llvm · lenary · Oct 16, 2025 · Oct 16, 2025
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
@@ -291,6 +291,8 @@ 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.
+- The `-mno-outline` flag will now add the `nooutline` IR attribute, so that
+  `-mno-outline` and `-moutline` objects can be mixed correctly during LTO.
 
 Removed Compiler Flags
 -------------------------

diff --git a/clang/include/clang/Basic/CodeGenOptions.def b/clang/include/clang/Basic/CodeGenOptions.def
@@ -489,6 +489,9 @@ CODEGENOPT(ResMayAlias, 1, 0, Benign)
 ENUM_CODEGENOPT(WinX64EHUnwindV2, WinX64EHUnwindV2Mode,
                 2, WinX64EHUnwindV2Mode::Disabled, Benign)
 
+/// Adds attributes that prevent outlining (`-mno-outline`)
+CODEGENOPT(DisableOutlining, 1, 0, Benign)
+
 /// FIXME: Make DebugOptions its own top-level .def file.
 #include "DebugOptions.def"
 

diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
@@ -5127,16 +5127,13 @@ 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,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)">;
+defm outline
+    : BoolMOption<
+          "outline", CodeGenOpts<"DisableOutlining">, DefaultFalse,
+          NegFlag<SetTrue, [], [ClangOption, CC1Option],
+                  "Disable function outlining (AArch64,Arm,RISC-V,X86 only)">,
+          PosFlag<SetFalse, [], [ClangOption],
+                  "Enable 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>,

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -2820,7 +2820,9 @@ void CodeGenModule::SetLLVMFunctionAttributesForDefinition(const Decl *D,
       B.addAttribute(llvm::Attribute::MinSize);
   }
 
-  if (D->hasAttr<NoOutlineAttr>())
+  // Add `nooutline` if Outlining is disabled with a command-line flag or a
+  // function attribute.
+  if (CodeGenOpts.DisableOutlining || D->hasAttr<NoOutlineAttr>())
     B.addAttribute(llvm::Attribute::NoOutline);
 
   F->addFnAttrs(B);

diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -2923,13 +2923,16 @@ void tools::addMachineOutlinerArgs(const Driver &D,
             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.
+        // Enable Pass in pipeline
         addArg(Twine("-enable-machine-outliner"));
       }
     } else {
-      // Disable all outlining behaviour.
+      if (!IsLTO)
+        // Disable all outlining behaviour using `nooutline` option, in case
+        // Linker Invocation lacks `-mno-outline`.
+        CmdArgs.push_back("-mno-outline");
+
+      // Disable Pass in Pipeline
       addArg(Twine("-enable-machine-outliner=never"));
     }
   }

diff --git a/clang/test/CodeGen/attr-nooutline.c b/clang/test/CodeGen/attr-nooutline.c
@@ -1,6 +1,17 @@
 // NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --check-attributes --version 6
-// RUN: %clang_cc1 -emit-llvm %s -triple x86_64-unknown-linux-gnu -o - | FileCheck %s --check-prefix=C
-// RUN: %clang_cc1 -emit-llvm -x c++ %s -triple x86_64-unknown-linux-gnu -o - | FileCheck %s --check-prefix=CXX
+// RUN: %clang_cc1 -emit-llvm %s -triple x86_64-unknown-linux-gnu -DTEST_ATTR -o - | FileCheck %s --check-prefix=C
+// RUN: %clang_cc1 -emit-llvm %s -triple x86_64-unknown-linux-gnu -mno-outline -o - | FileCheck %s --check-prefix=C
+// RUN: %clang_cc1 -emit-llvm %s -triple x86_64-unknown-linux-gnu -mno-outline -DTEST_ATTR -o - | FileCheck %s --check-prefix=C
+
+// RUN: %clang_cc1 -emit-llvm -x c++ %s -triple x86_64-unknown-linux-gnu -DTEST_ATTR -o - | FileCheck %s --check-prefix=CXX
+// RUN: %clang_cc1 -emit-llvm -x c++ %s -triple x86_64-unknown-linux-gnu -mno-outline -o - | FileCheck %s --check-prefix=CXX
+// RUN: %clang_cc1 -emit-llvm -x c++ %s -triple x86_64-unknown-linux-gnu -mno-outline -DTEST_ATTR -o - | FileCheck %s --check-prefix=CXX
+
+#ifdef TEST_ATTR
+#define NOOUTLINE [[clang::nooutline]]
+#else
+#define NOOUTLINE
+#endif
 
 // C: Function Attrs: noinline nooutline nounwind optnone
 // C-LABEL: define dso_local i32 @t1(
@@ -20,6 +31,6 @@
 // CXX-NEXT:    [[TMP0:%.*]] = load i32, ptr [[X_ADDR]], align 4
 // CXX-NEXT:    ret i32 [[TMP0]]
 //
-[[clang::nooutline]] int t1(int x) {
+NOOUTLINE int t1(int x) {
   return x;
 }
diff --git a/clang/test/Driver/aarch64-outliner.c b/clang/test/Driver/aarch64-outliner.c
@@ -3,4 +3,4 @@
 // ON: "-mllvm" "-enable-machine-outliner"
 // 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"
+// OFF: "-mno-outline" "-mllvm" "-enable-machine-outliner=never"
diff --git a/clang/test/Driver/arm-machine-outliner.c b/clang/test/Driver/arm-machine-outliner.c
@@ -3,6 +3,6 @@
 // RUN: %clang -target armv7-linux-gnueabihf -flto -moutline %s -### 2>&1 | FileCheck %s -check-prefix=ON-LTO
 // ON-LTO: "-plugin-opt=-enable-machine-outliner"
 // RUN: %clang -target armv7-linux-gnueabihf -moutline -mno-outline -c %s -### 2>&1 | FileCheck %s -check-prefix=OFF
-// OFF: "-mllvm" "-enable-machine-outliner=never"
+// OFF: "-mno-outline" "-mllvm" "-enable-machine-outliner=never"
 // RUN: %clang -target armv7-linux-gnueabihf -flto -moutline -mno-outline %s -### 2>&1 | FileCheck %s -check-prefix=OFF-LTO
 // OFF-LTO: "-plugin-opt=-enable-machine-outliner=never"
diff --git a/clang/test/Driver/riscv-outliner.c b/clang/test/Driver/riscv-outliner.c
@@ -4,4 +4,4 @@
 
 // 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"
+// OFF: "-mno-outline" "-mllvm" "-enable-machine-outliner=never"
diff --git a/clang/test/Driver/x86-outliner.c b/clang/test/Driver/x86-outliner.c
@@ -4,4 +4,4 @@
 
 // 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"
+// OFF: "-mno-outline" "-mllvm" "-enable-machine-outliner=never"