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Move previously dead code to handle computing the known bits of an alias

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up to where it actually works as intended. The problem is that
a GlobalAlias isa GlobalValue and so the prior block handled all of the
cases.

This allows us to constant fold based on the actual constant expression
in the global alias. As an example, see the last function in the newly
added test case which explicitly aligns an unaligned pointer using
constant expression math. Without this change, we fail to see that and
fold an alignment test to zero.

llvm-svn: 220164
This commit is contained in:
Chandler Carruth 2014-10-19 09:06:56 +00:00
parent 6e9cd62474
commit 9c87bbcef7
2 changed files with 51 additions and 10 deletions
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21
lib/Analysis/ValueTracking.cpp
View File

@ -790,6 +790,17 @@ void computeKnownBits(Value *V, APInt &KnownZero, APInt &KnownOne,
return;
}
// A weak GlobalAlias is totally unknown. A non-weak GlobalAlias has
// the bits of its aliasee.
if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
if (GA->mayBeOverridden()) {
KnownZero.clearAllBits(); KnownOne.clearAllBits();
} else {
computeKnownBits(GA->getAliasee(), KnownZero, KnownOne, TD, Depth+1, Q);
}
return;
}
// The address of an aligned GlobalValue has trailing zeros.
if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
unsigned Align = GV->getAlignment();
@ -815,16 +826,6 @@ void computeKnownBits(Value *V, APInt &KnownZero, APInt &KnownOne,
KnownOne.clearAllBits();
return;
}
// A weak GlobalAlias is totally unknown. A non-weak GlobalAlias has
// the bits of its aliasee.
if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
if (GA->mayBeOverridden()) {
KnownZero.clearAllBits(); KnownOne.clearAllBits();
} else {
computeKnownBits(GA->getAliasee(), KnownZero, KnownOne, TD, Depth+1, Q);
}
return;
}
if (Argument *A = dyn_cast<Argument>(V)) {
unsigned Align = A->getType()->isPointerTy() ? A->getParamAlignment() : 0;

40
test/Transforms/InstCombine/constant-fold-alias.ll Normal file
View File

@ -0,0 +1,40 @@
; RUN: opt -S < %s -instcombine | FileCheck %s
target datalayout = "e-p1:16:16-p2:32:32-p3:64:64"
@G1 = global i32 42, align 1
@G2 = global i32 42
@G3 = global [4 x i8] zeroinitializer, align 1
@A1 = alias bitcast (i8* getelementptr inbounds ([4 x i8]* @G3, i32 0, i32 2) to i32*)
@A2 = alias inttoptr (i64 and (i64 ptrtoint (i8* getelementptr inbounds ([4 x i8]* @G3, i32 0, i32 3) to i64), i64 -4) to i32*)
define i64 @f1() {
; This cannot be constant folded because G1 is underaligned.
; CHECK-LABEL: @f1(
; CHECK: ret i64 and
ret i64 and (i64 ptrtoint (i32* @G1 to i64), i64 1)
}
define i64 @f2() {
; The preferred alignment for G2 allows this one to foled to zero.
; CHECK-LABEL: @f2(
; CHECK: ret i64 0
ret i64 and (i64 ptrtoint (i32* @G2 to i64), i64 1)
}
define i64 @g1() {
; This cannot be constant folded because A1 aliases G3 which is underalaigned.
; CHECK-LABEL: @g1(
; CHECK: ret i64 and
ret i64 and (i64 ptrtoint (i32* @A1 to i64), i64 1)
}
define i64 @g2() {
; While A2 also aliases G3 which is underaligned, the math of A2 forces a
; certain alignment allowing this to fold to zero.
; CHECK-LABEL: @g2(
; CHECK: ret i64 0
ret i64 and (i64 ptrtoint (i32* @A2 to i64), i64 1)
}