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Fix PR3694: add an instcombine micro-optimization that helps

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clean up when using variable length arrays in llvm-gcc.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65832 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Duncan Sands 2009-03-02 09:18:21 +00:00
parent 540d73f0cb
commit 5b7cfb02f7
2 changed files with 31 additions and 5 deletions
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22
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -10766,15 +10766,25 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
// transform: GEP (bitcast [10 x i8]* X to [0 x i8]*), i32 0, ...
// into : GEP [10 x i8]* X, i32 0, ...
//
// Likewise, transform: GEP (bitcast i8* X to [0 x i8]*), i32 0, ...
// into : GEP i8* X, ...
//
// This occurs when the program declares an array extern like "int X[];"
//
const PointerType *CPTy = cast<PointerType>(PtrOp->getType());
const PointerType *XTy = cast<PointerType>(X->getType());
if (const ArrayType *XATy =
dyn_cast<ArrayType>(XTy->getElementType()))
if (const ArrayType *CATy =
dyn_cast<ArrayType>(CPTy->getElementType()))
if (const ArrayType *CATy =
dyn_cast<ArrayType>(CPTy->getElementType())) {
// GEP (bitcast i8* X to [0 x i8]*), i32 0, ... ?
if (CATy->getElementType() == XTy->getElementType()) {
// -> GEP i8* X, ...
SmallVector<Value*, 8> Indices(GEP.idx_begin()+1, GEP.idx_end());
return GetElementPtrInst::Create(X, Indices.begin(), Indices.end(),
GEP.getName());
} else if (const ArrayType *XATy =
dyn_cast<ArrayType>(XTy->getElementType())) {
// GEP (bitcast [10 x i8]* X to [0 x i8]*), i32 0, ... ?
if (CATy->getElementType() == XATy->getElementType()) {
// -> GEP [10 x i8]* X, i32 0, ...
// At this point, we know that the cast source type is a pointer
// to an array of the same type as the destination pointer
// array. Because the array type is never stepped over (there
@ -10782,6 +10792,8 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
GEP.setOperand(0, X);
return &GEP;
}
}
}
} else if (GEP.getNumOperands() == 2) {
// Transform things like:
// %t = getelementptr i32* bitcast ([2 x i32]* %str to i32*), i32 %V

14
test/Transforms/InstCombine/2009-03-02-VarLengthArrayGEP.ll Normal file
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@ -0,0 +1,14 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {getelementptr i32}
; PR3694
define i32 @e(i32 %m, i32 %n) nounwind {
entry:
%0 = alloca i32, i32 %n, align 4 ; <i32*> [#uses=2]
%1 = bitcast i32* %0 to [0 x i32]* ; <[0 x i32]*> [#uses=1]
call void @f(i32* %0) nounwind
%2 = getelementptr [0 x i32]* %1, i32 0, i32 %m ; <i32*> [#uses=1]
%3 = load i32* %2, align 4 ; <i32> [#uses=1]
ret i32 %3
}
declare void @f(i32*)