[llvm] 39c39e8 - [MemCpyOpt] Don't shorten memset if destination observable through unwinding

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2020-10-13T21:12:19+02:00
New Revision: 39c39e8a7f09ff8f0385ef9962d1075d9559d139

URL: https://github.com/llvm/llvm-project/commit/39c39e8a7f09ff8f0385ef9962d1075d9559d139
DIFF: https://github.com/llvm/llvm-project/commit/39c39e8a7f09ff8f0385ef9962d1075d9559d139.diff

LOG: [MemCpyOpt] Don't shorten memset if destination observable through unwinding

MemCpyOpt can shorten a memset if it is later partially overwritten
by a memcpy. It checks that the destination is not read in between,
but we also need to make sure that the destination cannot be observed
via unwinding.

Differential Revision: https://reviews.llvm.org/D89190

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
    llvm/test/Transforms/MemCpyOpt/memset-memcpy-redundant-memset.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
index c7bdf860f14b..9f9e9dfa1afd 100644
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1050,6 +1050,17 @@ bool MemCpyOptPass::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
   Value *DestSize = MemSet->getLength();
   Value *SrcSize = MemCpy->getLength();
 
+  // If the destination might be accessible by the caller, make sure we cannot
+  // unwind between the memset and the memcpy.
+  if (!MemCpy->getFunction()->doesNotThrow() &&
+      !isa<AllocaInst>(getUnderlyingObject(Dest))) {
+    for (const Instruction &I :
+         make_range(MemSet->getIterator(), MemCpy->getIterator())) {
+      if (I.mayThrow())
+        return false;
+    }
+  }
+
   // By default, create an unaligned memset.
   unsigned Align = 1;
   // If Dest is aligned, and SrcSize is constant, use the minimum alignment

diff  --git a/llvm/test/Transforms/MemCpyOpt/memset-memcpy-redundant-memset.ll b/llvm/test/Transforms/MemCpyOpt/memset-memcpy-redundant-memset.ll
index 4f9c8be32437..687f312fa15b 100644
--- a/llvm/test/Transforms/MemCpyOpt/memset-memcpy-redundant-memset.ll
+++ b/llvm/test/Transforms/MemCpyOpt/memset-memcpy-redundant-memset.ll
@@ -193,13 +193,9 @@ define void @test_intermediate_write(i8* %b) #0 {
 
 define void @test_throwing_call(i8* %src, i64 %src_size, i8* %dst, i64 %dst_size, i8 %c) {
 ; CHECK-LABEL: @test_throwing_call(
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[DST:%.*]], i8 [[C:%.*]], i64 [[DST_SIZE:%.*]], i1 false)
 ; CHECK-NEXT:    call void @call() [[ATTR2:#.*]]
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp ule i64 [[DST_SIZE:%.*]], [[SRC_SIZE:%.*]]
-; CHECK-NEXT:    [[TMP2:%.*]] = sub i64 [[DST_SIZE]], [[SRC_SIZE]]
-; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[TMP1]], i64 0, i64 [[TMP2]]
-; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr i8, i8* [[DST:%.*]], i64 [[SRC_SIZE]]
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* align 1 [[TMP4]], i8 [[C:%.*]], i64 [[TMP3]], i1 false)
-; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[DST]], i8* [[SRC:%.*]], i64 [[SRC_SIZE]], i1 false)
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[DST]], i8* [[SRC:%.*]], i64 [[SRC_SIZE:%.*]], i1 false)
 ; CHECK-NEXT:    ret void
 ;
   call void @llvm.memset.p0i8.i64(i8* %dst, i8 %c, i64 %dst_size, i1 false)