LLVM  11.0.0git
LiveDebugValues.cpp File Reference

LiveDebugValues is an optimistic "available expressions" dataflow algorithm. More...

#include "llvm/ADT/CoalescingBitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/UniqueVector.h"
#include "llvm/CodeGen/LexicalScopes.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/PseudoSourceValue.h"
#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/CodeGen/TargetFrameLowering.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <functional>
#include <queue>
#include <tuple>
#include <utility>
#include <vector>
Include dependency graph for LiveDebugValues.cpp:

Go to the source code of this file.

## Macros

#define DEBUG_TYPE   "livedebugvalues"

## Functions

STATISTIC (NumInserted, "Number of DBG_VALUE instructions inserted")

static Register isDbgValueDescribedByReg (const MachineInstr &MI)

static bool isRegOtherThanSPAndFP (const MachineOperand &Op, const MachineInstr &MI, const TargetRegisterInfo *TRI)
If Op is a stack or frame register return true, otherwise return false. More...

INITIALIZE_PASS (LiveDebugValues, DEBUG_TYPE, "Live DEBUG_VALUE analysis", false, false) LiveDebugValues
Default construct and initialize the pass. More...

static void collectRegDefs (const MachineInstr &MI, DefinedRegsSet &Regs, const TargetRegisterInfo *TRI)
Collect all register defines (including aliases) for the given instruction. More...

## Variables

static cl::opt< unsignedInputBBLimit ("livedebugvalues-input-bb-limit", cl::desc("Maximum input basic blocks before DBG_VALUE limit applies"), cl::init(10000), cl::Hidden)

static cl::opt< unsignedInputDbgValueLimit ("livedebugvalues-input-dbg-value-limit", cl::desc("Maximum input DBG_VALUE insts supported by debug range extension"), cl::init(50000), cl::Hidden)

## Detailed Description

LiveDebugValues is an optimistic "available expressions" dataflow algorithm.

The set of expressions is the set of machine locations (registers, spill slots, constants) that a variable fragment might be located, qualified by a DIExpression and indirect-ness flag, while each variable is identified by a DebugVariable object. The availability of an expression begins when a DBG_VALUE instruction specifies the location of a DebugVariable, and continues until that location is clobbered or re-specified by a different DBG_VALUE for the same DebugVariable.

The cannonical "available expressions" problem doesn't have expression clobbering, instead when a variable is re-assigned, any expressions using that variable get invalidated. LiveDebugValues can map onto "available expressions" by having every register represented by a variable, which is used in an expression that becomes available at a DBG_VALUE instruction. When the register is clobbered, its variable is effectively reassigned, and expressions computed from it become unavailable. A similar construct is needed when a DebugVariable has its location re-specified, to invalidate all other locations for that DebugVariable.

Using the dataflow analysis to compute the available expressions, we create a DBG_VALUE at the beginning of each block where the expression is live-in. This propagates variable locations into every basic block where the location can be determined, rather than only having DBG_VALUEs in blocks where locations are specified due to an assignment or some optimization. Movements of values between registers and spill slots are annotated with DBG_VALUEs too to track variable values bewteen locations. All this allows DbgEntityHistoryCalculator to focus on only the locations within individual blocks, facilitating testing and improving modularity.

We follow an optimisic dataflow approach, with this lattice:

///                    ┬ "Unknown"
///                          |
///                          v
///                         True
///                          |
///                          v
///                      ⊥ False
/// 

With "True" signifying that the expression is available (and thus a DebugVariable's location is the corresponding register), while "False" signifies that the expression is unavailable. "Unknown"s never survive to the end of the analysis (see below).

Formally, all DebugVariable locations that are live-out of a block are initialized to . A blocks live-in values take the meet of the lattice value for every predecessors live-outs, except for the entry block, where all live-ins are . The usual dataflow propagation occurs: the transfer function for a block assigns an expression for a DebugVariable to be "True" if a DBG_VALUE in the block specifies it; "False" if the location is clobbered; or the live-in value if it is unaffected by the block. We visit each block in reverse post order until a fixedpoint is reached. The solution produced is maximal.

Intuitively, we start by assuming that every expression / variable location is at least "True", and then propagate "False" from the entry block and any clobbers until there are no more changes to make. This gives us an accurate solution because all incorrect locations will have a "False" propagated into them. It also gives us a solution that copes well with loops by assuming that variable locations are live-through every loop, and then removing those that are not through dataflow.

Within LiveDebugValues: each variable location is represented by a VarLoc object that identifies the source variable, its current machine-location, and the DBG_VALUE inst that specifies the location. Each VarLoc is indexed in the (function-scope) VarLocMap, giving each VarLoc a unique index. Rather than operate directly on machine locations, the dataflow analysis in this pass identifies locations by their index in the VarLocMap, meaning all the variable locations in a block can be described by a sparse vector of VarLocMap indicies.

All the storage for the dataflow analysis is local to the ExtendRanges method and passed down to helper methods. "OutLocs" and "InLocs" record the in and out lattice values for each block. "OpenRanges" maintains a list of variable locations and, with the "process" method, evaluates the transfer function of each block. "flushPendingLocs" installs DBG_VALUEs for each live-in location at the start of blocks, while "Transfers" records transfers of values between machine-locations.

We avoid explicitly representing the "Unknown" () lattice value in the implementation. Instead, unvisited blocks implicitly have all lattice values set as "Unknown". After being visited, there will be path back to the entry block where the lattice value is "False", and as the transfer function cannot make new "Unknown" locations, there are no scenarios where a block can have an "Unknown" location after being visited. Similarly, we don't enumerate all possible variable locations before exploring the function: when a new location is discovered, all blocks previously explored were implicitly "False" but unrecorded, and become explicitly "False" when a new VarLoc is created with its bit not set in predecessor InLocs or OutLocs.

Definition in file LiveDebugValues.cpp.

## ◆ DEBUG_TYPE

 #define DEBUG_TYPE   "livedebugvalues"

Definition at line 154 of file LiveDebugValues.cpp.

## ◆ collectRegDefs()

 static void collectRegDefs ( const MachineInstr & MI, DefinedRegsSet & Regs, const TargetRegisterInfo * TRI )
static

Collect all register defines (including aliases) for the given instruction.

Definition at line 1762 of file LiveDebugValues.cpp.

## ◆ INITIALIZE_PASS()

 INITIALIZE_PASS ( LiveDebugValues , DEBUG_TYPE , "Live DEBUG_VALUE analysis" , false , false )

Default construct and initialize the pass.

Definition at line 832 of file LiveDebugValues.cpp.

## ◆ isDbgValueDescribedByReg()

 static Register isDbgValueDescribedByReg ( const MachineInstr & MI )
static

Definition at line 172 of file LiveDebugValues.cpp.

## ◆ isRegOtherThanSPAndFP()

 static bool isRegOtherThanSPAndFP ( const MachineOperand & Op, const MachineInstr & MI, const TargetRegisterInfo * TRI )
static

If Op is a stack or frame register return true, otherwise return false.

This is used to avoid basing the debug entry values on the registers, since we do not support it at the moment.

Definition at line 184 of file LiveDebugValues.cpp.

## ◆ STATISTIC()

 STATISTIC ( NumInserted , "Number of DBG_VALUE instructions inserted" )

## ◆ InputBBLimit

 cl::opt InputBBLimit("livedebugvalues-input-bb-limit", cl::desc("Maximum input basic blocks before DBG_VALUE limit applies"), cl::init(10000), cl::Hidden)
static

## ◆ InputDbgValueLimit

 cl::opt InputDbgValueLimit("livedebugvalues-input-dbg-value-limit", cl::desc( "Maximum input DBG_VALUE insts supported by debug range extension"), cl::init(50000), cl::Hidden)
static