[clang] [llvm] [LLVMABI] Implement the System V X86-64 ABI (PR #194718)

[via llvm-commits]

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@@ -344,18 +344,49 @@ bool CodeGenModule::shouldUseLLVMABILowering() const {
     return false;
   // Only opt in for targets that have an LLVMABI implementation; others
   // continue through the legacy ABIInfo path.
-  return getTriple().isBPF();
+  const llvm::Triple &T = getTriple();
+  return T.isBPF() || (T.getArch() == llvm::Triple::x86_64 && T.isOSLinux());
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adams381 wrote:

Suggested replacement that opens the new path to all SysV-x86_64 systems (Linux, FreeBSD, NetBSD, OpenBSD, DragonFly, Solaris, Haiku, Darwin) while still excluding Win64. Since `-fexperimental-abi-lowering` is opt-in, a permissive scope here is low-risk and gives reviewers more coverage to validate.

```suggestion
  return T.isBPF() || (T.getArch() == llvm::Triple::x86_64 &&
                       !T.isOSWindows() && !T.isOSCygMing());
```

If you'd rather keep Darwin out for now (it has subtle SysV deviations), happy with `... && !T.isOSDarwin()` added — but I'd lean toward letting it in given the experimental flag.

https://github.com/llvm/llvm-project/pull/194718