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

[Kito Cheng via cfe-commits]

================
@@ -2877,10 +2877,143 @@ 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");
+  }
+}
+
+static int getPriorityFromAttrString(StringRef AttrStr) {
+  SmallVector<StringRef, 8> Attrs;
+
+  AttrStr.split(Attrs, ";");
+
+  // Default Priority is zero.
+  int Priority = 0;
+  for (auto Attr : Attrs) {
+    if (Attr.consume_front("priority=")) {
+      int Result;
+      if (!Attr.getAsInteger(0, Result)) {
+        Priority = Result;
+      }
+    }
+  }
+
+  return Priority;
+}
+
+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;
+
+  SmallVector<CodeGenFunction::MultiVersionResolverOption, 10> CurrOptions(
+      Options);
+
+  llvm::stable_sort(
+      CurrOptions, [](const CodeGenFunction::MultiVersionResolverOption &LHS,
+                      const CodeGenFunction::MultiVersionResolverOption &RHS) {
+        return getPriorityFromAttrString(LHS.Conditions.Features[0]) >
+               getPriorityFromAttrString(RHS.Conditions.Features[0]);
+      });
+
+  // Check the each candidate function.
+  for (unsigned Index = 0; Index < CurrOptions.size(); Index++) {
+
+    if (CurrOptions[Index].Conditions.Features[0].starts_with("default")) {
+      HasDefault = true;
+      DefaultIndex = Index;
+      continue;
+    }
+
+    Builder.SetInsertPoint(CurBlock);
+
+    std::vector<std::string> TargetAttrFeats =
+        getContext()
+            .getTargetInfo()
+            .parseTargetAttr(CurrOptions[Index].Conditions.Features[0])
+            .Features;
+
+    if (TargetAttrFeats.empty())
+      continue;
+
+    // FeaturesCondition: The bitmask of the required extension has been
+    // enabled by the runtime object.
+    // (__riscv_feature_bits.features[i] & REQUIRED_BITMASK) ==
+    // REQUIRED_BITMASK
+    //
+    // When condition is met, return this version of the function.
+    // Otherwise, try the next version.
+    //
+    // if (FeaturesConditionVersion1)
+    //     return Version1;
+    // else if (FeaturesConditionVersion2)
+    //     return Version2;
+    // else if (FeaturesConditionVersion3)
+    //     return Version3;
+    // ...
+    // else
+    //     return DefaultVersion;
+
+    // TODO: Add a condition to check the length before accessing elements.
+    // Without checking the length first, we may access an incorrect memory
+    // address when using different versions.
+    llvm::SmallVector<StringRef, 8> CurrTargetAttrFeats;
+
+    for (auto &Feat : TargetAttrFeats) {
+      StringRef CurrFeat = Feat;
+      if (!CurrFeat.starts_with("+"))
+        continue;
+      CurrTargetAttrFeats.push_back(CurrFeat.substr(1));
----------------
kito-cheng wrote:

```suggestion
      if (CurrFeat.consume_front("+"))
        CurrTargetAttrFeats.push_back(CurrFeat);
```
and maybe assert in else??

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