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[clang][Driver] Support Outline Flags on RISC-V and X86 #163664

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[clang][Driver] Support Outline Flags on RISC-V and X86 #163664

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[clang][Driver] Support Outline Flags on RISC-V and X86
lenary Oct 15, 2025
12c0094
Release Note
lenary Oct 16, 2025
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1 change: 1 addition & 0 deletions clang/docs/ReleaseNotes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -287,6 +287,7 @@ Modified Compiler Flags
-----------------------
- The `-gkey-instructions` compiler flag is now enabled by default when DWARF is emitted for plain C/C++ and optimizations are enabled. (#GH149509)
- The `-fconstexpr-steps` compiler flag now accepts value `0` to opt out of this limit. (#GH160440)
- The `-mno-outline` and `-moutline` compiler flags are now allowed on RISC-V and X86, which both support the machine outliner.

Removed Compiler Flags
-------------------------
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16 changes: 10 additions & 6 deletions clang/include/clang/Driver/Options.td
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Expand Up @@ -5127,12 +5127,16 @@ def mms_bitfields : Flag<["-"], "mms-bitfields">, Group<m_Group>,
Visibility<[ClangOption, CC1Option]>,
HelpText<"Set the default structure layout to be compatible with the Microsoft compiler standard">,
MarshallingInfoFlag<LangOpts<"MSBitfields">>;
def moutline : Flag<["-"], "moutline">, Group<f_clang_Group>,
Visibility<[ClangOption, CC1Option]>,
HelpText<"Enable function outlining (AArch64 only)">;
def mno_outline : Flag<["-"], "mno-outline">, Group<f_clang_Group>,
Visibility<[ClangOption, CC1Option]>,
HelpText<"Disable function outlining (AArch64 only)">;
def moutline
: Flag<["-"], "moutline">,
Group<f_clang_Group>,
Visibility<[ClangOption, CC1Option]>,
HelpText<"Enable function outlining (AArch64,Arm,RISC-V,X86 only)">;
def mno_outline
: Flag<["-"], "mno-outline">,
Group<f_clang_Group>,
Visibility<[ClangOption, CC1Option]>,
HelpText<"Disable function outlining (AArch64,Arm,RISC-V,X86 only)">;
def mno_ms_bitfields : Flag<["-"], "mno-ms-bitfields">, Group<m_Group>,
HelpText<"Do not set the default structure layout to be compatible with the Microsoft compiler standard">;
def mskip_rax_setup : Flag<["-"], "mskip-rax-setup">, Group<m_Group>,
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12 changes: 8 additions & 4 deletions clang/lib/Driver/ToolChains/CommonArgs.cpp
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Expand Up @@ -2916,12 +2916,16 @@ void tools::addMachineOutlinerArgs(const Driver &D,
if (Arg *A = Args.getLastArg(options::OPT_moutline,
options::OPT_mno_outline)) {
if (A->getOption().matches(options::OPT_moutline)) {
// We only support -moutline in AArch64 and ARM targets right now. If
// we're not compiling for these, emit a warning and ignore the flag.
// Otherwise, add the proper mllvm flags.
if (!(Triple.isARM() || Triple.isThumb() || Triple.isAArch64())) {
// We only support -moutline in AArch64, ARM, RISC-V and X86 targets right
// now. If we're not compiling for these, emit a warning and ignore the
// flag. Otherwise, add the proper mllvm flags.
if (!(Triple.isARM() || Triple.isThumb() || Triple.isAArch64() ||
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Should we swap the if and else body to get rid of this !?

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Sure, will do.

Triple.isRISCV() || Triple.isX86())) {
D.Diag(diag::warn_drv_moutline_unsupported_opt) << Triple.getArchName();
} else {
// FIXME: This should probably use the `nooutline` attribute rather than
// tweaking Pipeline Pass flags, so `-mno-outline` and `-moutline`
// objects can be combined correctly during LTO.
addArg(Twine("-enable-machine-outliner"));
}
} else {
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3 changes: 0 additions & 3 deletions clang/test/Driver/aarch64-outliner.c
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Expand Up @@ -4,6 +4,3 @@
// RUN: %clang --target=aarch64 -moutline -mno-outline -S %s -### 2>&1 | FileCheck %s -check-prefix=OFF
// RUN: %clang --target=aarch64_be -moutline -mno-outline -S %s -### 2>&1 | FileCheck %s -check-prefix=OFF
// OFF: "-mllvm" "-enable-machine-outliner=never"
// RUN: %clang --target=x86_64 -moutline -S %s -### 2>&1 | FileCheck %s -check-prefix=WARN
// WARN: warning: 'x86_64' does not support '-moutline'; flag ignored [-Woption-ignored]
// WARN-NOT: "-mllvm" "-enable-machine-outliner"
7 changes: 7 additions & 0 deletions clang/test/Driver/riscv-outliner.c
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@@ -0,0 +1,7 @@
// RUN: %clang --target=riscv32 -moutline -S %s -### 2>&1 | FileCheck %s -check-prefix=ON
// RUN: %clang --target=riscv64 -moutline -S %s -### 2>&1 | FileCheck %s -check-prefix=ON
// ON: "-mllvm" "-enable-machine-outliner"

// RUN: %clang --target=riscv32 -moutline -mno-outline -S %s -### 2>&1 | FileCheck %s -check-prefix=OFF
// RUN: %clang --target=riscv64 -moutline -mno-outline -S %s -### 2>&1 | FileCheck %s -check-prefix=OFF
// OFF: "-mllvm" "-enable-machine-outliner=never"
3 changes: 3 additions & 0 deletions clang/test/Driver/unsupported-outliner.c
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@@ -0,0 +1,3 @@
// RUN: %clang --target=ppc64 -moutline -S %s -### 2>&1 | FileCheck %s -check-prefix=WARN
// WARN: warning: 'ppc64' does not support '-moutline'; flag ignored [-Woption-ignored]
// WARN-NOT: "-mllvm" "-enable-machine-outliner"
7 changes: 7 additions & 0 deletions clang/test/Driver/x86-outliner.c
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@@ -0,0 +1,7 @@
// RUN: %clang --target=i386 -moutline -S %s -### 2>&1 | FileCheck %s -check-prefix=ON
// RUN: %clang --target=x86_64 -moutline -S %s -### 2>&1 | FileCheck %s -check-prefix=ON
// ON: "-mllvm" "-enable-machine-outliner"

// RUN: %clang --target=i386 -moutline -mno-outline -S %s -### 2>&1 | FileCheck %s -check-prefix=OFF
// RUN: %clang --target=x86_64 -moutline -mno-outline -S %s -### 2>&1 | FileCheck %s -check-prefix=OFF
// OFF: "-mllvm" "-enable-machine-outliner=never"