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Don't assume that external global variables are aligned at their preferred

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alignment. Only the minimum alignment guaranteed by the ABI may be assumed.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78668 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2009-08-11 15:50:03 +00:00
parent 00b0a243bd
commit 004072508b
2 changed files with 32 additions and 2 deletions
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12
lib/Analysis/ValueTracking.cpp
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@ -81,8 +81,16 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
// The address of an aligned GlobalValue has trailing zeros. // The address of an aligned GlobalValue has trailing zeros.
if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) { if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
unsigned Align = GV->getAlignment(); unsigned Align = GV->getAlignment();
if (Align == 0 && TD && GV->getType()->getElementType()->isSized()) if (Align == 0 && TD && GV->getType()->getElementType()->isSized()) {
Align = TD->getPrefTypeAlignment(GV->getType()->getElementType()); const Type *ObjectType = GV->getType()->getElementType();
// If the object is defined in the current Module, we'll be giving
// it the preferred alignment. Otherwise, we have to assume that it
// may only have the minimum ABI alignment.
if (!GV->isDeclaration() && !GV->mayBeOverridden())
Align = TD->getPrefTypeAlignment(ObjectType);
else
Align = TD->getABITypeAlignment(ObjectType);
}
if (Align > 0) if (Align > 0)
KnownZero = Mask & APInt::getLowBitsSet(BitWidth, KnownZero = Mask & APInt::getLowBitsSet(BitWidth,
CountTrailingZeros_32(Align)); CountTrailingZeros_32(Align));

22
test/Transforms/InstCombine/align-external.ll Normal file
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@ -0,0 +1,22 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | FileCheck %s
; Don't assume that external global variables have their preferred
; alignment. They may only have the ABI minimum alignment.
; CHECK: %s = shl i64 %a, 3
; CHECK: %r = or i64 %s, ptrtoint (i32* @A to i64)
; CHECK: %q = add i64 %r, 1
; CHECK: ret i64 %q
target datalayout = "-i32:8:32"
@A = external global i32
@B = external global i32
define i64 @foo(i64 %a) {
%t = ptrtoint i32* @A to i64
%s = shl i64 %a, 3
%r = or i64 %t, %s
%q = add i64 %r, 1
ret i64 %q
}