g.im2col.[1-5]d’ intrinsics correspond to the cp.async.bulk.tensor.[1-5]d.* set of PTX instructions. These instructions initiate an asynchronous copy of tensor data from shared::cta to global memory (indicated by the s2g prefix) in im2col mode. In this mode, the tensor has to be at least three-dimensional. Unlike the g2s variants, there are no im2col_offsets for these intrinsics. The last argument to these intrinsics is a boolean flag, with the same functionality as described in the s2g.tile mode intrinsics above.

For more information, refer PTX ISA https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#data-movement-and-conversion-instructions-cp-async-bulk-tensor.

‘`llvm.nvvm.cp.async.bulk.tensor.prefetch.tile.[1-5]d`’¶

Syntax:¶

declare void @llvm.nvvm.cp.async.bulk.tensor.prefetch.tile.1d(ptr %tensor_map, i32 %d0, i64 %ch, i1 %flag_ch)
declare void @llvm.nvvm.cp.async.bulk.tensor.prefetch.tile.2d(..., i32 %d0, i32 %d1, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.prefetch.tile.3d(..., i32 %d0, i32 %d1, i32 %d2, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.prefetch.tile.4d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.prefetch.tile.5d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, i32 %d4, ...)

Overview:¶

The ‘@llvm.nvvm.cp.async.bulk.tensor.prefetch.tile.[1-5]d’ intrinsics correspond to the cp.async.bulk.prefetch.tensor.[1-5]d.L2.global* set of PTX instructions. These instructions initiate an asynchronous prefetch of tensor data from global memory to the L2 cache. In tile mode, the multi-dimensional layout of the source tensor is preserved at the destination. The dimension of the tensor data ranges from 1d to 5d with the coordinates specified by the i32 %d0 ... i32 %d4 arguments.

The last argument to these intrinsics is a boolean flag indicating support for cache_hint. This flag argument must be a compile-time constant. When set, it indicates a valid cache_hint (i64 %ch) and generates the .L2::cache_hint variant of the PTX instruction.

For more information, refer PTX ISA https://docs.nvidia.com/cuda/parallel-thread-execution/#data-movement-and-conversion-instructions-cp-async-bulk-prefetch-tensor.

‘`llvm.nvvm.cp.async.bulk.tensor.prefetch.im2col.[1-5]d`’¶

Syntax:¶

declare void @llvm.nvvm.cp.async.bulk.tensor.prefetch.im2col.3d(ptr %tensor_map, i32 %d0, i32 %d1, i32 %d2, i16 %im2col0, i64 %ch, i1 %flag_ch)
declare void @llvm.nvvm.cp.async.bulk.tensor.prefetch.im2col.4d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, i16 %im2col0, i16 %im2col1, ...)
declare void @llvm.nvvm.cp.async.bulk.tensor.prefetch.im2col.5d(..., i32 %d0, i32 %d1, i32 %d2, i32 %d3, i32 %d4, i16 %im2col0, i16 %im2col1, i16 %im2col2, ...)

Overview:¶

The ‘@llvm.nvvm.cp.async.bulk.tensor.prefetch.im2col.[1-5]d’ intrinsics correspond to the cp.async.bulk.prefetch.tensor.[1-5]d.L2.global* set of PTX instructions. These instructions initiate an asynchronous prefetch of tensor data from global memory to the L2 cache. In im2col mode, some dimensions of the source tensor are unrolled into a single dimensional column at the destination. In this mode, the tensor has to be at least three-dimensional. Along with the tensor coordinates, im2col offsets are also specified (denoted by i16 im2col0...i16 %im2col2). The number of im2col offsets is two less than the number of dimensions of the tensor operation. The last argument to these intrinsics is a boolean flag, with the same functionality as described in the tile mode intrinsics above.

For more information, refer PTX ISA https://docs.nvidia.com/cuda/parallel-thread-execution/#data-movement-and-conversion-instructions-cp-async-bulk-prefetch-tensor.

Other Intrinsics ¶

For the full set of NVPTX intrinsics, please see the include/llvm/IR/IntrinsicsNVVM.td file in the LLVM source tree.

Linking with Libdevice ¶

The CUDA Toolkit comes with an LLVM bitcode library called libdevice that implements many common mathematical functions. This library can be used as a high-performance math library for any compilers using the LLVM NVPTX target. The library can be found under nvvm/libdevice/ in the CUDA Toolkit and there is a separate version for each compute architecture.

For a list of all math functions implemented in libdevice, see libdevice Users Guide.

To accommodate various math-related compiler flags that can affect code generation of libdevice code, the library code depends on a special LLVM IR pass (NVVMReflect) to handle conditional compilation within LLVM IR. This pass looks for calls to the @__nvvm_reflect function and replaces them with constants based on the defined reflection parameters. Such conditional code often follows a pattern:

float my_function(float a) {
  if (__nvvm_reflect("FASTMATH"))
    return my_function_fast(a);
  else
    return my_function_precise(a);
}

The default value for all unspecified reflection parameters is zero.

The NVVMReflect pass should be executed early in the optimization pipeline, immediately after the link stage. The internalize pass is also recommended to remove unused math functions from the resulting PTX. For an input IR module module.bc, the following compilation flow is recommended:

The NVVMReflect pass will attempt to remove dead code even without optimizations. This allows potentially incompatible instructions to be avoided at all optimizations levels by using the __CUDA_ARCH argument.

Save list of external functions in module.bc
Link module.bc with libdevice.compute_XX.YY.bc
Internalize all functions not in list from (1)
Eliminate all unused internal functions
Run NVVMReflect pass
Run standard optimization pipeline

Note

linkonce and linkonce_odr linkage types are not suitable for the libdevice functions. It is possible to link two IR modules that have been linked against libdevice using different reflection variables.

Since the NVVMReflect pass replaces conditionals with constants, it will often leave behind dead code of the form:

entry:
  ..
  br i1 true, label %foo, label %bar
foo:
  ..
bar:
  ; Dead code
  ..

Therefore, it is recommended that NVVMReflect is executed early in the optimization pipeline before dead-code elimination.

The NVPTX TargetMachine knows how to schedule NVVMReflect at the beginning of your pass manager; just use the following code when setting up your pass manager and the PassBuilder will use registerPassBuilderCallbacks to let NVPTXTargetMachine::registerPassBuilderCallbacks add the pass to the pass manager:

std::unique_ptr<TargetMachine> TM = ...;
PassBuilder PB(TM);
ModulePassManager MPM;
PB.parsePassPipeline(MPM, ...);

Reflection Parameters ¶

The libdevice library currently uses the following reflection parameters to control code generation:

Flag	Description
`__CUDA_FTZ=[0,1]`	Use optimized code paths that flush subnormals to zero

The value of this flag is determined by the “nvvm-reflect-ftz” module flag. The following sets the ftz flag to 1.

!llvm.module.flags = !{!0}
!0 = !{i32 4, !"nvvm-reflect-ftz", i32 1}

(i32 4 indicates that the value set here overrides the value in another module we link with. See the LangRef <LangRef.html#module-flags-metadata> for details.)

Executing PTX ¶

The most common way to execute PTX assembly on a GPU device is to use the CUDA Driver API. This API is a low-level interface to the GPU driver and allows for JIT compilation of PTX code to native GPU machine code.

Initializing the Driver API:

CUdevice device;
CUcontext context;

// Initialize the driver API
cuInit(0);
// Get a handle to the first compute device
cuDeviceGet(&device, 0);
// Create a compute device context
cuCtxCreate(&context, 0, device);

JIT compiling a PTX string to a device binary:

CUmodule module;
CUfunction function;

// JIT compile a null-terminated PTX string
cuModuleLoadData(&module, (void*)PTXString);

// Get a handle to the "myfunction" kernel function
cuModuleGetFunction(&function, module, "myfunction");

For full examples of executing PTX assembly, please see the CUDA Samples distribution.

Common Issues ¶

ptxas complains of undefined function: __nvvm_reflect ¶

When linking with libdevice, the NVVMReflect pass must be used. See Linking with Libdevice for more information.

Tutorial: A Simple Compute Kernel ¶

To start, let us take a look at a simple compute kernel written directly in LLVM IR. The kernel implements vector addition, where each thread computes one element of the output vector C from the input vectors A and B. To make this easier, we also assume that only a single CTA (thread block) will be launched, and that it will be one dimensional.

The Kernel ¶

target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v16:16:16-v32:32:32-v64:64:64-v128:128:128-n16:32:64"
target triple = "nvptx64-nvidia-cuda"

; Intrinsic to read X component of thread ID
declare i32 @llvm.nvvm.read.ptx.sreg.tid.x() readnone nounwind

define void @kernel(ptr addrspace(1) %A,
                    ptr addrspace(1) %B,
                    ptr addrspace(1) %C) {
entry:
  ; What is my ID?
  %id = tail call i32 @llvm.nvvm.read.ptx.sreg.tid.x() readnone nounwind

  ; Compute pointers into A, B, and C
  %ptrA = getelementptr float, ptr addrspace(1) %A, i32 %id
  %ptrB = getelementptr float, ptr addrspace(1) %B, i32 %id
  %ptrC = getelementptr float, ptr addrspace(1) %C, i32 %id

  ; Read A, B
  %valA = load float, ptr addrspace(1) %ptrA, align 4
  %valB = load float, ptr addrspace(1) %ptrB, align 4

  ; Compute C = A + B
  %valC = fadd float %valA, %valB

  ; Store back to C
  store float %valC, ptr addrspace(1) %ptrC, align 4

  ret void
}

!nvvm.annotations = !{!0}
!0 = !{ptr @kernel, !"kernel", i32 1}

We can use the LLVM llc tool to directly run the NVPTX code generator:

# llc -mcpu=sm_20 kernel.ll -o kernel.ptx

Note

If you want to generate 32-bit code, change p:64:64:64 to p:32:32:32 in the module data layout string and use nvptx-nvidia-cuda as the target triple.

The output we get from llc (as of LLVM 3.4):

//
// Generated by LLVM NVPTX Back-End
//

.version 3.1
.target sm_20
.address_size 64

  // .globl kernel
                                        // @kernel
.visible .entry kernel(
  .param .u64 kernel_param_0,
  .param .u64 kernel_param_1,
  .param .u64 kernel_param_2
)
{
  .reg .f32   %f<4>;
  .reg .s32   %r<2>;
  .reg .s64   %rl<8>;

// %bb.0:                                // %entry
  ld.param.u64    %rl1, [kernel_param_0];
  mov.u32         %r1, %tid.x;
  mul.wide.s32    %rl2, %r1, 4;
  add.s64         %rl3, %rl1, %rl2;
  ld.param.u64    %rl4, [kernel_param_1];
  add.s64         %rl5, %rl4, %rl2;
  ld.param.u64    %rl6, [kernel_param_2];
  add.s64         %rl7, %rl6, %rl2;
  ld.global.f32   %f1, [%rl3];
  ld.global.f32   %f2, [%rl5];
  add.f32         %f3, %f1, %f2;
  st.global.f32   [%rl7], %f3;
  ret;
}

Dissecting the Kernel ¶

Now let us dissect the LLVM IR that makes up this kernel.

Data Layout ¶

The data layout string determines the size in bits of common data types, their ABI alignment, and their storage size. For NVPTX, you should use one of the following:

32-bit PTX:

target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v16:16:16-v32:32:32-v64:64:64-v128:128:128-n16:32:64"

64-bit PTX:

target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v16:16:16-v32:32:32-v64:64:64-v128:128:128-n16:32:64"

Target Intrinsics ¶

In this example, we use the @llvm.nvvm.read.ptx.sreg.tid.x intrinsic to read the X component of the current thread’s ID, which corresponds to a read of register %tid.x in PTX. The NVPTX back-end supports a large set of intrinsics. A short list is shown below; please see include/llvm/IR/IntrinsicsNVVM.td for the full list.

Intrinsic	CUDA Equivalent
`i32 @llvm.nvvm.read.ptx.sreg.tid.{x,y,z}`	threadIdx.{x,y,z}
`i32 @llvm.nvvm.read.ptx.sreg.ctaid.{x,y,z}`	blockIdx.{x,y,z}
`i32 @llvm.nvvm.read.ptx.sreg.ntid.{x,y,z}`	blockDim.{x,y,z}
`i32 @llvm.nvvm.read.ptx.sreg.nctaid.{x,y,z}`	gridDim.{x,y,z}
`void @llvm.nvvm.barrier0()`	__syncthreads()

Address Spaces ¶

You may have noticed that all of the pointer types in the LLVM IR example had an explicit address space specifier. What is address space 1? NVIDIA GPU devices (generally) have four types of memory:

Global: Large, off-chip memory
Shared: Small, on-chip memory shared among all threads in a CTA
Local: Per-thread, private memory
Constant: Read-only memory shared across all threads

These different types of memory are represented in LLVM IR as address spaces. There is also a fifth address space used by the NVPTX code generator that corresponds to the “generic” address space. This address space can represent addresses in any other address space (with a few exceptions). This allows users to write IR functions that can load/store memory using the same instructions. Intrinsics are provided to convert pointers between the generic and non-generic address spaces.

See Address Spaces and NVPTX Intrinsics for more information.

Kernel Metadata ¶

In PTX, a function can be either a kernel function (callable from the host program), or a device function (callable only from GPU code). You can think of kernel functions as entry-points in the GPU program. To mark an LLVM IR function as a kernel function, we make use of special LLVM metadata. The NVPTX back-end will look for a named metadata node called nvvm.annotations. This named metadata must contain a list of metadata that describe the IR. For our purposes, we need to declare a metadata node that assigns the “kernel” attribute to the LLVM IR function that should be emitted as a PTX kernel function. These metadata nodes take the form:

!{<function ref>, metadata !"kernel", i32 1}

For the previous example, we have:

!nvvm.annotations = !{!0}
!0 = !{ptr @kernel, !"kernel", i32 1}

Here, we have a single metadata declaration in nvvm.annotations. This metadata annotates our @kernel function with the kernel attribute.

Running the Kernel ¶

Generating PTX from LLVM IR is all well and good, but how do we execute it on a real GPU device? The CUDA Driver API provides a convenient mechanism for loading and JIT compiling PTX to a native GPU device, and launching a kernel. The API is similar to OpenCL. A simple example showing how to load and execute our vector addition code is shown below. Note that for brevity this code does not perform much error checking!

Note

You can also use the ptxas tool provided by the CUDA Toolkit to offline compile PTX to machine code (SASS) for a specific GPU architecture. Such binaries can be loaded by the CUDA Driver API in the same way as PTX. This can be useful for reducing startup time by precompiling the PTX kernels.

#include <iostream>
#include <fstream>
#include <cassert>
#include "cuda.h"


void checkCudaErrors(CUresult err) {
  assert(err == CUDA_SUCCESS);
}

/// main - Program entry point
int main(int argc, char **argv) {
  CUdevice    device;
  CUmodule    cudaModule;
  CUcontext   context;
  CUfunction  function;
  CUlinkState linker;
  int         devCount;

  // CUDA initialization
  checkCudaErrors(cuInit(0));
  checkCudaErrors(cuDeviceGetCount(&devCount));
  checkCudaErrors(cuDeviceGet(&device, 0));

  char name[128];
  checkCudaErrors(cuDeviceGetName(name, 128, device));
  std::cout << "Using CUDA Device [0]: " << name << "\n";

  int devMajor, devMinor;
  checkCudaErrors(cuDeviceComputeCapability(&devMajor, &devMinor, device));
  std::cout << "Device Compute Capability: "
            << devMajor << "." << devMinor << "\n";
  if (devMajor < 2) {
    std::cerr << "ERROR: Device 0 is not SM 2.0 or greater\n";
    return 1;
  }

  std::ifstream t("kernel.ptx");
  if (!t.is_open()) {
    std::cerr << "kernel.ptx not found\n";
    return 1;
  }
  std::string str((std::istreambuf_iterator<char>(t)),
                    std::istreambuf_iterator<char>());

  // Create driver context
  checkCudaErrors(cuCtxCreate(&context, 0, device));

  // Create module for object
  checkCudaErrors(cuModuleLoadDataEx(&cudaModule, str.c_str(), 0, 0, 0));

  // Get kernel function
  checkCudaErrors(cuModuleGetFunction(&function, cudaModule, "kernel"));

  // Device data
  CUdeviceptr devBufferA;
  CUdeviceptr devBufferB;
  CUdeviceptr devBufferC;

  checkCudaErrors(cuMemAlloc(&devBufferA, sizeof(float)*16));
  checkCudaErrors(cuMemAlloc(&devBufferB, sizeof(float)*16));
  checkCudaErrors(cuMemAlloc(&devBufferC, sizeof(float)*16));

  float* hostA = new float[16];
  float* hostB = new float[16];
  float* hostC = new float[16];

  // Populate input
  for (unsigned i = 0; i != 16; ++i) {
    hostA[i] = (float)i;
    hostB[i] = (float)(2*i);
    hostC[i] = 0.0f;
  }

  checkCudaErrors(cuMemcpyHtoD(devBufferA, &hostA[0], sizeof(float)*16));
  checkCudaErrors(cuMemcpyHtoD(devBufferB, &hostB[0], sizeof(float)*16));


  unsigned blockSizeX = 16;
  unsigned blockSizeY = 1;
  unsigned blockSizeZ = 1;
  unsigned gridSizeX  = 1;
  unsigned gridSizeY  = 1;
  unsigned gridSizeZ  = 1;

  // Kernel parameters
  void *KernelParams[] = { &devBufferA, &devBufferB, &devBufferC };

  std::cout << "Launching kernel\n";

  // Kernel launch
  checkCudaErrors(cuLaunchKernel(function, gridSizeX, gridSizeY, gridSizeZ,
                                 blockSizeX, blockSizeY, blockSizeZ,
                                 0, NULL, KernelParams, NULL));

  // Retrieve device data
  checkCudaErrors(cuMemcpyDtoH(&hostC[0], devBufferC, sizeof(float)*16));


  std::cout << "Results:\n";
  for (unsigned i = 0; i != 16; ++i) {
    std::cout << hostA[i] << " + " << hostB[i] << " = " << hostC[i] << "\n";
  }


  // Clean up after ourselves
  delete [] hostA;
  delete [] hostB;
  delete [] hostC;

  // Clean-up
  checkCudaErrors(cuMemFree(devBufferA));
  checkCudaErrors(cuMemFree(devBufferB));
  checkCudaErrors(cuMemFree(devBufferC));
  checkCudaErrors(cuModuleUnload(cudaModule));
  checkCudaErrors(cuCtxDestroy(context));

  return 0;
}

You will need to link with the CUDA driver and specify the path to cuda.h.

# clang++ sample.cpp -o sample -O2 -g -I/usr/local/cuda-5.5/include -lcuda

We don’t need to specify a path to libcuda.so since this is installed in a system location by the driver, not the CUDA toolkit.

If everything goes as planned, you should see the following output when running the compiled program:

Using CUDA Device [0]: GeForce GTX 680
Device Compute Capability: 3.0
Launching kernel
Results:
+ 0 = 0
+ 2 = 3
+ 4 = 6
+ 6 = 9
+ 8 = 12
+ 10 = 15
+ 12 = 18
+ 14 = 21
+ 16 = 24
+ 18 = 27
+ 20 = 30
+ 22 = 33
+ 24 = 36
+ 26 = 39
+ 28 = 42
+ 30 = 45

Note

You will likely see a different device identifier based on your hardware

Tutorial: Linking with Libdevice ¶

In this tutorial, we show a simple example of linking LLVM IR with the libdevice library. We will use the same kernel as the previous tutorial, except that we will compute C = pow(A, B) instead of C = A + B. Libdevice provides an __nv_powf function that we will use.

target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v16:16:16-v32:32:32-v64:64:64-v128:128:128-n16:32:64"
target triple = "nvptx64-nvidia-cuda"

; Intrinsic to read X component of thread ID
declare i32 @llvm.nvvm.read.ptx.sreg.tid.x() readnone nounwind
; libdevice function
declare float @__nv_powf(float, float)

define void @kernel(ptr addrspace(1) %A,
                    ptr addrspace(1) %B,
                    ptr addrspace(1) %C) {
entry:
  ; What is my ID?
  %id = tail call i32 @llvm.nvvm.read.ptx.sreg.tid.x() readnone nounwind

  ; Compute pointers into A, B, and C
  %ptrA = getelementptr float, ptr addrspace(1) %A, i32 %id
  %ptrB = getelementptr float, ptr addrspace(1) %B, i32 %id
  %ptrC = getelementptr float, ptr addrspace(1) %C, i32 %id

  ; Read A, B
  %valA = load float, ptr addrspace(1) %ptrA, align 4
  %valB = load float, ptr addrspace(1) %ptrB, align 4

  ; Compute C = pow(A, B)
  %valC = call float @__nv_powf(float %valA, float %valB)

  ; Store back to C
  store float %valC, ptr addrspace(1) %ptrC, align 4

  ret void
}

!nvvm.annotations = !{!0}
!0 = !{ptr @kernel, !"kernel", i32 1}

To compile this kernel, we perform the following steps:

Link with libdevice
Internalize all but the public kernel function
Run NVVMReflect and set __CUDA_FTZ to 0
Optimize the linked module
Codegen the module

These steps can be performed by the LLVM llvm-link, opt, and llc tools. In a complete compiler, these steps can also be performed entirely programmatically by setting up an appropriate pass configuration (see Linking with Libdevice).

# llvm-link t2.bc libdevice.compute_20.10.bc -o t2.linked.bc
# opt -internalize -internalize-public-api-list=kernel -nvvm-reflect-list=__CUDA_FTZ=0 -nvvm-reflect -O3 t2.linked.bc -o t2.opt.bc
# llc -mcpu=sm_20 t2.opt.bc -o t2.ptx

Note

The -nvvm-reflect-list=_CUDA_FTZ=0 is not strictly required, as any undefined variables will default to zero. It is shown here for evaluation purposes.

This gives us the following PTX (excerpt):

//
// Generated by LLVM NVPTX Back-End
//

.version 3.1
.target sm_20
.address_size 64

  // .globl kernel
                                        // @kernel
.visible .entry kernel(
  .param .u64 kernel_param_0,
  .param .u64 kernel_param_1,
  .param .u64 kernel_param_2
)
{
  .reg .pred  %p<30>;
  .reg .f32   %f<111>;
  .reg .s32   %r<21>;
  .reg .s64   %rl<8>;

// %bb.0:                                // %entry
  ld.param.u64  %rl2, [kernel_param_0];
  mov.u32   %r3, %tid.x;
  ld.param.u64  %rl3, [kernel_param_1];
  mul.wide.s32  %rl4, %r3, 4;
  add.s64   %rl5, %rl2, %rl4;
  ld.param.u64  %rl6, [kernel_param_2];
  add.s64   %rl7, %rl3, %rl4;
  add.s64   %rl1, %rl6, %rl4;
  ld.global.f32   %f1, [%rl5];
  ld.global.f32   %f2, [%rl7];
  setp.eq.f32 %p1, %f1, 0f3F800000;
  setp.eq.f32 %p2, %f2, 0f00000000;
  or.pred   %p3, %p1, %p2;
  @%p3 bra  BB0_1;
  bra.uni   BB0_2;
BB0_1:
  mov.f32   %f110, 0f3F800000;
  st.global.f32   [%rl1], %f110;
  ret;
BB0_2:                                  // %__nv_isnanf.exit.i
  abs.f32   %f4, %f1;
  setp.gtu.f32  %p4, %f4, 0f7F800000;
  @%p4 bra  BB0_4;
// %bb.3:                                // %__nv_isnanf.exit5.i
  abs.f32   %f5, %f2;
  setp.le.f32 %p5, %f5, 0f7F800000;
  @%p5 bra  BB0_5;
BB0_4:                                  // %.critedge1.i
  add.f32   %f110, %f1, %f2;
  st.global.f32   [%rl1], %f110;
  ret;
BB0_5:                                  // %__nv_isinff.exit.i

  ...

BB0_26:                                 // %__nv_truncf.exit.i.i.i.i.i
  mul.f32   %f90, %f107, 0f3FB8AA3B;
  cvt.rzi.f32.f32 %f91, %f90;
  mov.f32   %f92, 0fBF317200;
  fma.rn.f32  %f93, %f91, %f92, %f107;
  mov.f32   %f94, 0fB5BFBE8E;
  fma.rn.f32  %f95, %f91, %f94, %f93;
  mul.f32   %f89, %f95, 0f3FB8AA3B;
  // inline asm
  ex2.approx.ftz.f32 %f88,%f89;
  // inline asm
  add.f32   %f96, %f91, 0f00000000;
  ex2.approx.f32  %f97, %f96;
  mul.f32   %f98, %f88, %f97;
  setp.lt.f32 %p15, %f107, 0fC2D20000;
  selp.f32  %f99, 0f00000000, %f98, %p15;
  setp.gt.f32 %p16, %f107, 0f42D20000;
  selp.f32  %f110, 0f7F800000, %f99, %p16;
  setp.eq.f32 %p17, %f110, 0f7F800000;
  @%p17 bra   BB0_28;
// %bb.27:
  fma.rn.f32  %f110, %f110, %f108, %f110;
BB0_28:                                 // %__internal_accurate_powf.exit.i
  setp.lt.f32 %p18, %f1, 0f00000000;
  setp.eq.f32 %p19, %f3, 0f3F800000;
  and.pred    %p20, %p18, %p19;
  @!%p20 bra  BB0_30;
  bra.uni   BB0_29;
BB0_29:
  mov.b32    %r9, %f110;
  xor.b32   %r10, %r9, -2147483648;
  mov.b32    %f110, %r10;
BB0_30:                                 // %__nv_powf.exit
  st.global.f32   [%rl1], %f110;
  ret;
}

CUDA Builtin	PTX Special Register Intrinsic
`threadId`	`@llvm.nvvm.read.ptx.sreg.tid.*`
`blockIdx`	`@llvm.nvvm.read.ptx.sreg.ctaid.*`
`blockDim`	`@llvm.nvvm.read.ptx.sreg.ntid.*`
`gridDim`	`@llvm.nvvm.read.ptx.sreg.nctaid.*`

NVVM Intrinsic	PTX Instruction
`@llvm.nvvm.fence.proxy.tensormap_generic.release.*`	`fence.proxy.tensormap::generic.release.*`
`@llvm.nvvm.fence.proxy.tensormap_generic.acquire.*`	`fence.proxy.tensormap::generic.acquire.* [addr], size`