[all-commits] [llvm/llvm-project] a048ce: [X86] Default to -x86-pad-for-align=false to drop ...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: a048ce13e32daa255d26533c00da8abd0b67e819
      https://github.com/llvm/llvm-project/commit/a048ce13e32daa255d26533c00da8abd0b67e819
  Author: Fangrui Song <i at maskray.me>
  Date:   2021-01-16 (Sat, 16 Jan 2021)

  Changed paths:
    M llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
    M llvm/test/MC/X86/align-via-padding-corner.s
    M llvm/test/MC/X86/align-via-padding.s
    M llvm/test/MC/X86/align-via-relaxation.s
    M llvm/test/MC/X86/prefix-padding-32.s
    M llvm/test/MC/X86/prefix-padding-64.s

  Log Message:
  -----------
  [X86] Default to -x86-pad-for-align=false to drop assembler difference with or w/o -g

Fix PR48742: the D75203 assembler optimization locates MCRelaxableFragment's
within two MCSymbol's and relaxes some MCRelaxableFragment's to reduce the size
of a MCAlignFragment.  A -g build has more MCSymbol's and therefore may have
different assembler output (e.g. a MCRelaxableFragment (jmp) may have 5 bytes
with -O1 while 2 bytes with -O1 -g).

`.p2align 4, 0x90` is common due to loops. For a larger program, with a
lot of temporary labels, the assembly output difference is somewhat
destined. The cost seems to overweigh the benefits so we default to
-x86-pad-for-align=false until the heuristic is improved.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D94542