[all-commits] [llvm/llvm-project] 5a159e: [InstCombine] Negator: don't negate multi-use `sub`

[Roman Lebedev via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 5a159ed2a8e5a9a6ced73f78e4c64b01d76d3493
      https://github.com/llvm/llvm-project/commit/5a159ed2a8e5a9a6ced73f78e4c64b01d76d3493
  Author: Roman Lebedev <lebedev.ri at gmail.com>
  Date:   2020-04-23 (Thu, 23 Apr 2020)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineNegator.cpp
    M llvm/test/Transforms/InstCombine/sub-of-negatible.ll
    M llvm/test/Transforms/InstCombine/sub.ll

  Log Message:
  -----------
  [InstCombine] Negator: don't negate multi-use `sub`

While we can do that, it doesn't increase instruction count,
if the old `sub` sticks around then the transform is not only
not a unlikely win, but a likely regression, since we likely
now extended live range and use count of both of the `sub` operands,
as opposed to just the result of `sub`.

As Kostya Serebryany notes in post-commit review in
https://reviews.llvm.org/D68408#1998112
this indeed can degrade final assembly,
increase register pressure, and spilling.

This isn't what we want here,
so at least for now let's guard it with an use check.