[llvm] [WebAssembly] Remove FAKE_USEs before ExplicitLocals (PR #160768)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-webassembly

Author: Heejin Ahn (aheejin)

<details>
<summary>Changes</summary>

`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.

---

This is reapplication of #<!-- -->160228, which broke Wasm waterfall. This PR additionally prevents `FAKE_USE`s uses from being stackified.

Previously, a 'def' whose first use was a `FAKE_USE` was able to be stackified as `TEE`:
- Before
```
Reg = INST ...            // Def
FAKE_USE ..., Reg, ...    // Insert
INST ..., Reg, ...
INST ..., Reg, ...
```

- After RegStackify
```
DefReg = INST ...            // Def
TeeReg, Reg = TEE ... DefReg
FAKE_USE ..., TeeReg, ...    // Insert
INST ..., Reg, ...
INST ..., Reg, ...
```
And this assumes `DefReg` and `TeeReg` are stackified.

But this PR removes `FAKE_USE`s in the beginning of ExplicitLocals. And later in ExplicitLocals we have a routine to unstackify registers that have no uses left:
https://github.com/llvm/llvm-project/blob/7b28fcd2b182ba2c9d2d71c386be92fc0ee3cc9d/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp#L257-L269 (This was added in #<!-- -->149626. Then it didn't seem it would trigger the same assertions for `TEE`s because it was fixing the bug where a terminator was removed in CFGSort (#<!-- -->149097).
Details here: https://github.com/llvm/llvm-project/pull/149432#issuecomment-3091444141)

- After `FAKE_USE` removal and unstackification
```
DefReg = INST ...
TeeReg, Reg = TEE ... DefReg
INST ..., Reg, ...
INST ..., Reg, ...
```
And now `TeeReg` is unstackified. This triggered the assertion here, that `TeeReg` should be stackified:
https://github.com/llvm/llvm-project/blob/7b28fcd2b182ba2c9d2d71c386be92fc0ee3cc9d/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp#L316

This prevents `FAKE_USE`s' uses from being stackified altogether, including `TEE` transformation. Even when it is not a `TEE` transformation and just a single use stackification, it does not trigger the assertion but there's no point stackifying it given that it will be deleted.

---

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

---
Full diff: https://github.com/llvm/llvm-project/pull/160768.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 7591541779884..97545872e251b 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
@@ -870,6 +870,10 @@ 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/160768