[all-commits] [llvm/llvm-project] e6fed0: [RISCV] Make linker-relaxable instructions termina...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e6fed06335f4a05aac900380c975b2daf8cbb934
      https://github.com/llvm/llvm-project/commit/e6fed06335f4a05aac900380c975b2daf8cbb934
  Author: Fangrui Song <i at maskray.me>
  Date:   2023-06-29 (Thu, 29 Jun 2023)

  Changed paths:
    M llvm/include/llvm/MC/MCAsmBackend.h
    M llvm/include/llvm/MC/MCFragment.h
    M llvm/lib/MC/MCAsmBackend.cpp
    M llvm/lib/MC/MCELFStreamer.cpp
    M llvm/lib/MC/MCExpr.cpp
    M llvm/lib/MC/MCObjectStreamer.cpp
    M llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h
    M llvm/test/MC/ELF/RISCV/subsection.s

  Log Message:
  -----------
  [RISCV] Make linker-relaxable instructions terminate MCDataFragment

`MCExpr::evaluateAsAbsolute` has a longstanding bug. When the MCAssembler is
non-null and the MCAsmLayout is null, it may incorrectly fold A-B even if A and
B are separated by a linker-relaxable instruction. This behavior can suppress
some ADD/SUB relocations and lead to wrong results if the linker performs
relaxation.

To fix the bug, ensure that linker-relaxable instructions only appear at the end
of an MCDataFragment, thereby making them terminate the fragment. When computing
A-B, suppress folding if A and B are separated by a linker-relaxable
instruction.

* `.subsection` now correctly give errors for non-foldable expressions.
* gen-dwarf.s will pass even if we add back the .debug_line or .eh_frame/.debug_frame code from D150004
* This will fix suppressed relocation when we add R_RISCV_SET_ULEB128/R_RISCV_SUB_ULEB128.

In the future, we should investigate the desired behavior for
`MCExpr::evaluateAsAbsolute` when both MCAssembler and MCAsmLayout are non-null.

(Note: MCRelaxableFragment is only for assembler-relaxation. If we ever need
linker-relaxable MCRelaxableFragment, we would need to adjust RISCVMCExpr.cpp
(D58943/D73211).)

Depends on D153096

Differential Revision: https://reviews.llvm.org/D153097