[all-commits] [llvm/llvm-project] 3c8f3b: [WebAssembly] Treat 'rethrow' as terminator in cus...

[Heejin Ahn via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 3c8f3b91d898cb3f76e1e430da98972cdf8a4a1c
      https://github.com/llvm/llvm-project/commit/3c8f3b91d898cb3f76e1e430da98972cdf8a4a1c
  Author: Heejin Ahn <aheejin at gmail.com>
  Date:   2024-06-18 (Tue, 18 Jun 2024)

  Changed paths:
    M llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
    M llvm/test/CodeGen/WebAssembly/exception-legacy.ll

  Log Message:
  -----------
  [WebAssembly] Treat 'rethrow' as terminator in custom isel (#95967)

`rethrow` instruction is a terminator, but when when its DAG is built in
`SelectionDAGBuilder` in a custom routine, it was NOT treated as such.

```ll
rethrow:                                          ; preds = %catch.start
  invoke void @llvm.wasm.rethrow() #1 [ "funclet"(token %1) ]
          to label %unreachable unwind label %ehcleanup

ehcleanup:                                        ; preds = %rethrow, %catch.dispatch
  %tmp = phi i32 [ 10, %catch.dispatch ], [ 20, %rethrow ]
  ...
```

In this bitcode, because of the `phi`, a `CONST_I32` will be created in
the `rethrow` BB. Without this patch, the DAG for the `rethrow` BB looks
like this:
```
  t0: ch,glue = EntryToken
      t3: ch = CopyToReg t0, Register:i32 %9, Constant:i32<20>
      t5: ch = llvm.wasm.rethrow t0, TargetConstant:i32<12161>
    t6: ch = TokenFactor t3, t5
  t8: ch = br t6, BasicBlock:ch<unreachable 0x562532e43c50>
```
Note that `CopyToReg` and `llvm.wasm.rethrow` don't have dependence so
either can come first in the selected code, which can result in the code
like
```mir
bb.3.rethrow:
  RETHROW 0, implicit-def dead $arguments
  %9:i32 = CONST_I32 20, implicit-def dead $arguments
  BR %bb.6, implicit-def dead $arguments
```

After this patch, `llvm.wasm.rethrow` is treated as a terminator, and
the DAG will look like
```
        t0: ch,glue = EntryToken
      t3: ch = CopyToReg t0, Register:i32 %9, Constant:i32<20>
    t5: ch = llvm.wasm.rethrow t3, TargetConstant:i32<12161>
  t7: ch = br t5, BasicBlock:ch<unreachable 0x5555e3d32c70>
```
Note that now `rethrow` takes a token from `CopyToReg`, so `rethrow` has
to come after `CopyToReg`. And the resulting code will be
```mir
bb.3.rethrow:
  %9:i32 = CONST_I32 20, implicit-def dead $arguments
  RETHROW 0, implicit-def dead $arguments
  BR %bb.6, implicit-def dead $arguments
```

I'm not very familiar with the internals of `getRoot` vs.
`getControlRoot`, but other terminator instructions seem to use the
latter, and using it for `rethrow` too worked.



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