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

2fcaa549a824efeb56e807fcf750a56bf985296b (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: https://github.com/llvm/llvm-project/pull/109360

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/LLVMTargetMachine.cpp b/llvm/lib/CodeGen/LLVMTargetMachine.cpp
index 4ff2205..ea36fed 100644
--- a/llvm/lib/CodeGen/LLVMTargetMachine.cpp
+++ b/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;