[WebAssembly] Remove FAKE_USEs before ExplicitLocals (#160768)

`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.

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp
index e6486e2..5c3127e 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;