[RISCV][FMV] Support target_clones (llvm#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
riscv-non-isa/riscv-c-api-doc#48 (which has
landed), as modified by the proposed
riscv-non-isa/riscv-c-api-doc#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;
        }
    }
}
```

clang/include/clang/Basic/DiagnosticFrontendKinds.td

@@ -378,4 +378,8 @@ def warn_missing_symbol_graph_dir : Warning<
 def err_ast_action_on_llvm_ir : Error<
   "cannot apply AST actions to LLVM IR file '%0'">,
   DefaultFatal;
+
+def err_os_unsupport_riscv_fmv : Error<
+  "function multiversioning is currently only supported on Linux">;
+
 }

clang/include/clang/Basic/TargetInfo.h

@@ -1496,7 +1496,8 @@ class TargetInfo : public TransferrableTargetInfo,
   /// Identify whether this target supports multiversioning of functions,
   /// which requires support for cpu_supports and cpu_is functionality.
   bool supportsMultiVersioning() const {
-    return getTriple().isX86() || getTriple().isAArch64();
+    return getTriple().isX86() || getTriple().isAArch64() ||
+           getTriple().isRISCV();
   }
 
   /// Identify whether this target supports IFuncs.

clang/include/clang/Sema/SemaRISCV.h

@@ -43,6 +43,7 @@ class SemaRISCV : public SemaBase {
 
   void handleInterruptAttr(Decl *D, const ParsedAttr &AL);
   bool isAliasValid(unsigned BuiltinID, llvm::StringRef AliasName);
+  bool isValidFMVExtension(StringRef Ext);
 
   /// Indicate RISC-V vector builtin functions enabled or not.
   bool DeclareRVVBuiltins = false;

clang/lib/AST/ASTContext.cpp

@@ -14248,6 +14248,18 @@ void ASTContext::getFunctionFeatureMap(llvm::StringMap<bool> &FeatureMap,
                       Target->getTargetOpts().FeaturesAsWritten.begin(),
                       Target->getTargetOpts().FeaturesAsWritten.end());
       Target->initFeatureMap(FeatureMap, getDiagnostics(), TargetCPU, Features);
+    } else if (Target->getTriple().isRISCV()) {
+      StringRef VersionStr = TC->getFeatureStr(GD.getMultiVersionIndex());
+      std::vector<std::string> Features;
+      if (VersionStr != "default") {
+        ParsedTargetAttr ParsedAttr = Target->parseTargetAttr(VersionStr);
+        Features.insert(Features.begin(), ParsedAttr.Features.begin(),
+                        ParsedAttr.Features.end());
+      }
+      Features.insert(Features.begin(),
+                      Target->getTargetOpts().FeaturesAsWritten.begin(),
+                      Target->getTargetOpts().FeaturesAsWritten.end());
+      Target->initFeatureMap(FeatureMap, getDiagnostics(), TargetCPU, Features);
     } else {
       std::vector<std::string> Features;
       StringRef VersionStr = TC->getFeatureStr(GD.getMultiVersionIndex());

clang/lib/Basic/Targets/RISCV.cpp

@@ -459,6 +459,8 @@ ParsedTargetAttr RISCVTargetInfo::parseTargetAttr(StringRef Features) const {
         Ret.Duplicate = "tune=";
 
       Ret.Tune = AttrString;
+    } else if (Feature.starts_with("priority")) {
+      // Skip because it only use for FMV.
     }
   }
   return Ret;

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(

clang/lib/CodeGen/CodeGenFunction.h

@@ -5339,6 +5339,9 @@ class CodeGenFunction : public CodeGenTypeCache {
   void
   EmitAArch64MultiVersionResolver(llvm::Function *Resolver,
                                   ArrayRef<MultiVersionResolverOption> Options);
+  void
+  EmitRISCVMultiVersionResolver(llvm::Function *Resolver,
+                                ArrayRef<MultiVersionResolverOption> Options);
 
 private:
   QualType getVarArgType(const Expr *Arg);

clang/lib/CodeGen/CodeGenModule.cpp

@@ -4283,7 +4283,10 @@ void CodeGenModule::emitMultiVersionFunctions() {
               Feats.clear();
               if (getTarget().getTriple().isAArch64())
                 TC->getFeatures(Feats, I);
-              else {
+              else if (getTarget().getTriple().isRISCV()) {
+                StringRef Version = TC->getFeatureStr(I);
+                Feats.push_back(Version);
+              } else {
                 StringRef Version = TC->getFeatureStr(I);
                 if (Version.starts_with("arch="))
                   Architecture = Version.drop_front(sizeof("arch=") - 1);

clang/lib/CodeGen/Targets/RISCV.cpp

@@ -63,9 +63,53 @@ class RISCVABIInfo : public DefaultABIInfo {
                                                CharUnits Field2Off) const;
 
   ABIArgInfo coerceVLSVector(QualType Ty) const;
+
+  using ABIInfo::appendAttributeMangling;
+  void appendAttributeMangling(TargetClonesAttr *Attr, unsigned Index,
+                               raw_ostream &Out) const override;
+  void appendAttributeMangling(StringRef AttrStr,
+                               raw_ostream &Out) const override;
 };
 } // end anonymous namespace
 
+void RISCVABIInfo::appendAttributeMangling(TargetClonesAttr *Attr,
+                                           unsigned Index,
+                                           raw_ostream &Out) const {
+  appendAttributeMangling(Attr->getFeatureStr(Index), Out);
+}
+
+void RISCVABIInfo::appendAttributeMangling(StringRef AttrStr,
+                                           raw_ostream &Out) const {
+  if (AttrStr == "default") {
+    Out << ".default";
+    return;
+  }
+
+  Out << '.';
+
+  SmallVector<StringRef, 8> Attrs;
+  AttrStr.split(Attrs, ';');
+
+  // Only consider the arch string.
+  StringRef ArchStr;
+  for (auto &Attr : Attrs) {
+    if (Attr.starts_with("arch="))
+      ArchStr = Attr;
+  }
+
+  // Extract features string.
+  SmallVector<StringRef, 8> Features;
+  ArchStr.consume_front("arch=");
+  ArchStr.split(Features, ',');
+
+  llvm::stable_sort(Features);
+
+  for (auto Feat : Features) {
+    Feat.consume_front("+");
+    Out << "_" << Feat;
+  }
+}
+
 void RISCVABIInfo::computeInfo(CGFunctionInfo &FI) const {
   QualType RetTy = FI.getReturnType();
   if (!getCXXABI().classifyReturnType(FI))

clang/lib/Sema/SemaDeclAttr.cpp

@@ -3161,6 +3161,55 @@ bool Sema::checkTargetClonesAttrString(
           HasNotDefault = true;
         }
       }
+    } else if (TInfo.getTriple().isRISCV()) {
+      // Suppress warn_target_clone_mixed_values
+      HasCommas = false;
+
+      // Cur is split's parts of Str. RISC-V uses Str directly,
+      // so skip when encountered more than once.
+      if (!Str.starts_with(Cur))
+        continue;
+
+      llvm::SmallVector<StringRef, 8> AttrStrs;
+      Str.split(AttrStrs, ";");
+
+      bool IsPriority = false;
+      bool IsDefault = false;
+      for (auto &AttrStr : AttrStrs) {
+        // Only support arch=+ext,... syntax.
+        if (AttrStr.starts_with("arch=+")) {
+          ParsedTargetAttr TargetAttr =
+              Context.getTargetInfo().parseTargetAttr(AttrStr);
+
+          if (TargetAttr.Features.empty() ||
+              llvm::any_of(TargetAttr.Features, [&](const StringRef Ext) {
+                return !RISCV().isValidFMVExtension(Ext);
+              }))
+            return Diag(CurLoc, diag::warn_unsupported_target_attribute)
+                   << Unsupported << None << Str << TargetClones;
+        } else if (AttrStr.starts_with("default")) {
+          IsDefault = true;
+          DefaultIsDupe = HasDefault;
+          HasDefault = true;
+        } else if (AttrStr.consume_front("priority=")) {
+          IsPriority = true;
+          int Digit;
+          if (AttrStr.getAsInteger(0, Digit))
+            return Diag(CurLoc, diag::warn_unsupported_target_attribute)
+                   << Unsupported << None << Str << TargetClones;
+        } else {
+          return Diag(CurLoc, diag::warn_unsupported_target_attribute)
+                 << Unsupported << None << Str << TargetClones;
+        }
+      }
+
+      if (IsPriority && IsDefault)
+        return Diag(CurLoc, diag::warn_unsupported_target_attribute)
+               << Unsupported << None << Str << TargetClones;
+
+      if (llvm::is_contained(StringsBuffer, Str) || DefaultIsDupe)
+        Diag(CurLoc, diag::warn_target_clone_duplicate_options);
+      StringsBuffer.push_back(Str);
     } else {
       // Other targets ( currently X86 )
       if (Cur.starts_with("arch=")) {

clang/lib/Sema/SemaRISCV.cpp

@@ -25,6 +25,7 @@
 #include "clang/Sema/Sema.h"
 #include "clang/Support/RISCVVIntrinsicUtils.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/TargetParser/RISCVISAInfo.h"
 #include "llvm/TargetParser/RISCVTargetParser.h"
 #include <optional>
 #include <string>
@@ -1492,6 +1493,16 @@ bool SemaRISCV::isAliasValid(unsigned BuiltinID, StringRef AliasName) {
          BuiltinID <= RISCV::LastRVVBuiltin;
 }
 
+bool SemaRISCV::isValidFMVExtension(StringRef Ext) {
+  if (Ext.empty())
+    return false;
+
+  if (!Ext.consume_front("+"))
+    return false;
+
+  return -1 != RISCVISAInfo::getRISCVFeaturesBitsInfo(Ext).second;
+}
+
 SemaRISCV::SemaRISCV(Sema &S) : SemaBase(S) {}
 
 } // namespace clang

clang/test/CodeGen/attr-target-clones-riscv-invalid.c

@@ -0,0 +1,8 @@
+// RUN: not %clang_cc1 -triple riscv64 -target-feature +i -emit-llvm -o - %s 2>&1 | FileCheck %s --check-prefix=CHECK-UNSUPPORT-OS
+
+// CHECK-UNSUPPORT-OS: error: function multiversioning is currently only supported on Linux
+__attribute__((target_clones("default", "arch=+c"))) int foo(void) {
+  return 2;
+}
+
+int bar() { return foo(); }