[all-commits] [llvm/llvm-project] 856745: [RISCV][MC] Relax conditional branches to unresolv...

[Job Noorman via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 856745c5ebe5e3da35c4f08bc6090e2d348ea048
      https://github.com/llvm/llvm-project/commit/856745c5ebe5e3da35c4f08bc6090e2d348ea048
  Author: Job Noorman <jnoorman at igalia.com>
  Date:   2023-07-28 (Fri, 28 Jul 2023)

  Changed paths:
    M lld/test/ELF/riscv-branch.s
    M lld/test/ELF/riscv-undefined-weak.s
    M llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
    M llvm/test/MC/RISCV/compressed-relocations.s
    M llvm/test/MC/RISCV/long-conditional-jump.s
    M llvm/test/MC/RISCV/relocations.s

  Log Message:
  -----------
  [RISCV][MC] Relax conditional branches to unresolved symbols

D108961 introduced relaxation for out-of-range conditional branches.
However, relaxation was only performed when the branch target could be
resolved. I believe this has two undesired consequences:

- `b<cc> ... foo`, where `foo` is undefined, would not be relaxed
  although there is no guarantee the offset to `foo` will fit;
- Conditional branches are never relaxed with `-mattr=+relax` because MC
  considers fixups where `shouldForceRelocation` returns true (which
  will be the case with `+relax`) to be unresolved.

Note that binutils performs conditional branch relaxation in both cases.

This patch proposes to perform conditional branch relaxation even when
the target cannot be resolved.

Note on llvm/test/MC/RISCV/long-conditional-jump.s: I've removed the
`.p2align` because this causes alignment nops to be inserted for the
`+relax` tests. This in turn causes all the branch targets to change
compared to the non-`+relax` tests. Since `+relax` shouldn't change
these offsets, I found this confusing and hence chose to remove the
alignment.

Reviewed By: asb, MaskRay, reames

Differential Revision: https://reviews.llvm.org/D154958