[clang] Add cc1 --output-asm-variant= to set output syntax

2fcaa54 (2010) added cc1as option
`-output-asm-variant` (untested) to set the output syntax.
`clang -cc1as -filetype asm -output-asm-variant 1` allows AT&T input and
Intel output (`AssemblerDialect` is also used by non-x86 targets).

This patch renames the cc1as option (to avoid collision with -o) and
makes it available for cc1 to set output syntax. This allows different
input & output syntax:

```
echo 'asm("mov $1, %eax");' | clang -xc - -S -o - -Xclang --output-asm-variant=1
```

Note: `AsmWriterFlavor` (with a misleading name), used to initialize
MCAsmInfo::AssemblerDialect, is primarily used for assembly input, not
for output.
Therefore,
`echo 'asm("mov $1, %eax");' | clang -x c - -mllvm --x86-asm-syntax=intel -S -o -`,
which achieves a similar goal before Clang 19, was unintended.

Close #109157

Pull Request: #109360

Author: MaskRay

clang/include/clang/Basic/CodeGenOptions.def

@@ -96,6 +96,7 @@ CODEGENOPT(EmulatedTLS       , 1, 0) ///< Set by default or -f[no-]emulated-tls.
 ENUM_CODEGENOPT(EmbedBitcode, EmbedBitcodeKind, 2, Embed_Off)
 /// Inline asm dialect, -masm=(att|intel)
 ENUM_CODEGENOPT(InlineAsmDialect, InlineAsmDialectKind, 1, IAD_ATT)
+CODEGENOPT(OutputAsmVariant, 2, 3) ///< Set the asm variant for output (3: unspecified).
 CODEGENOPT(ForbidGuardVariables , 1, 0) ///< Issue errors if C++ guard variables
                                         ///< are required.
 CODEGENOPT(FunctionSections  , 1, 0) ///< Set when -ffunction-sections is enabled.

clang/include/clang/Driver/Options.td

@@ -7217,6 +7217,9 @@ def fuse_ctor_homing: Flag<["-"], "fuse-ctor-homing">,
 def as_secure_log_file : Separate<["-"], "as-secure-log-file">,
   HelpText<"Emit .secure_log_unique directives to this filename.">,
   MarshallingInfoString<CodeGenOpts<"AsSecureLogFile">>;
+def output_asm_variant : Joined<["--"], "output-asm-variant=">,
+  HelpText<"Select the asm variant (integer) to use for output (3: unspecified)">,
+  MarshallingInfoInt<CodeGenOpts<"OutputAsmVariant">, "3">;
 
 } // let Visibility = [CC1Option, CC1AsOption]
 
@@ -8307,8 +8310,6 @@ def filetype : Separate<["-"], "filetype">,
     HelpText<"Specify the output file type ('asm', 'null', or 'obj')">;
 
 // Transliterate Options
-def output_asm_variant : Separate<["-"], "output-asm-variant">,
-    HelpText<"Select the asm variant index to use for output">;
 def show_encoding : Flag<["-"], "show-encoding">,
     HelpText<"Show instruction encoding information in transliterate mode">;
 def show_inst : Flag<["-"], "show-inst">,

clang/lib/CodeGen/BackendUtil.cpp

@@ -509,6 +509,8 @@ static bool initTargetOptions(DiagnosticsEngine &Diags,
   Options.MCOptions.X86RelaxRelocations = CodeGenOpts.X86RelaxRelocations;
   Options.MCOptions.CompressDebugSections =
       CodeGenOpts.getCompressDebugSections();
+  if (CodeGenOpts.OutputAsmVariant != 3) // 3 (default): not specified
+    Options.MCOptions.OutputAsmVariant = CodeGenOpts.OutputAsmVariant;
   Options.MCOptions.ABIName = TargetOpts.ABI;
   for (const auto &Entry : HSOpts.UserEntries)
     if (!Entry.IsFramework &&

clang/test/CodeGen/inline-asm-output-variant.c

@@ -0,0 +1,26 @@
+// REQUIRES: x86-registered-target
+/// AT&T input
+// RUN: %clang_cc1 -triple x86_64 -S --output-asm-variant=0 %s -o - | FileCheck --check-prefix=ATT %s
+// RUN: %clang_cc1 -triple x86_64 -S --output-asm-variant=1 %s -o - | FileCheck --check-prefix=INTEL %s
+
+/// Intel input
+// RUN: %clang_cc1 -triple x86_64 -S -D INTEL -mllvm -x86-asm-syntax=intel -inline-asm=intel %s -o - | FileCheck --check-prefix=INTEL %s
+// RUN: %clang_cc1 -triple x86_64 -S -D INTEL -mllvm -x86-asm-syntax=intel -inline-asm=intel --output-asm-variant=1 %s -o - | FileCheck --check-prefix=INTEL %s
+
+// ATT: movl $1, %eax
+// ATT: movl $2, %eax
+
+// INTEL: mov eax, 1
+// INTEL: mov eax, 2
+
+#ifdef INTEL
+asm("mov eax, 1");
+void foo() {
+  asm("mov eax, 2");
+}
+#else
+asm("mov $1, %eax");
+void foo() {
+  asm("mov $2, %eax");
+}
+#endif

clang/test/Misc/cc1as-output-asm-variant.c

@@ -0,0 +1,8 @@
+// REQUIRES: x86-registered-target
+// RUN: %clang -cc1as -triple x86_64 %s -o - | FileCheck %s --check-prefix=ATT
+// RUN: %clang -cc1as -triple x86_64 %s --output-asm-variant=1 -o - | FileCheck %s --check-prefix=INTEL
+
+// ATT: movl $1, %eax
+// INTEL: mov eax, 1
+
+mov $1, %eax

llvm/include/llvm/MC/MCTargetOptions.h

@@ -72,6 +72,8 @@ class MCTargetOptions {
 
   bool X86Sse2Avx = false;
 
+  std::optional<unsigned> OutputAsmVariant;
+
   EmitDwarfUnwindType EmitDwarfUnwind;
 
   int DwarfVersion = 0;

llvm/lib/CodeGen/LLVMTargetMachine.cpp

@@ -160,7 +160,9 @@ Expected<std::unique_ptr<MCStreamer>> LLVMTargetMachine::createMCStreamer(
   switch (FileType) {
   case CodeGenFileType::AssemblyFile: {
     MCInstPrinter *InstPrinter = getTarget().createMCInstPrinter(
-        getTargetTriple(), MAI.getAssemblerDialect(), MAI, MII, MRI);
+        getTargetTriple(),
+        Options.MCOptions.OutputAsmVariant.value_or(MAI.getAssemblerDialect()),
+        MAI, MII, MRI);
 
     // Create a code emitter if asked to show the encoding.
     std::unique_ptr<MCCodeEmitter> MCE;