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Fix a bug in the over-index constant folding. When over-indexing an 

array member of a struct, it's possible to land in an arbitrary position
inside that struct, such that attempting to find further getelementptr
indices will fail. In such cases, folding cannot be done.

llvm-svn: 79485
This commit is contained in:
Dan Gohman 2009-08-19 22:46:59 +00:00
parent 1d764f61ef
commit c59ba42b0f
2 changed files with 41 additions and 8 deletions
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32
llvm/lib/Analysis/ConstantFolding.cpp
View File

@ -165,8 +165,11 @@ static Constant *SymbolicallyEvaluateGEP(Constant* const* Ops, unsigned NumOps,
// Also, this helps GlobalOpt do SROA on GlobalVariables.
const Type *Ty = Ptr->getType();
SmallVector<Constant*, 32> NewIdxs;
for (unsigned Index = 1; Index != NumOps; ++Index) {
do {
if (const SequentialType *ATy = dyn_cast<SequentialType>(Ty)) {
// The only pointer indexing we'll do is on the first index of the GEP.
if (isa<PointerType>(ATy) && ATy != Ptr->getType())
break;
// Determine which element of the array the offset points into.
uint64_t ElemSize = TD->getTypeAllocSize(ATy->getElementType());
if (ElemSize == 0)
@ -183,20 +186,33 @@ static Constant *SymbolicallyEvaluateGEP(Constant* const* Ops, unsigned NumOps,
Offset -= SL.getElementOffset(ElIdx);
Ty = STy->getTypeAtIndex(ElIdx);
} else {
return 0;
// We've reached some non-indexable type.
break;
}
}
} while (Ty != cast<PointerType>(ResultTy)->getElementType());
// If we haven't used up the entire offset by descending the static
// type, then the offset is pointing into the middle of an indivisible
// member, so we can't simplify it.
if (Offset != 0)
return 0;
// If the base is the start of a GlobalVariable and all the array indices
// remain in their static bounds, the GEP is inbounds. We can check that
// all indices are in bounds by just checking the first index only
// because we've just normalized all the indices.
if (isa<GlobalVariable>(Ptr) && NewIdxs[0]->isNullValue())
return ConstantExpr::getInBoundsGetElementPtr(Ptr,
&NewIdxs[0], NewIdxs.size());
Constant *C = isa<GlobalVariable>(Ptr) && NewIdxs[0]->isNullValue() ?
ConstantExpr::getInBoundsGetElementPtr(Ptr, &NewIdxs[0], NewIdxs.size()) :
ConstantExpr::getGetElementPtr(Ptr, &NewIdxs[0], NewIdxs.size());
assert(cast<PointerType>(C->getType())->getElementType() == Ty &&
"Computed GetElementPtr has unexpected type!");
// Otherwise it may not be inbounds.
return ConstantExpr::getGetElementPtr(Ptr, &NewIdxs[0], NewIdxs.size());
// If we ended up indexing a member with a type that doesn't match
// type type of what the original indices indexed, add a cast.
if (Ty != cast<PointerType>(ResultTy)->getElementType())
C = ConstantExpr::getBitCast(C, ResultTy);
return C;
}
/// FoldBitCast - Constant fold bitcast, symbolically evaluating it with

17
llvm/test/Transforms/InstCombine/constant-fold-gep-overindex.ll Normal file
View File

@ -0,0 +1,17 @@
; RUN: llvm-as < %s | opt -instcombine
target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
target triple = "i386-apple-darwin10.0"
%0 = type { i8*, [19 x i8] }
%1 = type { i8*, [0 x i8] }
@s = external global %0 ; <%0*> [#uses=1]
@"\01LC8" = external constant [17 x i8] ; <[17 x i8]*> [#uses=1]
define i32 @main() nounwind {
entry:
%0 = call i32 (i8*, ...)* @printf(i8* getelementptr ([17 x i8]* @"\01LC8", i32 0, i32 0), i8* undef, i8* getelementptr (%1* bitcast (%0* @s to %1*), i32 0, i32 1, i32 0)) nounwind ; <i32> [#uses=0]
ret i32 0
}
declare i32 @printf(i8*, ...) nounwind