[llvm] [WebAssembly] Remove FAKE_USEs before ExplicitLocals (PR #160228)

[Heejin Ahn via llvm-commits]

https://github.com/aheejin created https://github.com/llvm/llvm-project/pull/160228

`FAKE_USE`s are essentially no-ops, so they have to be removed before running ExplicitLocals so that `drop`s will be correctly inserted to drop those values used by the `FAKE_USE`s.

Fixes https://github.com/emscripten-core/emscripten/issues/25301.

>From f167fc836688d9b01f7a9014b7ac5f2b86bbae29 Mon Sep 17 00:00:00 2001
From: Heejin Ahn <aheejin at gmail.com>
Date: Tue, 23 Sep 2025 03:53:34 +0000
Subject: [PATCH] [WebAssembly] Remove FAKE_USEs before ExplicitLocals

`FAKE_USE`s are essentially no-ops, so they have to be removed before
running ExplicitLocals so that `drop`s will be correctly inserted to
drop those values used by the `FAKE_USE`s.

Fixes https://github.com/emscripten-core/emscripten/issues/25301.
---
 .../WebAssembly/WebAssemblyExplicitLocals.cpp     | 14 ++++++++++++++
 llvm/test/CodeGen/WebAssembly/fake-use.ll         | 15 +++++++++++++++
 2 files changed, 29 insertions(+)
 create mode 100644 llvm/test/CodeGen/WebAssembly/fake-use.ll

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
index e6486e247209b..014cd99b9aa23 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
@@ -216,6 +216,18 @@ static MachineInstr *findStartOfTree(MachineOperand &MO,
   return Def;
 }
 
+// FAKE_USEs are no-ops, so remove them here so that the values used by them
+// will be correctly dropped later.
+static void removeFakeUses(MachineFunction &MF) {
+  SmallVector<MachineInstr*> ToDelete;
+  for (auto &MBB : MF)
+    for (auto &MI : MBB)
+      if (MI.isFakeUse())
+        ToDelete.push_back(&MI);
+  for (auto *MI : ToDelete)
+    MI->eraseFromParent();
+}
+
 bool WebAssemblyExplicitLocals::runOnMachineFunction(MachineFunction &MF) {
   LLVM_DEBUG(dbgs() << "********** Make Locals Explicit **********\n"
                        "********** Function: "
@@ -226,6 +238,8 @@ bool WebAssemblyExplicitLocals::runOnMachineFunction(MachineFunction &MF) {
   WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
   const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
 
+  removeFakeUses(MF);
+
   // Map non-stackified virtual registers to their local ids.
   DenseMap<unsigned, unsigned> Reg2Local;
 
diff --git a/llvm/test/CodeGen/WebAssembly/fake-use.ll b/llvm/test/CodeGen/WebAssembly/fake-use.ll
new file mode 100644
index 0000000000000..d5732e628440a
--- /dev/null
+++ b/llvm/test/CodeGen/WebAssembly/fake-use.ll
@@ -0,0 +1,15 @@
+; RUN: llc < %s | llvm-mc -triple=wasm32-unknown-unknown
+
+target triple = "wasm32-unknown-unknown"
+
+define void @fake_use_test() {
+  %t = call i32 @foo()
+  tail call void (...) @llvm.fake.use(i32 %t)
+  ret void
+}
+
+declare void @foo()
+; Function Attrs: mustprogress nocallback nofree nosync nounwind willreturn memory(inaccessiblemem: readwrite)
+declare void @llvm.fake.use(...) #0
+
+attributes #0 = { mustprogress nocallback nofree nosync nounwind willreturn memory(inaccessiblemem: readwrite) }