Commit 34b60d8a authored by Fangrui Song's avatar Fangrui Song
Browse files

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
parent 1e634f39

clang/docs/ReleaseNotes.rst

+4 −0
Original line number Diff line number Diff line
@@ -98,6 +98,10 @@ New Compiler Flags 
    -Wl,--gc-sections on ELF platforms to the linking command, and possibly

    adding -fdata-sections -ffunction-sections to the command generating

    the shared object).

- New option ``-fbinutils-version=`` specifies the targeted binutils version.

  For example, ``-fbinutils-version=2.35`` means compatibility with GNU as/ld

  before 2.35 is not needed: new features can be used and there is no need to

  work around old GNU as/ld bugs.



Deprecated Compiler Flags

-------------------------

clang/include/clang/Basic/CodeGenOptions.h

+6 −0
Original line number Diff line number Diff line
@@ -128,6 +128,12 @@ public: 
  // "none":        Disable sections/labels for basic blocks.

  std::string BBSections;



  // If set, override the default value of MCAsmInfo::BinutilsVersion. If

  // DisableIntegratedAS is specified, the assembly output will consider GNU as

  // support. "none" means that all ELF features can be used, regardless of

  // binutils support.

  std::string BinutilsVersion;



  enum class FramePointerKind {

    None,        // Omit all frame pointers.

    NonLeaf,     // Keep non-leaf frame pointers.

clang/include/clang/Driver/Options.td

+7 −0
Original line number Diff line number Diff line
@@ -3969,6 +3969,13 @@ defm : BooleanFFlag<"keep-inline-functions">, Group<clang_ignored_gcc_optimizati 


def fprofile_dir : Joined<["-"], "fprofile-dir=">, Group<f_Group>;



// The default value matches BinutilsVersion in MCAsmInfo.h.

def fbinutils_version_EQ : Joined<["-"], "fbinutils-version=">,

  MetaVarName<"<major.minor>">, Group<f_Group>, Flags<[CC1Option]>,

  HelpText<"Produced object files can use all ELF features supported by this "

  "binutils version and newer. If -fno-integrated-as is specified, the "

  "generated assembly will consider GNU as support. 'none' means that all ELF "

  "features can be used, regardless of binutils support. Defaults to 2.26.">;

def fuse_ld_EQ : Joined<["-"], "fuse-ld=">, Group<f_Group>, Flags<[CoreOption, LinkOption]>;

def ld_path_EQ : Joined<["--"], "ld-path=">, Group<Link_Group>;

clang/lib/CodeGen/BackendUtil.cpp

+2 −0
Original line number Diff line number Diff line
@@ -490,6 +490,8 @@ static bool initTargetOptions(DiagnosticsEngine &Diags, 
    break;

  }



  Options.BinutilsVersion =

      llvm::TargetMachine::parseBinutilsVersion(CodeGenOpts.BinutilsVersion);

  Options.UseInitArray = CodeGenOpts.UseInitArray;

  Options.DisableIntegratedAS = CodeGenOpts.DisableIntegratedAS;

  Options.CompressDebugSections = CodeGenOpts.getCompressDebugSections();

clang/lib/Driver/ToolChains/Clang.cpp

+16 −0
Original line number Diff line number Diff line
@@ -4840,6 +4840,22 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA, 
                    IsIntegratedAssemblerDefault))

    CmdArgs.push_back("-fno-verbose-asm");



  // Parse 'none' or '$major.$minor'. Disallow -fbinutils-version=0 because we

  // use that to indicate the MC default in the backend.

  if (Arg *A = Args.getLastArg(options::OPT_fbinutils_version_EQ)) {

    StringRef V = A->getValue();

    unsigned Num;

    if (V == "none")

      A->render(Args, CmdArgs);

    else if (!V.consumeInteger(10, Num) && Num > 0 &&

             (V.empty() || (V.consume_front(".") &&

                            !V.consumeInteger(10, Num) && V.empty())))

      A->render(Args, CmdArgs);

    else

      D.Diag(diag::err_drv_invalid_argument_to_option)

          << A->getValue() << A->getOption().getName();

  }



  if (!TC.useIntegratedAs())

    CmdArgs.push_back("-no-integrated-as");