[clang] [llvm] [RISCV][FMV] Support target_clones (PR #85786)

[Piyou Chen via cfe-commits]

================
@@ -2854,10 +2854,121 @@ void CodeGenFunction::EmitMultiVersionResolver(
   case llvm::Triple::aarch64:
     EmitAArch64MultiVersionResolver(Resolver, Options);
     return;
+  case llvm::Triple::riscv32:
+  case llvm::Triple::riscv64:
+    EmitRISCVMultiVersionResolver(Resolver, Options);
+    return;
 
   default:
-    assert(false && "Only implemented for x86 and AArch64 targets");
+    assert(false && "Only implemented for x86, AArch64 and RISC-V targets");
+  }
+}
+
+void CodeGenFunction::EmitRISCVMultiVersionResolver(
+    llvm::Function *Resolver, ArrayRef<MultiVersionResolverOption> Options) {
+
+  if (getContext().getTargetInfo().getTriple().getOS() !=
+      llvm::Triple::OSType::Linux) {
+    CGM.getDiags().Report(diag::err_os_unsupport_riscv_target_clones);
+    return;
+  }
+
+  llvm::BasicBlock *CurBlock = createBasicBlock("resolver_entry", Resolver);
+  Builder.SetInsertPoint(CurBlock);
+  EmitRISCVCpuInit();
+
+  bool SupportsIFunc = getContext().getTargetInfo().supportsIFunc();
+  bool HasDefault = false;
+  unsigned DefaultIndex = 0;
+  // Check the each candidate function.
+  for (unsigned Index = 0; Index < Options.size(); Index++) {
----------------
BeMg wrote:

For RISC-V target, I'm not sure that it is possible to define the score for all extension combination, then we could sort the order base on it. Or we could extend the `target_clones` syntax with `Priority=<NUM>` to indicate the order by user. 

```
__attribute__((target_clones("default", "arch=+zba;Priority=1", "arch=+zba,+zbb;Priority=2"))) int bar() {  
    return 1;
}
```

And Yes, It will third version (+zba,+zbb) will be dead in current implementation. It will always get into +zba version or default.

https://github.com/llvm/llvm-project/pull/85786