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[Driver] Don't default to -mrelax-all for non-RISCV -O0 #90013

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Apr 25, 2024
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[Driver] Don't default to -mrelax-all for non-RISCV -O0 #90013

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52 changes: 10 additions & 42 deletions clang/lib/Driver/ToolChains/Clang.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -835,46 +835,6 @@ static void addPGOAndCoverageFlags(const ToolChain &TC, Compilation &C,
}
}

/// Check whether the given input tree contains any compilation actions.
static bool ContainsCompileAction(const Action *A) {
if (isa<CompileJobAction>(A) || isa<BackendJobAction>(A))
return true;

return llvm::any_of(A->inputs(), ContainsCompileAction);
}

/// Check if -relax-all should be passed to the internal assembler.
/// This is done by default when compiling non-assembler source with -O0.
static bool UseRelaxAll(Compilation &C, const ArgList &Args) {
bool RelaxDefault = true;

if (Arg *A = Args.getLastArg(options::OPT_O_Group))
RelaxDefault = A->getOption().matches(options::OPT_O0);

// RISC-V requires an indirect jump for offsets larger than 1MiB. This cannot
// be done by assembler branch relaxation as it needs a free temporary
// register. Because of this, branch relaxation is handled by a MachineIR
// pass before the assembler. Forcing assembler branch relaxation for -O0
// makes the MachineIR branch relaxation inaccurate and it will miss cases
// where an indirect branch is necessary. To avoid this issue we are
// sacrificing the compile time improvement of using -mrelax-all for -O0.
if (C.getDefaultToolChain().getTriple().isRISCV())
RelaxDefault = false;

if (RelaxDefault) {
RelaxDefault = false;
for (const auto &Act : C.getActions()) {
if (ContainsCompileAction(Act)) {
RelaxDefault = true;
break;
}
}
}

return Args.hasFlag(options::OPT_mrelax_all, options::OPT_mno_relax_all,
RelaxDefault);
}

static void
RenderDebugEnablingArgs(const ArgList &Args, ArgStringList &CmdArgs,
llvm::codegenoptions::DebugInfoKind DebugInfoKind,
Expand Down Expand Up @@ -2472,8 +2432,16 @@ static void CollectArgsForIntegratedAssembler(Compilation &C,
const ArgList &Args,
ArgStringList &CmdArgs,
const Driver &D) {
if (UseRelaxAll(C, Args))
CmdArgs.push_back("-mrelax-all");
// Default to -mno-relax-all.
//
// Note: RISC-V requires an indirect jump for offsets larger than 1MiB. This
// cannot be done by assembler branch relaxation as it needs a free temporary
// register. Because of this, branch relaxation is handled by a MachineIR pass
// before the assembler. Forcing assembler branch relaxation for -O0 makes the
// MachineIR branch relaxation inaccurate and it will miss cases where an
// indirect branch is necessary.
Args.addOptInFlag(CmdArgs, options::OPT_mrelax_all,
options::OPT_mno_relax_all);

// Only default to -mincremental-linker-compatible if we think we are
// targeting the MSVC linker.
Expand Down
2 changes: 1 addition & 1 deletion clang/test/Driver/integrated-as.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@
// RUN: %clang -### -c -save-temps -integrated-as --target=x86_64 %s 2>&1 | FileCheck %s

// CHECK: cc1as
// CHECK: -mrelax-all
// CHECK-NOT: -mrelax-all

// RISC-V does not enable -mrelax-all
// RUN: %clang -### -c -save-temps -integrated-as --target=riscv64 %s 2>&1 | FileCheck %s -check-prefix=RISCV-RELAX
Expand Down
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