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Constant propagation after hiting llvm.assume
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After hitting @llvm.assume(X) we can:
- propagate equality that X == true
- if X is icmp/fcmp (with eq operation), and one of operand
  is constant we can change all variables with constants in the same BasicBlock

http://reviews.llvm.org/D11918

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245265 91177308-0d34-0410-b5e6-96231b3b80d8
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@prazek
prazek committed Aug 18, 2015
1 parent 8d5e5d4 commit 5982c0a
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Showing 2 changed files with 190 additions and 3 deletions.
71 changes: 68 additions & 3 deletions lib/Transforms/Scalar/GVN.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -608,6 +608,10 @@ namespace {
DenseMap<uint32_t, LeaderTableEntry> LeaderTable;
BumpPtrAllocator TableAllocator;

// Block-local map of equivalent values to their leader, does not
// propagate to any successors. Entries added mid-block are applied
// to the remaining instructions in the block.
SmallMapVector<llvm::Value *, llvm::Constant *, 4> ReplaceWithConstMap;
SmallVector<Instruction*, 8> InstrsToErase;

typedef SmallVector<NonLocalDepResult, 64> LoadDepVect;
Expand Down Expand Up @@ -699,6 +703,7 @@ namespace {
// Helper functions of redundant load elimination
bool processLoad(LoadInst *L);
bool processNonLocalLoad(LoadInst *L);
bool processAssumeIntrinsic(IntrinsicInst *II);
void AnalyzeLoadAvailability(LoadInst *LI, LoadDepVect &Deps,
AvailValInBlkVect &ValuesPerBlock,
UnavailBlkVect &UnavailableBlocks);
Expand All @@ -719,6 +724,7 @@ namespace {
void verifyRemoved(const Instruction *I) const;
bool splitCriticalEdges();
BasicBlock *splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ);
bool replaceOperandsWithConsts(Instruction *I) const;
bool propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root);
bool processFoldableCondBr(BranchInst *BI);
void addDeadBlock(BasicBlock *BB);
Expand Down Expand Up @@ -1759,6 +1765,38 @@ bool GVN::processNonLocalLoad(LoadInst *LI) {
return PerformLoadPRE(LI, ValuesPerBlock, UnavailableBlocks);
}

bool GVN::processAssumeIntrinsic(IntrinsicInst *IntrinsicI) {
assert(IntrinsicI->getIntrinsicID() == Intrinsic::assume &&
"This function can only be called with llvm.assume intrinsic");
Value *V = IntrinsicI->getArgOperand(0);
Constant *True = ConstantInt::getTrue(V->getContext());
bool Changed = false;
for (BasicBlock *Successor : successors(IntrinsicI->getParent())) {
BasicBlockEdge Edge(IntrinsicI->getParent(), Successor);

// This property is only true in dominated successors, propagateEquality
// will check dominance for us.
Changed |= propagateEquality(V, True, Edge);
}

if (auto *CmpI = dyn_cast<CmpInst>(V)) {
if (CmpI->getPredicate() == CmpInst::Predicate::ICMP_EQ ||
CmpI->getPredicate() == CmpInst::Predicate::FCMP_OEQ ||
(CmpI->getPredicate() == CmpInst::Predicate::FCMP_UEQ &&
CmpI->getFastMathFlags().noNaNs())) {
Value *CmpLHS = CmpI->getOperand(0);
Value *CmpRHS = CmpI->getOperand(1);
if (isa<Constant>(CmpLHS))
std::swap(CmpLHS, CmpRHS);
auto *RHSConst = dyn_cast<Constant>(CmpRHS);

// If only one operand is constant.
if (RHSConst != nullptr && !isa<Constant>(CmpLHS))
ReplaceWithConstMap[CmpLHS] = RHSConst;
}
}
return Changed;
}

static void patchReplacementInstruction(Instruction *I, Value *Repl) {
// Patch the replacement so that it is not more restrictive than the value
Expand Down Expand Up @@ -2031,6 +2069,21 @@ static bool isOnlyReachableViaThisEdge(const BasicBlockEdge &E,
return Pred != nullptr;
}

// Tries to replace instruction with const, using information from
// ReplaceWithConstMap.
bool GVN::replaceOperandsWithConsts(Instruction *Instr) const {
bool Changed = false;
for (unsigned OpNum = 0; OpNum < Instr->getNumOperands(); ++OpNum) {
Value *Operand = Instr->getOperand(OpNum);
auto it = ReplaceWithConstMap.find(Operand);
if (it != ReplaceWithConstMap.end()) {
Instr->setOperand(OpNum, it->second);
Changed = true;
}
}
return Changed;
}

/// The given values are known to be equal in every block
/// dominated by 'Root'. Exploit this, for example by replacing 'LHS' with
/// 'RHS' everywhere in the scope. Returns whether a change was made.
Expand All @@ -2047,11 +2100,13 @@ bool GVN::propagateEquality(Value *LHS, Value *RHS,
std::pair<Value*, Value*> Item = Worklist.pop_back_val();
LHS = Item.first; RHS = Item.second;

if (LHS == RHS) continue;
if (LHS == RHS)
continue;
assert(LHS->getType() == RHS->getType() && "Equality but unequal types!");

// Don't try to propagate equalities between constants.
if (isa<Constant>(LHS) && isa<Constant>(RHS)) continue;
if (isa<Constant>(LHS) && isa<Constant>(RHS))
continue;

// Prefer a constant on the right-hand side, or an Argument if no constants.
if (isa<Constant>(LHS) || (isa<Argument>(LHS) && !isa<Constant>(RHS)))
Expand Down Expand Up @@ -2202,6 +2257,10 @@ bool GVN::processInstruction(Instruction *I) {
return true;
}

if (IntrinsicInst *IntrinsicI = dyn_cast<IntrinsicInst>(I))
if (IntrinsicI->getIntrinsicID() == Intrinsic::assume)
return processAssumeIntrinsic(IntrinsicI);

if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
if (processLoad(LI))
return true;
Expand Down Expand Up @@ -2266,7 +2325,8 @@ bool GVN::processInstruction(Instruction *I) {

// Instructions with void type don't return a value, so there's
// no point in trying to find redundancies in them.
if (I->getType()->isVoidTy()) return false;
if (I->getType()->isVoidTy())
return false;

uint32_t NextNum = VN.getNextUnusedValueNumber();
unsigned Num = VN.lookup_or_add(I);
Expand Down Expand Up @@ -2373,10 +2433,15 @@ bool GVN::processBlock(BasicBlock *BB) {
if (DeadBlocks.count(BB))
return false;

// Clearing map before every BB because it can be used only for single BB.
ReplaceWithConstMap.clear();
bool ChangedFunction = false;

for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
BI != BE;) {
if (!ReplaceWithConstMap.empty())
ChangedFunction |= replaceOperandsWithConsts(BI);

ChangedFunction |= processInstruction(BI);
if (InstrsToErase.empty()) {
++BI;
Expand Down
122 changes: 122 additions & 0 deletions test/Transforms/GVN/assume-equal.ll
View file
Original file line number Diff line number Diff line change
@@ -0,0 +1,122 @@
; RUN: opt < %s -gvn -S | FileCheck %s

%struct.A = type { i32 (...)** }
@_ZTV1A = available_externally unnamed_addr constant [4 x i8*] [i8* null, i8* bitcast (i8** @_ZTI1A to i8*), i8* bitcast (i32 (%struct.A*)* @_ZN1A3fooEv to i8*), i8* bitcast (i32 (%struct.A*)* @_ZN1A3barEv to i8*)], align 8
@_ZTI1A = external constant i8*

; Checks if indirect calls can be replaced with direct
; assuming that %vtable == @_ZTV1A (with alignment).
; Checking const propagation across other BBs
; CHECK-LABEL: define void @_Z1gb(

define void @_Z1gb(i1 zeroext %p) {
entry:
%call = tail call noalias i8* @_Znwm(i64 8) #4
%0 = bitcast i8* %call to %struct.A*
tail call void @_ZN1AC1Ev(%struct.A* %0) #1
%1 = bitcast i8* %call to i8***
%vtable = load i8**, i8*** %1, align 8
%cmp.vtables = icmp eq i8** %vtable, getelementptr inbounds ([4 x i8*], [4 x i8*]* @_ZTV1A, i64 0, i64 2)
tail call void @llvm.assume(i1 %cmp.vtables)
br i1 %p, label %if.then, label %if.else

if.then: ; preds = %entry
%vtable1.cast = bitcast i8** %vtable to i32 (%struct.A*)**
%2 = load i32 (%struct.A*)*, i32 (%struct.A*)** %vtable1.cast, align 8

; CHECK: call i32 @_ZN1A3fooEv(
%call2 = tail call i32 %2(%struct.A* %0) #1

br label %if.end

if.else: ; preds = %entry
%vfn47 = getelementptr inbounds i8*, i8** %vtable, i64 1
%vfn4 = bitcast i8** %vfn47 to i32 (%struct.A*)**

; CHECK: call i32 @_ZN1A3barEv(
%3 = load i32 (%struct.A*)*, i32 (%struct.A*)** %vfn4, align 8

%call5 = tail call i32 %3(%struct.A* %0) #1
br label %if.end

if.end: ; preds = %if.else, %if.then
ret void
}

; Checking const propagation in the same BB
; CHECK-LABEL: define i32 @main()

define i32 @main() {
entry:
%call = tail call noalias i8* @_Znwm(i64 8)
%0 = bitcast i8* %call to %struct.A*
tail call void @_ZN1AC1Ev(%struct.A* %0)
%1 = bitcast i8* %call to i8***
%vtable = load i8**, i8*** %1, align 8
%cmp.vtables = icmp eq i8** %vtable, getelementptr inbounds ([4 x i8*], [4 x i8*]* @_ZTV1A, i64 0, i64 2)
tail call void @llvm.assume(i1 %cmp.vtables)
%vtable1.cast = bitcast i8** %vtable to i32 (%struct.A*)**

; CHECK: call i32 @_ZN1A3fooEv(
%2 = load i32 (%struct.A*)*, i32 (%struct.A*)** %vtable1.cast, align 8

%call2 = tail call i32 %2(%struct.A* %0)
ret i32 0
}

; This tests checks const propatation with fcmp instruction.
; CHECK-LABEL: define float @_Z1gf(float %p)

define float @_Z1gf(float %p) {
entry:
%p.addr = alloca float, align 4
%f = alloca float, align 4
store float %p, float* %p.addr, align 4

store float 3.000000e+00, float* %f, align 4
%0 = load float, float* %p.addr, align 4
%1 = load float, float* %f, align 4
%cmp = fcmp oeq float %1, %0 ; note const on lhs
call void @llvm.assume(i1 %cmp)

; CHECK: ret float 3.000000e+00
ret float %0
}

; CHECK-LABEL: define float @_Z1hf(float %p)

define float @_Z1hf(float %p) {
entry:
%p.addr = alloca float, align 4
store float %p, float* %p.addr, align 4

%0 = load float, float* %p.addr, align 4
%cmp = fcmp nnan ueq float %0, 3.000000e+00
call void @llvm.assume(i1 %cmp)

; CHECK: ret float 3.000000e+00
ret float %0
}

; CHECK-LABEL: define float @_Z1if(float %p)


define float @_Z1if(float %p) {
entry:
%p.addr = alloca float, align 4
store float %p, float* %p.addr, align 4

%0 = load float, float* %p.addr, align 4
%cmp = fcmp ueq float %0, 3.000000e+00 ; no nnan flag - can't propagate
call void @llvm.assume(i1 %cmp)

; CHECK-NOT: ret float 3.000000e+00
ret float %0
}

declare noalias i8* @_Znwm(i64)
declare void @_ZN1AC1Ev(%struct.A*)
declare void @llvm.assume(i1)
declare i32 @_ZN1A3fooEv(%struct.A*)
declare i32 @_ZN1A3barEv(%struct.A*)

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