[LowerGlobalDtors] Skip __cxa_atexit call completely when arg0 is unused

[sbc100]

In emscripten we have a build mode (the default actually) where the runtime never exits and therefore __cxa_atexit is a dummy/stub function that does nothing. In this case we would like to be able completely DCE any otherwise-unused global dtor functions.

Fixes: emscripten-core/emscripten#19993

[llvmbot]

@llvm/pr-subscribers-lld-wasm
@llvm/pr-subscribers-lld

@llvm/pr-subscribers-llvm-transforms

Author: Sam Clegg (sbc100)

Changes

In emscripten we have a build mode (the default actually) where the runtime never exits and there for __cxa_atexit is a dummy/stub function that does nothing. In this case we would like to be able completely DCE any otherwise-unused global dtor functions.

Fixes: emscripten-core/emscripten#19993

---

Full diff: https://github.com/llvm/llvm-project/pull/68758.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Utils/LowerGlobalDtors.cpp (+11)
• (added) llvm/test/Transforms/LowerGlobalDestructors/lower-global-dtors-unused.ll (+17)

diff --git a/llvm/lib/Transforms/Utils/LowerGlobalDtors.cpp b/llvm/lib/Transforms/Utils/LowerGlobalDtors.cpp
index 195c274ff18e2cd..f67a1eb53b8976a 100644
--- a/llvm/lib/Transforms/Utils/LowerGlobalDtors.cpp
+++ b/llvm/lib/Transforms/Utils/LowerGlobalDtors.cpp
@@ -140,6 +140,17 @@ static bool runImpl(Module &M) {
                         {PointerType::get(AtExitFuncTy, 0), VoidStar, VoidStar},
                         /*isVarArg=*/false));
 
+  // If __cxa_atexit is defined (e.g. in the case of LTO) and arg0 is not
+  // actually used (i.e. it's dummy/stub function as used in emscripten when
+  // the program never exits) we can simply return early and clear out
+  // @llvm.global_dtors.
+  if (auto F = dyn_cast<Function>(AtExit.getCallee())) {
+    if (F && F->hasExactDefinition() && F->getArg(0)->getNumUses() == 0) {
+      GV->eraseFromParent();
+      return true;
+    }
+  }
+
   // Declare __dso_local.
   Type *DsoHandleTy = Type::getInt8Ty(C);
   Constant *DsoHandle = M.getOrInsertGlobal("__dso_handle", DsoHandleTy, [&] {
diff --git a/llvm/test/Transforms/LowerGlobalDestructors/lower-global-dtors-unused.ll b/llvm/test/Transforms/LowerGlobalDestructors/lower-global-dtors-unused.ll
new file mode 100644
index 000000000000000..a6e7133b2947ee4
--- /dev/null
+++ b/llvm/test/Transforms/LowerGlobalDestructors/lower-global-dtors-unused.ll
@@ -0,0 +1,17 @@
+; RUN: opt -passes=lower-global-dtors -S < %s | FileCheck %s --implicit-check-not=llvm.global_dtors
+
+; Test that @llvm.global_dtors is completely removed if __cxa_atexit
+; is a no-op (i.e. doesn't use its first argument).
+
+declare void @orig_dtor()
+
+define i32 @__cxa_atexit(ptr, ptr, ptr) {
+  ret i32 0
+}
+
+@llvm.global_dtors = appending global [1 x { i32, ptr, ptr }] [
+  { i32, ptr, ptr } { i32 0, ptr @orig_dtor, ptr null }
+]
+
+; CHECK-NOT: @llvm.global_dtors
+; CHECK-NOT: call void @orig_dtor()

[sbc100]

I think another approach would be to somehow run GlobalDCE (or something like that) after we generate all the __cxa_atexit calls.. I couldn't find a way to do that from WebAssemblyPassConfig::addIRPasses though, and this seems like a more direct appoach.

[sbc100]

@kripken

[tlively]

LGTM, but I wonder if anyone else has any opinions, since I'm not too familiar with this part of the code base.

[tlively]

Those test failures on CI look real, though.

[sbc100]

Adding @sunfishcode who originally wrote this pass and @yln who made it into a generic pass

[sunfishcode]

The change here looks good to me. I like how it does automatic detection so it avoids needing a flag.