Opaque Pointers

The Opaque Pointer Type

Traditionally, LLVM IR pointer types have contained a pointee type. For example, i32* is a pointer that points to an i32 somewhere in memory. However, due to a lack of pointee type semantics and various issues with having pointee types, there is a desire to remove pointee types from pointers.

The opaque pointer type project aims to replace all pointer types containing pointee types in LLVM with an opaque pointer type. The new pointer type is tentatively represented textually as ptr.

Address spaces are still used to distinguish between different kinds of pointers where the distinction is relevant for lowering (e.g. data vs function pointers have different sizes on some architectures). Opaque pointers are not changing anything related to address spaces and lowering. For more information, see DataLayout.

Issues with explicit pointee types

LLVM IR pointers can be cast back and forth between pointers with different pointee types. The pointee type does not necessarily actually represent the actual underlying type in memory. In other words, the pointee type contains no real semantics.

Lots of operations do not actually care about the underlying type. These operations, typically intrinsics, usually end up taking an i8*. This causes lots of redundant no-op bitcasts in the IR to and from a pointer with a different pointee type. The extra bitcasts take up space and require extra work to look through in optimizations. And more bitcasts increases the chances of incorrect bitcasts, especially in regards to address spaces.

Some instructions still need to know what type to treat the memory pointed to by the pointer as. For example, a load needs to know how many bytes to load from memory. In these cases, instructions themselves contain a type argument. For example the load instruction from older versions of LLVM

load i64* %p

becomes

load i64, ptr %p

A nice analogous transition that happened earlier in LLVM is integer signedness. There is no distinction between signed and unsigned integer types, rather the integer operations themselves contain what to treat the integer as. Initially, LLVM IR distinguished between unsigned and signed integer types. The transition from manifesting signedness in types to instructions happened early on in LLVM’s life to the betterment of LLVM IR.

Opaque Pointers Mode

During the transition phase, LLVM can be used in two modes: In typed pointer mode (currently still the default) all pointer types have a pointee type and opaque pointers cannot be used. In opaque pointers mode, all pointers are opaque. The opaque pointer mode can be enabled using -opaque-pointers in LLVM tools like opt, or -mllvm -opaque-pointers in clang.

In opaque pointer mode, all typed pointers used in IR, bitcode, or created using PointerType::get() and similar APIs are automatically converted into opaque pointers. This simplifies migration and allows testing existing IR with opaque pointers.

define i8* @test(i8* %p) {
  %p2 = getelementptr i8, i8* %p, i64 1
  ret i8* %p2
}

; Is automatically converted into the following if -opaque-pointers
; is enabled:

define ptr @test(ptr %p) {
  %p2 = getelementptr i8, ptr %p, i64 1
  ret ptr %p2
}

I Still Need Pointee Types!

The frontend should already know what type each operation operates on based on the input source code. However, some frontends like Clang may end up relying on LLVM pointer pointee types to keep track of pointee types. The frontend needs to keep track of frontend pointee types on its own.

For optimizations around frontend types, pointee types are not useful due their lack of semantics. Rather, since LLVM IR works on untyped memory, for a frontend to tell LLVM about frontend types for the purposes of alias analysis, extra metadata is added to the IR. For more information, see TBAA.

Some specific operations still need to know what type a pointer types to. For the most part, this is codegen and ABI specific. For example, byval arguments are pointers, but backends need to know the underlying type of the argument to properly lower it. In cases like these, the attributes contain a type argument. For example,

call void @f(ptr byval(i32) %p)

signifies that %p as an argument should be lowered as an i32 passed indirectly.

If you have use cases that this sort of fix doesn’t cover, please email llvm-dev.

Migration Instructions

In order to support opaque pointers, two types of changes tend to be necessary. The first is the removal of all calls to PointerType::getElementType() and Type::getPointerElementType().

In the LLVM middle-end and backend, this is usually accomplished by inspecting the type of relevant operations instead. For example, memory access related analyses and optimizations should use the types encoded in the load and store instructions instead of querying the pointer type.

Frontends need to be adjusted to track pointee types independently of LLVM, insofar as they are necessary for lowering. For example, clang now tracks the pointee type in the Address structure.

While direct usage of pointer element types is immediately apparent in code, there is a more subtle issue that opaque pointers need to contend with: A lot of code assumes that pointer equality also implies that the used load/store type is the same. Consider the following examples with typed an opaque pointers:

define i32 @test(i32* %p) {
  store i32 0, i32* %p
  %bc = bitcast i32* %p to i64*
  %v = load i64, i64* %bc
  ret i64 %v
}

define i32 @test(ptr %p) {
  store i32 0, ptr %p
  %v = load i64, ptr %p
  ret i64 %v
}

Without opaque pointers, a check that the pointer operand of the load and store are the same also ensures that the accessed type is the same. Using a different type requires a bitcast, which will result in distinct pointer operands.

With opaque pointers, the bitcast is not present, and this check is no longer sufficient. In the above example, it could result in store to load forwarding of an incorrect type. Code making such assumptions needs to be adjusted to check the accessed type explicitly: LI->getType() == SI->getValueOperand()->getType().

Frontends using the C API through an FFI interface should be aware that a number of C API functions are deprecated and will be removed as part of the opaque pointer transition:

LLVMBuildLoad -> LLVMBuildLoad2
LLVMBuildCall -> LLVMBuildCall2
LLVMBuildInvoke -> LLVMBuildInvoke2
LLVMBuildGEP -> LLVMBuildGEP2
LLVMBuildInBoundsGEP -> LLVMBuildInBoundsGEP2
LLVMBuildStructGEP -> LLVMBuildStructGEP2
LLVMConstGEP -> LLVMConstGEP2
LLVMConstInBoundsGEP -> LLVMConstInBoundsGEP2
LLVMAddAlias -> LLVMAddAlias2

Additionally, it will no longer be possible to call LLVMGetElementType() on a pointer type.

Transition State

As of January 2022 large parts of LLVM support opaque pointers, but there are still some major open problems:

  • Bitcode already fully supports opaque pointers, and reading up-to-date typed pointer bitcode in opaque pointers mode also works. However, we currently do not support pointee type based auto-upgrade of old bitcode in opaque pointer mode.
  • While clang has limited support for opaque pointers (sufficient to compile CTMark on Linux), a major effort will be needed to systematically remove all uses of getPointerElementType() and the deprecated Address() constructor.
  • We do not yet have a testing strategy for how we can test both typed and opaque pointers during the migration. Currently, individual tests for opaque pointers are being added, but the bulk of tests still uses typed pointers.
  • Loop access analysis does not support opaque pointers yet, and is currently the main source of assertion failures in optimized builds.
  • Miscellanous uses of pointer element types remain everywhere.