[WebAssembly] Assertion failure at -O0

[nikic]

; RUN: llc -O0 < %s
target triple = "wasm32-unknown-unknown"

define void @test(i1 %x) {
  %y = xor i1 %x, true
  br i1 %y, label %exit, label %exit

exit:
  ret void
}

llc: /home/npopov/repos/llvm-project/llvm/lib/Target/WebAssembly/WebAssemblyDebugFixup.cpp:152: virtual bool (anonymous namespace)::WebAssemblyDebugFixup::runOnMachineFunction(MachineFunction &): Assertion `Stack.empty() && "WebAssemblyDebugFixup: Stack not empty at end of basic block!"' failed.

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: Nikita Popov (nikic)

```llvm ; RUN: llc -O0 < %s target triple = "wasm32-unknown-unknown"

define void @test(i1 %x) {
%y = xor i1 %x, true
br i1 %y, label %exit, label %exit

exit:
ret void
}

llc: /home/npopov/repos/llvm-project/llvm/lib/Target/WebAssembly/WebAssemblyDebugFixup.cpp:152: virtual bool (anonymous namespace)::WebAssemblyDebugFixup::runOnMachineFunction(MachineFunction &): Assertion `Stack.empty() && "WebAssemblyDebugFixup: Stack not empty at end of basic block!"' failed.

</details>

[nikic]

Presumably caused by #136510. (Edit: Confirmed that reverting it fixes the issue.)

After stackify:

bb.0 (%ir-block.0):
  successors: %bb.1
  liveins: $arguments, $value_stack
  %0:i32 = ARGUMENT_i32 0, implicit $arguments
  %4:i32 = CONST_I32 -1, implicit-def $arguments, implicit-def $value_stack, implicit $value_stack
  %5:i32 = XOR_I32 %0:i32, %4:i32, implicit-def $arguments, implicit-def $value_stack, implicit $value_stack
  %2:i32 = CONST_I32 1, implicit-def $arguments, implicit-def $value_stack, implicit $value_stack
  %3:i32 = AND_I32 %5:i32, %2:i32, implicit-def $arguments, implicit-def $value_stack, implicit $value_stack
  BR_IF %bb.1, %3:i32, implicit-def $arguments, implicit-def $value_stack, implicit $value_stack

bb.1.exit:
; predecessors: %bb.0
  liveins: $value_stack
  RETURN implicit-def $arguments

And then after wasm-cfg-sort:

bb.0 (%ir-block.0):
  successors: %bb.1

  %0:i32 = ARGUMENT_i32 0, implicit $arguments
  %4:i32 = CONST_I32 -1, implicit-def $arguments, implicit-def $value_stack, implicit $value_stack
  %5:i32 = XOR_I32 %0:i32, %4:i32, implicit-def $arguments, implicit-def $value_stack, implicit $value_stack
  %2:i32 = CONST_I32 1, implicit-def $arguments, implicit-def $value_stack, implicit $value_stack
  %3:i32 = AND_I32 %5:i32, %2:i32, implicit-def $arguments, implicit-def $value_stack, implicit $value_stack

bb.1.exit:
; predecessors: %bb.0

  RETURN implicit-def $arguments

cc @SingleAccretion @aheejin

I'm not familiar with wasms stack model -- is the assumption that the stack always returns to the block-entry state at the end of the block? In that case it would be the wasm-cfg-sort transform (or specifically wasm's removeBranch implementation) at fault.

[SingleAccretion]

I'm not familiar with wasms stack model -- is the assumption that the stack always returns to the block-entry state at the end of the block?

It is certainly not right to leave anything additional on the stack with a local transformation like this (no-op-ing a degenerate branch).

We don't expect any unused stack defs after stackifying since the algorithm builds a tree of uses (therefore the root is never stackified). If the root ends up unused, it would then be explicitly dropped by WebAssemblyExplicitLocals.

At this point though, I don't know what the right fix is... Inserting drops in removeBranch seems heavy-handed since it would essentially duplicate the work done by WebAssemblyExplicitLocals (the method could be called before it, presumably).

[nikic]

I thought that unstackifying the vreg for the branch operand might be a good option, which would then leave explicit-locals to insert the drop. However, it looks like this causes issues with TEE instructions, which require one of the registers to be stackified.