[all-commits] [llvm/llvm-project] 96c85f: [RISCV] Don't outline pcrel-lo operand.

Kito Cheng via All-commits all-commits at lists.llvm.org
Wed Aug 24 06:48:05 PDT 2022

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 96c85f80f0d615ffde0f85d8270e0a8c9f4e5430
  Author: Kito Cheng <kito.cheng at sifive.com>
  Date:   2022-08-24 (Wed, 24 Aug 2022)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
    M llvm/test/CodeGen/RISCV/machineoutliner-pcrel-lo.mir

  Log Message:
  [RISCV] Don't outline pcrel-lo operand.

This issue is found by build llvm-testsuite with `-Oz`, linker will complain
`dangerous relocation: %pcrel_lo missing matching %pcrel_hi` and that
turn out cause by we outlined pcrel-lo, but leave pcrel-hi there, that's
not problem in general, but the problem is they put into different section, they
pcrel-hi and pcrel-lo pair (e.g. AUIPC+ADDI) *MUST* put be present in same
section due to the implementation.

Outlined function will put into .text name, but the source functions
will put in .text.<function-name> if function-section is enabled or the
function has `comdat` attribute.

There are few solutions for this issue:
1. Always disallow instructions with pcrel-lo flags.
2. Only disallow instructions with pcrel-lo flags that when function-section is
   enabled or this function has `comdat` attribute.
3. Check the corresponding instruction with pcrel-high also included in the
   outlining candidate sequence or not, and allow that only when pcrel-high is
   included in the outlining candidate.

First one is most conservative, that might lose some optimization
opportunities, and second one could save those opportunities, and last
one is hard to implement, and don't have any benefits since pcrel-high
are using different label even accessing same symbol.

Use custom section name might also cause this problem, but that already
filtered by RISCVInstrInfo::isFunctionSafeToOutlineFrom.

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D132528

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