[llvm-branch-commits] [llvm] release/21.x: [WebAssembly] Remove FAKE_USEs before ExplicitLocals (#160768) (PR #171184)

[via llvm-branch-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-webassembly

Author: None (llvmbot)

<details>
<summary>Changes</summary>

Backport e5b2a06546eb20662156b8a59b77aca086301486

Requested by: @<!-- -->dschuff

---
Full diff: https://github.com/llvm/llvm-project/pull/171184.diff


3 Files Affected:

- (modified) llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp (+14) 
- (modified) llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp (+4) 
- (added) llvm/test/CodeGen/WebAssembly/fake-use.ll (+25) 


``````````diff
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
index e6486e247209b..5c3127e2d3dc6 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/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
index bc91c6424b63e..fd13ef9a1921d 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
@@ -866,6 +866,10 @@ bool WebAssemblyRegStackify::runOnMachineFunction(MachineFunction &MF) {
       if (Insert->isDebugValue())
         continue;
 
+      // Ignore FAKE_USEs, which are no-ops and will be deleted later.
+      if (Insert->isFakeUse())
+        continue;
+
       // Iterate through the inputs in reverse order, since we'll be pulling
       // operands off the stack in LIFO order.
       CommutingState Commuting;
diff --git a/llvm/test/CodeGen/WebAssembly/fake-use.ll b/llvm/test/CodeGen/WebAssembly/fake-use.ll
new file mode 100644
index 0000000000000..a18ce33566df0
--- /dev/null
+++ b/llvm/test/CodeGen/WebAssembly/fake-use.ll
@@ -0,0 +1,25 @@
+; RUN: llc < %s | llvm-mc -triple=wasm32-unknown-unknown
+
+target triple = "wasm32-unknown-unknown"
+
+define void @fake_use() {
+  %t = call i32 @foo()
+  tail call void (...) @llvm.fake.use(i32 %t)
+  ret void
+}
+
+; %t shouldn't be converted to TEE in RegStackify, because the FAKE_USE will be
+; deleted in the beginning of ExplicitLocals.
+define void @fake_use_no_tee() {
+  %t = call i32 @foo()
+  tail call void (...) @llvm.fake.use(i32 %t)
+  call void @use(i32 %t)
+  ret void
+}
+
+declare i32 @foo()
+declare void @use(i32 %t)
+; 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) }

``````````

</details>


https://github.com/llvm/llvm-project/pull/171184