Missed DSE of non-inlineable call

[qchateau]

Minimal working example:

@x = global i32 0, align 4

define void @bar(ptr %0) noinline {
  %2 = load i32, ptr @x, align 4
  store i32 %2, ptr %0, align 4
  ret void
}

define void @foo() {
  %1 = alloca i32, align 4
  call void @bar(ptr %1)
  ret void
}

@bar stores the value of global @x in its argument. It is marked noinline.
@foo calls @bar with an alloca variable, and does nothing else. At -O3, opt (trunk) is not able to kill the call to @bar.

In practice, I noticed this with an indirect call instead of a call to "@bar", and could not make DSE work as soon as the called function read memory (understand: the call gets DSE'ed with memory(argmem: readwrite), but not with memory(read, argmem: readwrite).

If I remvoe noinline , the call gets inline, then DSE'd correctly.

https://godbolt.org/z/9MEGrYdsa

[nikic]

I think this particular case would be handled by InstCombine if we relax the MemoryLocation::getForDest requirements.

[qchateau]

@nikic Indeed the following patch seems to do the trick in my branch. Hopefully this works on the main branch and does not cause any regression.

diff --git a/llvm/lib/Analysis/MemoryLocation.cpp b/llvm/lib/Analysis/MemoryLocation.cpp
index e0cd320e9..acdae5755 100644
--- a/llvm/lib/Analysis/MemoryLocation.cpp
+++ b/llvm/lib/Analysis/MemoryLocation.cpp
@@ -120,7 +120,8 @@ MemoryLocation MemoryLocation::getForDest(const AnyMemIntrinsic *MI) {

 std::optional<MemoryLocation>
 MemoryLocation::getForDest(const CallBase *CB, const TargetLibraryInfo &TLI) {
-  if (!CB->onlyAccessesArgMemory())
+  auto const CBME = CB->getMemoryEffects();
+  if (!(CBME - MemoryEffects::argMemOnly()).onlyReadsMemory())
     return std::nullopt;

   if (CB->hasOperandBundles())

[qchateau]

Thanks, this will help greatly with DSE of sret functions