[PATCH] D101736: [WebAssembly] Allow DBG_VALUE after terminator in MachineVerifier

[Jeremy Morse via Phabricator via llvm-commits]

jmorse added a comment.

Hi,

I'm not familiar with WebAssembly, but thought I'd describe how this would works in other backends -- it might inspire you to take a different route. Sorry if it doesn't apply to WebAssembly.

For the problem of:

  But wasm is special in this way, because uses, not defs, can change the
  value stack location's contents. Our terminator instructions, such as
  BR_IF don't write to registers but uses registers, which can be
  stackified, changing the value stack and thus possibly invalidating some
  variables' value in previous DBG_VALUEs, so they have to be cleared by
  additional DBG_VALUEs after the terminator in that case.

There's a function / class, DbgEntityHistoryCalculator, that would handle this for other architectures. It translates DBG_VALUE instructions into location lists described using MCSymbols. It starts a location range at the point where a DBG_VALUE occurs, and then terminates that range at:

- Another DBG_VALUE for the same variable,
- The end of the block,
- When the relevant machine register is clobbered.

It seems to me that if the WebAssembly value stack can be tracked within a block in a similar way, then when a value is no longer present, DbgEntityHistoryCalculator could terminate the location range as if a register had been clobbered. X86 tail-calls are an example: they clobber registers, but we don't insert DBG_VALUE $noregs after them, because DbgEntityHistoryCalculator observes the clobber and terminates the range.

If WebAssembly can't use DbgEntityHistoryCalculator for some reason, then yes, something like this patch would be needed.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D101736/new/

https://reviews.llvm.org/D101736