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

Authored by MaskRay on Jan 26 2021, 12:28 PM.

Description

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