[NewGVN] Mark function as changed if we erase instructions.

[fhahn]

Summary

Currently eliminateInstructions only returns true if any instruction got
replaced. In the test case for this patch, we eliminate the trivially
dead calls, for which eliminateInstructions not do a replacement and the
function is not marked as changed, which is why the inliner crashes
while traversing the call graph.

Alternatively we could also change eliminateInstructions to return true
in case we mark instructions for deletion, but that's slightly more code
and doing it at the place where the replacement happens seems safer.

Fixes PR37517.

[fhahn]

ping

[bjope]

An alternative could perhaps be to do something like this

Changed |= !InstructionsToErase.empty();

before the loop (assuming that if we got something in InstructionsToErase then we also have done some modifications).
But I think your solution (setting Changed whenever we do a change) is easier to understand.

Thanks for the patch. LGTM!

[fhahn]

In D51169#1225675, @bjope wrote:

An alternative could perhaps be to do something like this

Changed |= !InstructionsToErase.empty();

before the loop (assuming that if we got something in InstructionsToErase then we also have done some modifications).
But I think your solution (setting Changed whenever we do a change) is easier to understand.

Thanks for the patch. LGTM!

Thanks! I just had another look and I think even though we have 2 ifs in the loop over InstructionsToErase, we should always do modifications if it is not empty. In particular, if an instruction to erase does not have a parent any more, that would mean the parent BB has been deleted earlier, so we already did modifications. I think I'll go with your suggestion!

[bjope]

Still LGTM!

lib/Transforms/Scalar/NewGVN.cpp

bool NewGVN::runGVN() {
   verifyMemoryCongruency();
   verifyIterationSettled(F);
   verifyStoreExpressions();
 
   Changed |= eliminateInstructions(F);
 
   // Delete all instructions marked for deletion.
   for (Instruction *ToErase : InstructionsToErase) {
-    if (!ToErase->use_empty())
+    if (!ToErase->use_empty()) {
       ToErase->replaceAllUsesWith(UndefValue::get(ToErase->getType()));
+      Changed = true;
+    }
 
-    if (ToErase->getParent())
+    if (ToErase->getParent()) {
       ToErase->eraseFromParent();
+      Changed = true;
+    }
   }
 
   // Delete all unreachable blocks.
   auto UnreachableBlockPred = [&](const BasicBlock &BB) {
     return !ReachableBlocks.count(&BB);
   };
 
   for (auto &BB : make_filter_range(F, UnreachableBlockPred)) {

test/Transforms/NewGVN/eliminate-callsite-inline.ll

+; RUN: opt -inline -newgvn -S < %s | FileCheck %s
+
+; CHECK-LABEL: @f2()
+; CHECK-NEXT:    ret void
+; CHECK-NOT: @f1
+
+define void @f2() {
+  call void @f1()
+  call void @f1()
+  ret void
+}
+
+define internal void @f1() #1 {
+entry:
+  ret void
+}
+
+attributes #1 = { noinline nounwind readnone }