[InstCombine] Avoid deleting volatile memcpys.

[pcwalton]

Summary

InstCombine can replace memcpy to an alloca with a pointer directly to the
source in certain cases. Unfortunately, it also did so for volatile memcpys.
This patch makes it stop doing that.

This was discovered in D136822.

[pcwalton]

I noticed that the memcpy in the asan test case is volatile without any changes, so I shouldn't need to touch that test case now.

[nikic]

LGTM

FYI If you add "Depends on D137029." when submitting the review, this will add a child revision and make the pre-commit CI work.

llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

for (auto &U : ValuePair.first->uses()) {
       }
 
       // If this is isn't our memcpy/memmove, reject it as something we can't
       // handle.
       MemTransferInst *MI = dyn_cast<MemTransferInst>(I);
       if (!MI)
         return false;
 
+      // If the transfer is volatile, reject it.
+      if (MI->isVolatile())
+        return false;
+
       // If the transfer is using the alloca as a source of the transfer, then
       // ignore it since it is a load (unless the transfer is volatile).
-      if (U.getOperandNo() == 1) {
-        if (MI->isVolatile()) return false;
+      if (U.getOperandNo() == 1)
         continue;
-      }
 
       // If we already have seen a copy, reject the second one.
       if (TheCopy) return false;
 
       // If the pointer has been offset from the start of the alloca, we can't
       // safely handle this.
       if (IsOffset) return false;
 

llvm/test/Transforms/InstCombine/memcpy-from-global.ll

 ; CHECK-NEXT:    ret void
 ;
   %alloca = alloca %T, align 8
   call void @llvm.memcpy.p0.p0.i64(ptr %alloca, ptr %arg, i64 124, i1 false)
   call void @bar(ptr %alloca) readonly
   ret void
 }
 
-; FIXME: Test that we incorrectly elide a volatile memcpy.
+; Test that we don't elide a volatile memcpy.
 define void @volatile_memcpy() {
 ; CHECK-LABEL: @volatile_memcpy(
-; CHECK-NEXT:    call void @bar(ptr nonnull @H) #[[ATTR3]]
+; CHECK-NEXT:    [[A:%.*]] = alloca [[U:%.*]], align 16
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 4 [[A]], ptr align 4 @H, i64 20, i1 true)
+; CHECK-NEXT:    call void @bar(ptr nonnull [[A]]) #[[ATTR3]]
 ; CHECK-NEXT:    ret void
 ;
   %A = alloca %U, align 16
   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %A, ptr align 4 @H, i64 20, i1 true)
   call void @bar(ptr %A) readonly
   ret void
 }
 
 attributes #0 = { null_pointer_is_valid }