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Make use of @llvm.assume in ValueTracking (computeKnownBits, etc.)

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This change, which allows @llvm.assume to be used from within computeKnownBits
(and other associated functions in ValueTracking), adds some (optional)
parameters to computeKnownBits and friends. These functions now (optionally)
take a "context" instruction pointer, an AssumptionTracker pointer, and also a
DomTree pointer, and most of the changes are just to pass this new information
when it is easily available from InstSimplify, InstCombine, etc.

As explained below, the significant conceptual change is that known properties
of a value might depend on the control-flow location of the use (because we
care that the @llvm.assume dominates the use because assumptions have
control-flow dependencies). This means that, when we ask if bits are known in a
value, we might get different answers for different uses.

The significant changes are all in ValueTracking. Two main changes: First, as
with the rest of the code, new parameters need to be passed around. To make
this easier, I grouped them into a structure, and I made internal static
versions of the relevant functions that take this structure as a parameter. The
new code does as you might expect, it looks for @llvm.assume calls that make
use of the value we're trying to learn something about (often indirectly),
attempts to pattern match that expression, and uses the result if successful.
By making use of the AssumptionTracker, the process of finding @llvm.assume
calls is not expensive.

Part of the structure being passed around inside ValueTracking is a set of
already-considered @llvm.assume calls. This is to prevent a query using, for
example, the assume(a == b), to recurse on itself. The context and DT params
are used to find applicable assumptions. An assumption needs to dominate the
context instruction, or come after it deterministically. In this latter case we
only handle the specific case where both the assumption and the context
instruction are in the same block, and we need to exclude assumptions from
being used to simplify their own ephemeral values (those which contribute only
to the assumption) because otherwise the assumption would prove its feeding
comparison trivial and would be removed.

This commit adds the plumbing and the logic for a simple masked-bit propagation
(just enough to write a regression test). Future commits add more patterns
(and, correspondingly, more regression tests).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217342 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hal Finkel
2014-09-07 18:57:58 +00:00
parent 22f8dcb2b5
commit 851b04c920
48 changed files with 1457 additions and 545 deletions
Download Patch File Download Diff File Expand all files Collapse all files

43
lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -17,6 +17,7 @@
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionTracker.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/InstructionSimplify.h"
@ -194,7 +195,9 @@ namespace {
/// represented in the result.
static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
ExtensionKind &Extension,
const DataLayout &DL, unsigned Depth) {
const DataLayout &DL, unsigned Depth,
AssumptionTracker *AT,
DominatorTree *DT) {
assert(V->getType()->isIntegerTy() && "Not an integer value");
// Limit our recursion depth.
@ -211,23 +214,24 @@ static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
case Instruction::Or:
// X|C == X+C if all the bits in C are unset in X. Otherwise we can't
// analyze it.
if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &DL))
if (!MaskedValueIsZero(BOp->getOperand(0), RHSC->getValue(), &DL, 0,
AT, BOp, DT))
break;
// FALL THROUGH.
case Instruction::Add:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
DL, Depth+1);
DL, Depth+1, AT, DT);
Offset += RHSC->getValue();
return V;
case Instruction::Mul:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
DL, Depth+1);
DL, Depth+1, AT, DT);
Offset *= RHSC->getValue();
Scale *= RHSC->getValue();
return V;
case Instruction::Shl:
V = GetLinearExpression(BOp->getOperand(0), Scale, Offset, Extension,
DL, Depth+1);
DL, Depth+1, AT, DT);
Offset <<= RHSC->getValue().getLimitedValue();
Scale <<= RHSC->getValue().getLimitedValue();
return V;
@ -248,7 +252,7 @@ static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
Extension = isa<SExtInst>(V) ? EK_SignExt : EK_ZeroExt;
Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
DL, Depth+1);
DL, Depth+1, AT, DT);
Scale = Scale.zext(OldWidth);
Offset = Offset.zext(OldWidth);
@ -278,7 +282,8 @@ static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
static const Value *
DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
SmallVectorImpl<VariableGEPIndex> &VarIndices,
bool &MaxLookupReached, const DataLayout *DL) {
bool &MaxLookupReached, const DataLayout *DL,
AssumptionTracker *AT, DominatorTree *DT) {
// Limit recursion depth to limit compile time in crazy cases.
unsigned MaxLookup = MaxLookupSearchDepth;
MaxLookupReached = false;
@ -309,7 +314,10 @@ DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
// If it's not a GEP, hand it off to SimplifyInstruction to see if it
// can come up with something. This matches what GetUnderlyingObject does.
if (const Instruction *I = dyn_cast<Instruction>(V))
// TODO: Get a DominatorTree and use it here.
// TODO: Get a DominatorTree and AssumptionTracker and use them here
// (these are both now available in this function, but this should be
// updated when GetUnderlyingObject is updated). TLI should be
// provided also.
if (const Value *Simplified =
SimplifyInstruction(const_cast<Instruction *>(I), DL)) {
V = Simplified;
@ -368,7 +376,7 @@ DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
// Use GetLinearExpression to decompose the index into a C1*V+C2 form.
APInt IndexScale(Width, 0), IndexOffset(Width, 0);
Index = GetLinearExpression(Index, IndexScale, IndexOffset, Extension,
*DL, 0);
*DL, 0, AT, DT);
// The GEP index scale ("Scale") scales C1*V+C2, yielding (C1*V+C2)*Scale.
// This gives us an aggregate computation of (C1*Scale)*V + C2*Scale.
@ -449,6 +457,7 @@ namespace {
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AliasAnalysis>();
AU.addRequired<AssumptionTracker>();
AU.addRequired<TargetLibraryInfo>();
}
@ -571,6 +580,7 @@ char BasicAliasAnalysis::ID = 0;
INITIALIZE_AG_PASS_BEGIN(BasicAliasAnalysis, AliasAnalysis, "basicaa",
"Basic Alias Analysis (stateless AA impl)",
false, true, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
INITIALIZE_AG_PASS_END(BasicAliasAnalysis, AliasAnalysis, "basicaa",
"Basic Alias Analysis (stateless AA impl)",
@ -884,6 +894,11 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
bool GEP1MaxLookupReached;
SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
AssumptionTracker *AT = &getAnalysis<AssumptionTracker>();
DominatorTreeWrapperPass *DTWP =
getAnalysisIfAvailable<DominatorTreeWrapperPass>();
DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
// If we have two gep instructions with must-alias or not-alias'ing base
// pointers, figure out if the indexes to the GEP tell us anything about the
// derived pointer.
@ -907,10 +922,10 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
const Value *GEP2BasePtr =
DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
GEP2MaxLookupReached, DL);
GEP2MaxLookupReached, DL, AT, DT);
const Value *GEP1BasePtr =
DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
GEP1MaxLookupReached, DL);
GEP1MaxLookupReached, DL, AT, DT);
// DecomposeGEPExpression and GetUnderlyingObject should return the
// same result except when DecomposeGEPExpression has no DataLayout.
if (GEP1BasePtr != UnderlyingV1 || GEP2BasePtr != UnderlyingV2) {
@ -939,14 +954,14 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
// about the relation of the resulting pointer.
const Value *GEP1BasePtr =
DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
GEP1MaxLookupReached, DL);
GEP1MaxLookupReached, DL, AT, DT);
int64_t GEP2BaseOffset;
bool GEP2MaxLookupReached;
SmallVector<VariableGEPIndex, 4> GEP2VariableIndices;
const Value *GEP2BasePtr =
DecomposeGEPExpression(GEP2, GEP2BaseOffset, GEP2VariableIndices,
GEP2MaxLookupReached, DL);
GEP2MaxLookupReached, DL, AT, DT);
// DecomposeGEPExpression and GetUnderlyingObject should return the
// same result except when DecomposeGEPExpression has no DataLayout.
@ -985,7 +1000,7 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
const Value *GEP1BasePtr =
DecomposeGEPExpression(GEP1, GEP1BaseOffset, GEP1VariableIndices,
GEP1MaxLookupReached, DL);
GEP1MaxLookupReached, DL, AT, DT);
// DecomposeGEPExpression and GetUnderlyingObject should return the
// same result except when DecomposeGEPExpression has no DataLayout.