[all-commits] [llvm/llvm-project] 34b60d: Add -fbinutils-version= to gate ELF features on th...

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 34b60d8a5684edabc06543c89ddd11874a927886
      https://github.com/llvm/llvm-project/commit/34b60d8a5684edabc06543c89ddd11874a927886
  Author: Fangrui Song <maskray at google.com>
  Date:   2021-01-26 (Tue, 26 Jan 2021)

  Changed paths:
    M clang/docs/ReleaseNotes.rst
    M clang/include/clang/Basic/CodeGenOptions.h
    M clang/include/clang/Driver/Options.td
    M clang/lib/CodeGen/BackendUtil.cpp
    M clang/lib/Driver/ToolChains/Clang.cpp
    M clang/lib/Frontend/CompilerInvocation.cpp
    A clang/test/Driver/fbinutils-version.c
    M llvm/include/llvm/MC/MCAsmInfo.h
    M llvm/include/llvm/Target/TargetMachine.h
    M llvm/include/llvm/Target/TargetOptions.h
    M llvm/lib/CodeGen/LLVMTargetMachine.cpp
    M llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
    M llvm/lib/Target/TargetMachine.cpp
    M llvm/test/CodeGen/X86/explicit-section-mergeable.ll
    A llvm/test/tools/llc/binutils-version.ll
    M llvm/tools/llc/llc.cpp

  Log Message:
  -----------
  Add -fbinutils-version= to gate ELF features on the specified binutils version

There are two use cases.

Assembler
We have accrued some code gated on MCAsmInfo::useIntegratedAssembler().  Some
features are supported by latest GNU as, but we have to use
MCAsmInfo::useIntegratedAs() because the newer versions have not been widely
adopted (e.g. SHF_LINK_ORDER 'o' and 'unique' linkage in 2.35, --compress-debug-sections= in 2.26).

Linker
We want to use features supported only by LLD or very new GNU ld, or don't want
to work around older GNU ld. We currently can't represent that "we don't care
about old GNU ld".  You can find such workarounds in a few other places, e.g.
Mips/MipsAsmprinter.cpp PowerPC/PPCTOCRegDeps.cpp X86/X86MCInstrLower.cpp
AArch64 TLS workaround for R_AARCH64_TLSLD_MOVW_DTPREL_* (PR ld/18276),
R_AARCH64_TLSLE_LDST8_TPREL_LO12 (https://bugs.llvm.org/show_bug.cgi?id=36727 https://sourceware.org/bugzilla/show_bug.cgi?id=22969)

Mixed SHF_LINK_ORDER and non-SHF_LINK_ORDER components (supported by LLD in D84001;
GNU ld feature request https://sourceware.org/bugzilla/show_bug.cgi?id=16833 may take a while before available).
This feature allows to garbage collect some unused sections (e.g. fragmented .gcc_except_table).

This patch adds `-fbinutils-version=` to clang and `-binutils-version` to llc.
It changes one codegen place in SHF_MERGE to demonstrate its usage.
`-fbinutils-version=2.35` means the produced object file does not care about GNU
ld<2.35 compatibility. When `-fno-integrated-as` is specified, the produced
assembly can be consumed by GNU as>=2.35, but older versions may not work.

`-fbinutils-version=none` means that we can use all ELF features, regardless of
GNU as/ld support.

Both clang and llc need `parseBinutilsVersion`. Such command line parsing is
usually implemented in `llvm/lib/CodeGen/CommandFlags.cpp` (LLVMCodeGen),
however, ClangCodeGen does not depend on LLVMCodeGen. So I add
`parseBinutilsVersion` to `llvm/lib/Target/TargetMachine.cpp` (LLVMTarget).

Differential Revision: https://reviews.llvm.org/D85474