[all-commits] [llvm/llvm-project] c5ba61: [BOLT][RISCV] Add support for linker relaxation

Job Noorman via All-commits all-commits at lists.llvm.org
Fri Sep 15 02:57:45 PDT 2023

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c5ba61978c3364b31c00130561285c28306655b7
  Author: Job Noorman <jnoorman at igalia.com>
  Date:   2023-09-15 (Fri, 15 Sep 2023)

  Changed paths:
    M bolt/include/bolt/Core/MCPlusBuilder.h
    M bolt/include/bolt/Passes/FixRISCVCallsPass.h
    M bolt/lib/Core/BinaryContext.cpp
    M bolt/lib/Passes/FixRISCVCallsPass.cpp
    M bolt/lib/Rewrite/JITLinkLinker.cpp
    M bolt/lib/Target/RISCV/RISCVMCPlusBuilder.cpp
    M bolt/test/RISCV/internal-func-reloc.s
    M bolt/test/RISCV/lit.local.cfg
    A bolt/test/RISCV/relax.s
    M bolt/test/RISCV/reloc-call.s

  Log Message:
  [BOLT][RISCV] Add support for linker relaxation

Calls on RISC-V are typically compiled to `auipc`/`jalr` pairs to allow
a maximum target range (32-bit pc-relative). In order to optimize calls
to near targets, linker relaxation may replace those pairs with, for
example, single `jal` instructions.

To allow BOLT to freely reassign function addresses in relaxed binaries,
this patch proposes the following approach:
- Expand all relaxed calls back to `auipc`/`jalr`;
- Rely on JITLink to relax those back to shorter forms where possible.

This is implemented by detecting all possible call instructions and
replacing them with `PseudoCALL` (or `PseudoTAIL`) instructions. The
RISC-V backend then expands those and adds the necessary relocations for

Since BOLT generally ignores pseudo instruction, this patch makes
`MCPlusBuilder::isPseudo` virtual so that `RISCVMCPlusBuilder` can
override it to exclude `PseudoCALL` and `PseudoTAIL`.

To ensure JITLink knows about the correct section addresses while
relaxing, reassignment of addresses has been moved to a post-allocation
pass. Note that this is probably the time it had to be done in the
first place since in `notifyResolved` (where it was done before), all
symbols are supposed to be resolved already.

Depends on D159082

Reviewed By: maksfb

Differential Revision: https://reviews.llvm.org/D159089

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