[MemCpyOpt] Do not exchange llvm.lifetime.start and llvm.memcpy

[timshen]

Summary

The transform:

llvm.lifetime.start(%tmp5)
llvm.lifetime.memcpy(%arg1, %tmp5)

->

llvm.lifeitme.memcpy(%arg1, %tmp5)
llvm.lifetime.start(%arg1)

is wrong, since then lifetime.start doesn't operate on an alloca, and lifetime.start and lifetime.end don't match anymore.

[echristo]

lib/Transforms/Scalar/MemCpyOptimizer.cpp

792 ↗

(On Diff #59913)

We might be able to move this check to line 1184 before we call the function... or maybe hoist the conditions a bit and handle it for processStore as well?

Thoughts?

[timshen]

lib/Transforms/Scalar/MemCpyOptimizer.cpp

792 ↗

(On Diff #59913)

It'll be hard to check this in a higher level, because the CallInst doesn't necessarily exist.

Checking at every caller side will also work, but I can't think of a case where we don't need this check - exchanging a lifetime mark with the memcpy is clearly wrong.

[echristo]

lib/Transforms/Scalar/MemCpyOptimizer.cpp

792 ↗

(On Diff #59913)

I was mostly trying to figure out other ways to get down here than where we are. I guess this will work though...

llvm/trunk/lib/Transforms/Scalar/MemCpyOptimizer.cpp

bool MemCpyOpt::performCallSlotOptzn(Instruction *cpy,
   //
   //   memcpy(dest, src, ...)
   //   call @func(..., dest, ...)
   //
   // Since moving the memcpy is technically awkward, we additionally check that
   // src only holds uninitialized values at the moment of the call, meaning that
   // the memcpy can be discarded rather than moved.
 
+  // Lifetime marks shouldn't be operated on.
+  if (Function *F = C->getCalledFunction())
+    if (F->isIntrinsic() && F->getIntrinsicID() == Intrinsic::lifetime_start)
+      return false;
+
   // Deliberately get the source and destination with bitcasts stripped away,
   // because we'll need to do type comparisons based on the underlying type.
   CallSite CS(C);
 
   // Require that src be an alloca.  This simplifies the reasoning considerably.
   AllocaInst *srcAlloca = dyn_cast<AllocaInst>(cpySrc);
   if (!srcAlloca)
     return false;

llvm/trunk/test/Transforms/MemCpyOpt/lifetime.ll

+; RUN: opt < %s -O1 -S | FileCheck %s
+
+; performCallSlotOptzn in MemCpy should not exchange the calls to
+; @llvm.lifetime.start and @llvm.memcpy.
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture readonly, i64, i32, i1) #1
+declare void @llvm.lifetime.start(i64, i8* nocapture) #1
+declare void @llvm.lifetime.end(i64, i8* nocapture) #1
+
+define void @_ZN4CordC2EOS_(i8* nocapture dereferenceable(16) %arg1) {
+bb:
+; CHECK-LABEL: @_ZN4CordC2EOS_
+; CHECK-NOT: call void @llvm.lifetime.start
+; CHECK: ret void
+  %tmp = alloca [8 x i8], align 8
+  %tmp5 = bitcast [8 x i8]* %tmp to i8*
+  call void @llvm.lifetime.start(i64 16, i8* %tmp5)
+  %tmp10 = getelementptr inbounds i8, i8* %tmp5, i64 7
+  store i8 0, i8* %tmp10, align 1
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %arg1, i8* %tmp5, i64 16, i32 8, i1 false)
+  call void @llvm.lifetime.end(i64 16, i8* %tmp5)
+  ret void
+}
+
+attributes #1 = { argmemonly nounwind }