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InstCombine: Make OptimizePointerDifference more aggressive.

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- Ignore pointer casts.
- Also expand GEPs that aren't constantexprs when they have one use or only constant indices.

- We now compile "&foo[i] - &foo[j]" into "i - j".

llvm-svn: 150961
This commit is contained in:
Benjamin Kramer 2012-02-20 14:34:57 +00:00
parent 9963b8be92
commit 3d87f26b44
2 changed files with 53 additions and 27 deletions
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54
lib/Transforms/InstCombine/InstCombineAddSub.cpp
View File

@ -479,57 +479,57 @@ Value *InstCombiner::OptimizePointerDifference(Value *LHS, Value *RHS,
// If LHS is a gep based on RHS or RHS is a gep based on LHS, we can optimize
// this.
bool Swapped = false;
GetElementPtrInst *GEP = 0;
ConstantExpr *CstGEP = 0;
// TODO: Could also optimize &A[i] - &A[j] -> "i-j", and "&A.foo[i] - &A.foo".
GEPOperator *GEP1 = 0, *GEP2 = 0;
// For now we require one side to be the base pointer "A" or a constant
// expression derived from it.
if (GetElementPtrInst *LHSGEP = dyn_cast<GetElementPtrInst>(LHS)) {
// GEP derived from it.
if (GEPOperator *LHSGEP = dyn_cast<GEPOperator>(LHS)) {
// (gep X, ...) - X
if (LHSGEP->getOperand(0) == RHS) {
GEP = LHSGEP;
GEP1 = LHSGEP;
Swapped = false;
} else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(RHS)) {
// (gep X, ...) - (ce_gep X, ...)
if (CE->getOpcode() == Instruction::GetElementPtr &&
LHSGEP->getOperand(0) == CE->getOperand(0)) {
CstGEP = CE;
GEP = LHSGEP;
} else if (GEPOperator *RHSGEP = dyn_cast<GEPOperator>(RHS)) {
// (gep X, ...) - (gep X, ...)
if (LHSGEP->getOperand(0)->stripPointerCasts() ==
RHSGEP->getOperand(0)->stripPointerCasts()) {
GEP2 = RHSGEP;
GEP1 = LHSGEP;
Swapped = false;
}
}
}
if (GetElementPtrInst *RHSGEP = dyn_cast<GetElementPtrInst>(RHS)) {
if (GEPOperator *RHSGEP = dyn_cast<GEPOperator>(RHS)) {
// X - (gep X, ...)
if (RHSGEP->getOperand(0) == LHS) {
GEP = RHSGEP;
GEP1 = RHSGEP;
Swapped = true;
} else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(LHS)) {
// (ce_gep X, ...) - (gep X, ...)
if (CE->getOpcode() == Instruction::GetElementPtr &&
RHSGEP->getOperand(0) == CE->getOperand(0)) {
CstGEP = CE;
GEP = RHSGEP;
} else if (GEPOperator *LHSGEP = dyn_cast<GEPOperator>(LHS)) {
// (gep X, ...) - (gep X, ...)
if (RHSGEP->getOperand(0)->stripPointerCasts() ==
LHSGEP->getOperand(0)->stripPointerCasts()) {
GEP2 = LHSGEP;
GEP1 = RHSGEP;
Swapped = true;
}
}
}
if (GEP == 0)
// Avoid duplicating the arithmetic if GEP2 has non-constant indices and
// multiple users.
if (GEP1 == 0 ||
(GEP2 != 0 && !GEP2->hasAllConstantIndices() && !GEP2->hasOneUse()))
return 0;
// Emit the offset of the GEP and an intptr_t.
Value *Result = EmitGEPOffset(GEP);
Value *Result = EmitGEPOffset(GEP1);
// If we had a constant expression GEP on the other side offsetting the
// pointer, subtract it from the offset we have.
if (CstGEP) {
Value *CstOffset = EmitGEPOffset(CstGEP);
Result = Builder->CreateSub(Result, CstOffset);
if (GEP2) {
Value *Offset = EmitGEPOffset(GEP2);
Result = Builder->CreateSub(Result, Offset);
}
// If we have p - gep(p, ...) then we have to negate the result.
if (Swapped)

26
test/Transforms/InstCombine/sub.ll
View File

@ -301,3 +301,29 @@ define i32 @test28(i32 %x, i32 %y, i32 %z) {
; CHECK-NEXT: add i32
; CHECK-NEXT: ret i32
}
define i64 @test29(i8* %foo, i64 %i, i64 %j) {
%gep1 = getelementptr inbounds i8* %foo, i64 %i
%gep2 = getelementptr inbounds i8* %foo, i64 %j
%cast1 = ptrtoint i8* %gep1 to i64
%cast2 = ptrtoint i8* %gep2 to i64
%sub = sub i64 %cast1, %cast2
ret i64 %sub
; CHECK: @test29
; CHECK-NEXT: sub i64 %i, %j
; CHECK-NEXT: ret i64
}
define i64 @test30(i8* %foo, i64 %i, i64 %j) {
%bit = bitcast i8* %foo to i32*
%gep1 = getelementptr inbounds i32* %bit, i64 %i
%gep2 = getelementptr inbounds i8* %foo, i64 %j
%cast1 = ptrtoint i32* %gep1 to i64
%cast2 = ptrtoint i8* %gep2 to i64
%sub = sub i64 %cast1, %cast2
ret i64 %sub
; CHECK: @test30
; CHECK-NEXT: %gep1.idx = shl nuw i64 %i, 2
; CHECK-NEXT: sub i64 %gep1.idx, %j
; CHECK-NEXT: ret i64
}