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

This patch enable the function multiversion(FMV) and `target_clones`
attribute for RISC-V target.

The proposal of `target_clones` syntax can be found at the
https://github.com/riscv-non-isa/riscv-c-api-doc/pull/48 (which has
landed), as modified by the proposed
https://github.com/riscv-non-isa/riscv-c-api-doc/pull/85 (which adds the
priority syntax).

It supports the `target_clones` function attribute and function
multiversioning feature for RISC-V target. It will generate the ifunc
resolver function for the function that declared with target_clones
attribute.

The resolver function will check the version support by runtime object
`__riscv_feature_bits`.

For example:

```
__attribute__((target_clones("default", "arch=+ver1", "arch=+ver2"))) int bar() {
    return 1;
}
```

the corresponding resolver will be like:

```
bar.resolver() {
    __init_riscv_feature_bits();
    // Check arch=+ver1
    if ((__riscv_feature_bits.features[0] & BITMASK_OF_VERSION1) == BITMASK_OF_VERSION1) {
        return bar.arch=+ver1;
    } else {
        // Check arch=+ver2
        if ((__riscv_feature_bits.features[0] & BITMASK_OF_VERSION2) == BITMASK_OF_VERSION2) {
            return bar.arch=+ver2;
        } else {
            // Default
            return bar.default;
        }
    }
}
```

1 files changed, 133 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index fae26d1..eda96f3 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -2889,10 +2889,142 @@ 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_fmv);
+    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('+'))
+        CurrTargetAttrFeats.push_back(CurrFeat.substr(1));
+    }
+
+    Builder.SetInsertPoint(CurBlock);
+    llvm::Value *FeatsCondition = EmitRISCVCpuSupports(CurrTargetAttrFeats);
+
+    llvm::BasicBlock *RetBlock = createBasicBlock("resolver_return", Resolver);
+    CGBuilderTy RetBuilder(*this, RetBlock);
+    CreateMultiVersionResolverReturn(
+        CGM, Resolver, RetBuilder, CurrOptions[Index].Function, SupportsIFunc);
+    llvm::BasicBlock *ElseBlock = createBasicBlock("resolver_else", Resolver);
+
+    Builder.SetInsertPoint(CurBlock);
+    Builder.CreateCondBr(FeatsCondition, RetBlock, ElseBlock);
+
+    CurBlock = ElseBlock;
+  }
+
+  // Finally, emit the default one.
+  if (HasDefault) {
+    Builder.SetInsertPoint(CurBlock);
+    CreateMultiVersionResolverReturn(CGM, Resolver, Builder,
+                                     CurrOptions[DefaultIndex].Function,
+                                     SupportsIFunc);
+    return;
+  }
+
+  // If no generic/default, emit an unreachable.
+  Builder.SetInsertPoint(CurBlock);
+  llvm::CallInst *TrapCall = EmitTrapCall(llvm::Intrinsic::trap);
+  TrapCall->setDoesNotReturn();
+  TrapCall->setDoesNotThrow();
+  Builder.CreateUnreachable();
+  Builder.ClearInsertionPoint();
 }
 
 void CodeGenFunction::EmitAArch64MultiVersionResolver(