[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, 4 insertions, 0 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
index 08ca20b5..97f2ed0 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
@@ -867,6 +867,10 @@ 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;