[all-commits] [llvm/llvm-project] a07c08: [WebAssembly] Lower llvm.debugtrap properly

[Thomas Lively via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: a07c08f74fafcbf196cda4b20f0761538fca3dbe
      https://github.com/llvm/llvm-project/commit/a07c08f74fafcbf196cda4b20f0761538fca3dbe
  Author: Thomas Lively <tlively at google.com>
  Date:   2020-06-04 (Thu, 04 Jun 2020)

  Changed paths:
    M llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
    M llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
    A llvm/test/CodeGen/WebAssembly/debugtrap.ll

  Log Message:
  -----------
  [WebAssembly] Lower llvm.debugtrap properly

Summary:
Unlike normal traps, debug traps are allowed to return and can have
additional instructions in the same basic block. Without explicit
backend support for debug traps, they are lowered in ISel as normal
traps. Since normal traps are lowered in the WebAssembly backend to
the UNREACHABLE instruction, which is a terminator, using debug traps
could lead to invalid MBBs when there are additional instructions
after the trap. This patch fixes the issue by lowering debug traps to
a new version of the UNREACHABLE instruction, DEBUG_UNREACHABLE, that
is not a terminator.

An alternative approach would have been to make UNREACHABLE not a
terminator, but that breaks a large number of tests. In particular, it
would require removing the traps inserted after noreturn calls to
@llvm.wasm.throw because otherwise the terminator throw would be
followed by a non-terminator UNREACHABLE and we would be back to
having invalid MBBs. Overall the approach in this patch seems simpler.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81055