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[msan] Fix PR32842 

It turned out that MSan was incorrectly calculating the shadow for int comparisons: it was done by truncating the result of (Shadow1 OR Shadow2) to i1, effectively rendering all bits except LSB useless.
This approach doesn't work e.g. in the case where the values being compared are even (i.e. have the LSB of the shadow equal to zero).
Instead, if CreateShadowCast() has to cast a bigger int to i1, we replace the truncation with an ICMP to 0.

This patch doesn't affect the code generated for SPEC 2006 binaries, i.e. there's no performance impact.

For the test case reported in PR32842 MSan with the patch generates a slightly more efficient code:

  orq     %rcx, %rax
  jne     .LBB0_6
, instead of:

  orl     %ecx, %eax
  testb   $1, %al
  jne     .LBB0_6

llvm-svn: 302787
This commit is contained in:
Alexander Potapenko 2017-05-11 11:07:48 +00:00
parent 29006dc724
commit a658ae8fe2
2 changed files with 25 additions and 2 deletions
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7
llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
View File

@ -1576,13 +1576,16 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
Value *CreateShadowCast(IRBuilder<> &IRB, Value *V, Type *dstTy,
bool Signed = false) {
Type *srcTy = V->getType();
size_t srcSizeInBits = VectorOrPrimitiveTypeSizeInBits(srcTy);
size_t dstSizeInBits = VectorOrPrimitiveTypeSizeInBits(dstTy);
if (srcSizeInBits > 1 && dstSizeInBits == 1)
return IRB.CreateICmpNE(V, getCleanShadow(V));
if (dstTy->isIntegerTy() && srcTy->isIntegerTy())
return IRB.CreateIntCast(V, dstTy, Signed);
if (dstTy->isVectorTy() && srcTy->isVectorTy() &&
dstTy->getVectorNumElements() == srcTy->getVectorNumElements())
return IRB.CreateIntCast(V, dstTy, Signed);
size_t srcSizeInBits = VectorOrPrimitiveTypeSizeInBits(srcTy);
size_t dstSizeInBits = VectorOrPrimitiveTypeSizeInBits(dstTy);
Value *V1 = IRB.CreateBitCast(V, Type::getIntNTy(*MS.C, srcSizeInBits));
Value *V2 =
IRB.CreateIntCast(V1, Type::getIntNTy(*MS.C, dstSizeInBits), Signed);

20
llvm/test/Instrumentation/MemorySanitizer/pr32842.ll Normal file
View File

@ -0,0 +1,20 @@
; Regression test for https://bugs.llvm.org/show_bug.cgi?id=32842
;
; RUN: opt < %s -msan -S | FileCheck %s
;target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
define zeroext i1 @_Z1fii(i32 %x, i32 %y) sanitize_memory {
entry:
%cmp = icmp slt i32 %x, %y
ret i1 %cmp
}
; CHECK: [[X:[^ ]+]] = load{{.*}}__msan_param_tls{{.*}}
; CHECK: [[Y:[^ ]+]] = load{{.*}}__msan_param_tls{{.*}}
; CHECK: [[OR:[^ ]+]] = or i32 [[Y]], [[X]]
; Make sure the shadow of the (x < y) comparison isn't truncated to i1.
; CHECK-NOT: trunc i32 [[OR]] to i1
; CHECK: [[CMP:[^ ]+]] = icmp ne i32 [[OR]], 0
; CHECK: store i1 [[CMP]],{{.*}}__msan_retval_tls