[all-commits] [llvm/llvm-project] ad25f8: X86InstrInfo: Support immediates that are +1/-1 di...

[Matthias Braun via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ad25f8a556d239d8b7d17383cf1a0771359521fd
      https://github.com/llvm/llvm-project/commit/ad25f8a556d239d8b7d17383cf1a0771359521fd
  Author: Matthias Braun <matthiasb at fb.com>
  Date:   2022-01-11 (Tue, 11 Jan 2022)

  Changed paths:
    M llvm/lib/Target/X86/X86InstrInfo.cpp
    M llvm/lib/Target/X86/X86InstrInfo.h
    M llvm/test/CodeGen/X86/optimize-compare.mir
    A llvm/test/CodeGen/X86/peep-test-5.ll
    M llvm/test/CodeGen/X86/use-cr-result-of-dom-icmp-st.ll

  Log Message:
  -----------
  X86InstrInfo: Support immediates that are +1/-1 different in optimizeCompareInstr

This is a re-commit of e2c7ee0743592e39274e28dbe0d0c213ba342317 which
was reverted in a2a58d91e82db38fbdf88cc317dcb3753d79d492 and
ea81cea8163a1a0e54df42103ee1c657bbf03791. This includes a fix to
consistently check for EFLAGS being live-out. See phabricator
review.

Original Summary:

This extends `optimizeCompareInstr` to re-use previous comparison
results if the previous comparison was with an immediate that was 1
bigger or smaller. Example:

    CMP x, 13
    ...
    CMP x, 12   ; can be removed if we change the SETg
    SETg ...    ; x > 12  changed to `SETge` (x >= 13) removing CMP

Motivation: This often happens because SelectionDAG canonicalization
tends to add/subtract 1 often when optimizing for fallthrough blocks.
Example for `x > C` the fallthrough optimization switches true/false
blocks with `!(x > C)` --> `x <= C` and canonicalization turns this into
`x < C + 1`.

Differential Revision: https://reviews.llvm.org/D110867