[FMV][GlobalOpt] Bypass the IFunc Resolver of MultiVersioned functions.

[labrinea]

To deduce whether the optimization is legal we need to compare the target
features between caller and callee versions. The criteria for bypassing
the resolver are the following:

• If the callee's feature set is a subset of the caller's feature set,
  then the callee is a candidate for direct call.

• Among such candidates the one of highest priority is the best match
  and it shall be picked, unless there is a version of the callee with
  higher priority than the best match which cannot be picked from a
  higher priority caller (directly or through the resolver).

• For every higher priority callee version than the best match, there
  is a higher priority caller version whose feature set availability
  is implied by the callee's feature set.

[llvmbot]

@llvm/pr-subscribers-backend-aarch64
@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Alexandros Lamprineas (labrinea)

Changes

To deduce whether the optimization is legal we need to compare the target
features between caller and callee versions. The criteria for bypassing
the resolver are the following:

• If the callee's feature set is a subset of the caller's feature set,
  then the callee is a candidate for direct call.

• Among such candidates the one of highest priority is the best match
  and it shall be picked, unless there is a version of the callee with
  higher priority than the best match which cannot be picked because
  there is no corresponding caller for whom it would have been the best
  match.

Implementation details:

First we collect all the callee versions in feature priority order. We do
the same for all the callsites. Then we try to constant fold the resolver
for every callsite starting from higher priority callers. This guarantees
that as soon as we find a callee whose priority is lower than the expected
best match then there is no point in continuing further.

The constant folding works for single basic block resolvers as well as
for resolvers consisting of multiple basic blocks. The set of instructions
we attempt to fold are a handful give or take (return, binop, compare,
select, branch, phi) and we only follow single user use-def chains. For
callsites residing in the same caller we cache the folded result to avoid
redundant computation.

---

Patch is 24.13 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/87939.diff

8 Files Affected:

• (modified) llvm/include/llvm/Analysis/TargetTransformInfo.h (+23)
• (modified) llvm/include/llvm/Analysis/TargetTransformInfoImpl.h (+6)
• (modified) llvm/lib/Analysis/TargetTransformInfo.cpp (+10)
• (modified) llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp (+12)
• (modified) llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h (+6)
• (modified) llvm/lib/TargetParser/AArch64TargetParser.cpp (+7-2)
• (modified) llvm/lib/Transforms/IPO/GlobalOpt.cpp (+225-1)
• (added) llvm/test/Transforms/GlobalOpt/resolve-fmv-ifunc.ll (+211)

diff --git a/llvm/include/llvm/Analysis/TargetTransformInfo.h b/llvm/include/llvm/Analysis/TargetTransformInfo.h
index fa9392b86c15b9..530935fd63d326 100644
--- a/llvm/include/llvm/Analysis/TargetTransformInfo.h
+++ b/llvm/include/llvm/Analysis/TargetTransformInfo.h
@@ -1762,6 +1762,16 @@ class TargetTransformInfo {
   /// false, but it shouldn't matter what it returns anyway.
   bool hasArmWideBranch(bool Thumb) const;
 
+  /// Returns true if the target supports Function MultiVersioning.
+  bool hasFMV() const;
+
+  /// Returns the MultiVersion priority of a given function.
+  uint64_t getFMVPriority(Function &F) const;
+
+  /// Returns the symbol which contains the cpu feature mask used by
+  /// the Function MultiVersioning resolver.
+  GlobalVariable *getCPUFeatures(Module &M) const;
+
   /// \return The maximum number of function arguments the target supports.
   unsigned getMaxNumArgs() const;
 
@@ -2152,6 +2162,9 @@ class TargetTransformInfo::Concept {
   virtual VPLegalization
   getVPLegalizationStrategy(const VPIntrinsic &PI) const = 0;
   virtual bool hasArmWideBranch(bool Thumb) const = 0;
+  virtual bool hasFMV() const = 0;
+  virtual uint64_t getFMVPriority(Function &F) const = 0;
+  virtual GlobalVariable *getCPUFeatures(Module &M) const = 0;
   virtual unsigned getMaxNumArgs() const = 0;
 };
 
@@ -2904,6 +2917,16 @@ class TargetTransformInfo::Model final : public TargetTransformInfo::Concept {
     return Impl.hasArmWideBranch(Thumb);
   }
 
+  bool hasFMV() const override { return Impl.hasFMV(); }
+
+  uint64_t getFMVPriority(Function &F) const override {
+    return Impl.getFMVPriority(F);
+  }
+
+  GlobalVariable *getCPUFeatures(Module &M) const override {
+    return Impl.getCPUFeatures(M);
+  }
+
   unsigned getMaxNumArgs() const override {
     return Impl.getMaxNumArgs();
   }
diff --git a/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h b/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
index 63c2ef8912b29c..746c09f0d50370 100644
--- a/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
+++ b/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
@@ -941,6 +941,12 @@ class TargetTransformInfoImplBase {
 
   bool hasArmWideBranch(bool) const { return false; }
 
+  bool hasFMV() const { return false; }
+
+  uint64_t getFMVPriority(Function &F) const { return 0; }
+
+  GlobalVariable *getCPUFeatures(Module &M) const { return nullptr; }
+
   unsigned getMaxNumArgs() const { return UINT_MAX; }
 
 protected:
diff --git a/llvm/lib/Analysis/TargetTransformInfo.cpp b/llvm/lib/Analysis/TargetTransformInfo.cpp
index 5f933b4587843c..39da6cc4445759 100644
--- a/llvm/lib/Analysis/TargetTransformInfo.cpp
+++ b/llvm/lib/Analysis/TargetTransformInfo.cpp
@@ -1296,6 +1296,16 @@ bool TargetTransformInfo::hasArmWideBranch(bool Thumb) const {
   return TTIImpl->hasArmWideBranch(Thumb);
 }
 
+bool TargetTransformInfo::hasFMV() const { return TTIImpl->hasFMV(); }
+
+uint64_t TargetTransformInfo::getFMVPriority(Function &F) const {
+  return TTIImpl->getFMVPriority(F);
+}
+
+GlobalVariable *TargetTransformInfo::getCPUFeatures(Module &M) const {
+  return TTIImpl->getCPUFeatures(M);
+}
+
 unsigned TargetTransformInfo::getMaxNumArgs() const {
   return TTIImpl->getMaxNumArgs();
 }
diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
index ee7137b92445bb..a92f859b59a3de 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
@@ -21,6 +21,7 @@
 #include "llvm/IR/IntrinsicsAArch64.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/TargetParser/AArch64TargetParser.h"
 #include "llvm/Transforms/InstCombine/InstCombiner.h"
 #include "llvm/Transforms/Vectorize/LoopVectorizationLegality.h"
 #include <algorithm>
@@ -231,6 +232,17 @@ static bool hasPossibleIncompatibleOps(const Function *F) {
   return false;
 }
 
+uint64_t AArch64TTIImpl::getFMVPriority(Function &F) const {
+  StringRef FeatureStr = F.getFnAttribute("target-features").getValueAsString();
+  SmallVector<StringRef, 8> Features;
+  FeatureStr.split(Features, ",");
+  return AArch64::getCpuSupportsMask(Features);
+}
+
+GlobalVariable *AArch64TTIImpl::getCPUFeatures(Module &M) const {
+  return M.getGlobalVariable("__aarch64_cpu_features");
+}
+
 bool AArch64TTIImpl::areInlineCompatible(const Function *Caller,
                                          const Function *Callee) const {
   SMEAttrs CallerAttrs(*Caller), CalleeAttrs(*Callee);
diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
index de39dea2be43e1..51ad79690679f5 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
@@ -83,6 +83,12 @@ class AArch64TTIImpl : public BasicTTIImplBase<AArch64TTIImpl> {
   unsigned getInlineCallPenalty(const Function *F, const CallBase &Call,
                                 unsigned DefaultCallPenalty) const;
 
+  bool hasFMV() const { return ST->hasFMV(); }
+
+  uint64_t getFMVPriority(Function &F) const;
+
+  GlobalVariable *getCPUFeatures(Module &M) const;
+
   /// \name Scalar TTI Implementations
   /// @{
 
diff --git a/llvm/lib/TargetParser/AArch64TargetParser.cpp b/llvm/lib/TargetParser/AArch64TargetParser.cpp
index 71099462d5ecff..7a3d2fc5f0c9db 100644
--- a/llvm/lib/TargetParser/AArch64TargetParser.cpp
+++ b/llvm/lib/TargetParser/AArch64TargetParser.cpp
@@ -50,8 +50,13 @@ std::optional<AArch64::ArchInfo> AArch64::ArchInfo::findBySubArch(StringRef SubA
 uint64_t AArch64::getCpuSupportsMask(ArrayRef<StringRef> FeatureStrs) {
   uint64_t FeaturesMask = 0;
   for (const StringRef &FeatureStr : FeatureStrs) {
-    if (auto Ext = parseArchExtension(FeatureStr))
-      FeaturesMask |= (1ULL << Ext->CPUFeature);
+    StringRef Feat = resolveExtAlias(FeatureStr);
+    for (const auto &E : Extensions) {
+      if (Feat == E.Name || Feat == E.Feature) {
+        FeaturesMask |= (1ULL << E.CPUFeature);
+        break;
+      }
+    }
   }
   return FeaturesMask;
 }
diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index da714c9a75701b..75ff270bb09b90 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -89,7 +89,7 @@ STATISTIC(NumAliasesRemoved, "Number of global aliases eliminated");
 STATISTIC(NumCXXDtorsRemoved, "Number of global C++ destructors removed");
 STATISTIC(NumInternalFunc, "Number of internal functions");
 STATISTIC(NumColdCC, "Number of functions marked coldcc");
-STATISTIC(NumIFuncsResolved, "Number of statically resolved IFuncs");
+STATISTIC(NumIFuncsResolved, "Number of resolved IFuncs");
 STATISTIC(NumIFuncsDeleted, "Number of IFuncs removed");
 
 static cl::opt<bool>
@@ -2462,6 +2462,227 @@ DeleteDeadIFuncs(Module &M,
   return Changed;
 }
 
+static Function *foldResolverForCallSite(CallBase *CS, uint64_t Priority,
+                                         TargetTransformInfo &TTI) {
+  // Look for the instruction which feeds the feature mask to the users.
+  auto findRoot = [&TTI](Function *F) -> Instruction * {
+    for (Instruction &I : F->getEntryBlock())
+      if (auto *Load = dyn_cast<LoadInst>(&I))
+        if (Load->getPointerOperand() == TTI.getCPUFeatures(*F->getParent()))
+          return Load;
+    return nullptr;
+  };
+
+  auto *IF = cast<GlobalIFunc>(CS->getCalledOperand());
+  Instruction *Root = findRoot(IF->getResolverFunction());
+  // There is no such instruction. Bail.
+  if (!Root)
+    return nullptr;
+
+  // Create a constant mask to use as seed for the constant propagation.
+  Constant *Seed = Constant::getIntegerValue(
+      Root->getType(), APInt(Root->getType()->getIntegerBitWidth(), Priority));
+
+  auto DL = CS->getModule()->getDataLayout();
+
+  // Recursively propagate on single use chains.
+  std::function<Constant *(Instruction *, Instruction *, Constant *,
+                           BasicBlock *)>
+      constFoldInst = [&](Instruction *I, Instruction *Use, Constant *C,
+                          BasicBlock *Pred) -> Constant * {
+    // Base case.
+    if (auto *Ret = dyn_cast<ReturnInst>(I))
+      if (Ret->getReturnValue() == Use)
+        return C;
+
+    // Minimal set of instruction types to handle.
+    if (auto *BinOp = dyn_cast<BinaryOperator>(I)) {
+      bool Swap = BinOp->getOperand(1) == Use;
+      if (auto *Other = dyn_cast<Constant>(BinOp->getOperand(Swap ? 0 : 1)))
+        C = Swap ? ConstantFoldBinaryInstruction(BinOp->getOpcode(), Other, C)
+                 : ConstantFoldBinaryInstruction(BinOp->getOpcode(), C, Other);
+    } else if (auto *Cmp = dyn_cast<CmpInst>(I)) {
+      bool Swap = Cmp->getOperand(1) == Use;
+      if (auto *Other = dyn_cast<Constant>(Cmp->getOperand(Swap ? 0 : 1)))
+        C = Swap ? ConstantFoldCompareInstOperands(Cmp->getPredicate(), Other,
+                                                   C, DL)
+                 : ConstantFoldCompareInstOperands(Cmp->getPredicate(), C,
+                                                   Other, DL);
+    } else if (auto *Sel = dyn_cast<SelectInst>(I)) {
+      if (Sel->getCondition() == Use)
+        C = dyn_cast<Constant>(C->isZeroValue() ? Sel->getFalseValue()
+                                                : Sel->getTrueValue());
+    } else if (auto *Phi = dyn_cast<PHINode>(I)) {
+      if (Pred)
+        C = dyn_cast<Constant>(Phi->getIncomingValueForBlock(Pred));
+    } else if (auto *Br = dyn_cast<BranchInst>(I)) {
+      if (Br->getCondition() == Use) {
+        BasicBlock *BB = Br->getSuccessor(C->isZeroValue());
+        return constFoldInst(&BB->front(), Root, Seed, Br->getParent());
+      }
+    } else {
+      // Don't know how to handle. Bail.
+      return nullptr;
+    }
+
+    // Folding succeeded. Continue.
+    if (C && I->hasOneUse())
+      if (auto *UI = dyn_cast<Instruction>(I->user_back()))
+        return constFoldInst(UI, I, C, nullptr);
+
+    return nullptr;
+  };
+
+  // Collect all users in the entry block ordered by proximity. The rest of
+  // them can be discovered later. Unfortunately we cannot simply traverse
+  // the Root's 'users()' as their order is not the same as execution order.
+  unsigned NUsersLeft = std::distance(Root->user_begin(), Root->user_end());
+  SmallVector<Instruction *> Users;
+  for (Instruction &I : *Root->getParent()) {
+    if (any_of(I.operands(), [Root](auto &Op) { return Op == Root; })) {
+      Users.push_back(&I);
+      if (--NUsersLeft == 0)
+        break;
+    }
+  }
+
+  // Return as soon as we find a foldable user. It has the highest priority.
+  for (Instruction *I : Users) {
+    Constant *C = constFoldInst(I, Root, Seed, nullptr);
+    if (C)
+      return cast<Function>(C);
+  }
+
+  return nullptr;
+}
+
+// Bypass the IFunc Resolver of MultiVersioned functions when possible. To
+// deduce whether the optimization is legal we need to compare the target
+// features between caller and callee versions. The criteria for bypassing
+// the resolver are the following:
+//
+// * If the callee's feature set is a subset of the caller's feature set,
+//   then the callee is a candidate for direct call.
+//
+// * Among such candidates the one of highest priority is the best match
+//   and it shall be picked, unless there is a version of the callee with
+//   higher priority than the best match which cannot be picked because
+//   there is no corresponding caller for whom it would have been the best
+//   match.
+//
+static bool OptimizeNonTrivialIFuncs(
+    Module &M, function_ref<TargetTransformInfo &(Function &)> GetTTI) {
+  bool Changed = false;
+
+  std::function<void(Value *, SmallVectorImpl<Function *> &)> visitValue =
+      [&](Value *V, SmallVectorImpl<Function *> &FuncVersions) {
+        if (auto *Func = dyn_cast<Function>(V)) {
+          FuncVersions.push_back(Func);
+        } else if (auto *Sel = dyn_cast<SelectInst>(V)) {
+          visitValue(Sel->getTrueValue(), FuncVersions);
+          visitValue(Sel->getFalseValue(), FuncVersions);
+        } else if (auto *Phi = dyn_cast<PHINode>(V))
+          for (unsigned I = 0, E = Phi->getNumIncomingValues(); I != E; ++I)
+            visitValue(Phi->getIncomingValue(I), FuncVersions);
+      };
+
+  for (GlobalIFunc &IF : M.ifuncs()) {
+    if (IF.isInterposable())
+      continue;
+
+    Function *Resolver = IF.getResolverFunction();
+    if (!Resolver)
+      continue;
+
+    if (Resolver->isInterposable())
+      continue;
+
+    TargetTransformInfo &TTI = GetTTI(*Resolver);
+    if (!TTI.hasFMV())
+      return false;
+
+    // Discover the callee versions.
+    SmallVector<Function *> Callees;
+    for (BasicBlock &BB : *Resolver)
+      if (auto *Ret = dyn_cast_or_null<ReturnInst>(BB.getTerminator()))
+        visitValue(Ret->getReturnValue(), Callees);
+
+    if (Callees.empty())
+      continue;
+
+    // Cache the feature mask for each callee version.
+    DenseMap<Function *, uint64_t> CalleePriorityMap;
+    for (Function *Callee : Callees) {
+      auto [It, Inserted] = CalleePriorityMap.try_emplace(Callee);
+      if (Inserted)
+        It->second = TTI.getFMVPriority(*Callee);
+    }
+
+    // Sort the callee versions in increasing feature priority order.
+    // Every time we find a caller that matches the highest priority
+    // callee we pop_back() one from this ordered list.
+    llvm::stable_sort(Callees, [&](auto *LHS, auto *RHS) {
+      return CalleePriorityMap[LHS] < CalleePriorityMap[RHS];
+    });
+
+    // Find the callsites and cache the feature mask for each caller.
+    DenseMap<Function *, uint64_t> CallerPriorityMap;
+    SmallVector<CallBase *> CallSites;
+    for (User *U : IF.users()) {
+      if (auto *CB = dyn_cast<CallBase>(U)) {
+        if (CB->getCalledOperand() == &IF) {
+          Function *Caller = CB->getFunction();
+          auto [It, Inserted] = CallerPriorityMap.try_emplace(Caller);
+          if (Inserted)
+            It->second = TTI.getFMVPriority(*Caller);
+          CallSites.push_back(CB);
+        }
+      }
+    }
+
+    // Sort the callsites in decreasing feature priority order.
+    llvm::stable_sort(CallSites, [&](auto *LHS, auto *RHS) {
+      return CallerPriorityMap[LHS->getFunction()] >
+             CallerPriorityMap[RHS->getFunction()];
+    });
+
+    // Now try to constant fold the resolver for every callsite starting
+    // from higher priority callers. This guarantees that as soon as we
+    // find a callee whose priority is lower than the expected best match
+    // then there is no point in continuing further.
+    DenseMap<uint64_t, Function *> foldedResolverCache;
+    for (CallBase *CS : CallSites) {
+      uint64_t CallerPriority = CallerPriorityMap[CS->getFunction()];
+      auto [It, Inserted] = foldedResolverCache.try_emplace(CallerPriority);
+      Function *&Callee = It->second;
+      if (Inserted)
+        Callee = foldResolverForCallSite(CS, CallerPriority, TTI);
+      if (Callee) {
+        if (!Callees.empty()) {
+          // If the priority of the candidate is greater or equal to
+          // the expected best match then it shall be picked. Otherwise
+          // there is a higher priority callee without a corresponding
+          // caller, in which case abort.
+          uint64_t CalleePriority = CalleePriorityMap[Callee];
+          if (CalleePriority == CalleePriorityMap[Callees.back()])
+            Callees.pop_back();
+          else if (CalleePriority < CalleePriorityMap[Callees.back()])
+            break;
+        }
+        CS->setCalledOperand(Callee);
+        Changed = true;
+      } else {
+        // Oops, something went wrong. We couldn't fold. Abort.
+        break;
+      }
+    }
+    if (IF.use_empty() ||
+        all_of(IF.users(), [](User *U) { return isa<GlobalAlias>(U); }))
+      NumIFuncsResolved++;
+  }
+  return Changed;
+}
+
 static bool
 optimizeGlobalsInModule(Module &M, const DataLayout &DL,
                         function_ref<TargetLibraryInfo &(Function &)> GetTLI,
@@ -2525,6 +2746,9 @@ optimizeGlobalsInModule(Module &M, const DataLayout &DL,
     // Optimize IFuncs whose callee's are statically known.
     LocalChange |= OptimizeStaticIFuncs(M);
 
+    // Optimize IFuncs based on the target features of the caller.
+    LocalChange |= OptimizeNonTrivialIFuncs(M, GetTTI);
+
     // Remove any IFuncs that are now dead.
     LocalChange |= DeleteDeadIFuncs(M, NotDiscardableComdats);
 
diff --git a/llvm/test/Transforms/GlobalOpt/resolve-fmv-ifunc.ll b/llvm/test/Transforms/GlobalOpt/resolve-fmv-ifunc.ll
new file mode 100644
index 00000000000000..bcc73c8e44970f
--- /dev/null
+++ b/llvm/test/Transforms/GlobalOpt/resolve-fmv-ifunc.ll
@@ -0,0 +1,211 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --filter "call i32 @(test_single_bb_resolver|test_multi_bb_resolver)" --version 4
+; RUN: opt --passes=globalopt -o - -S < %s | FileCheck %s
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64-unknown-linux-gnu"
+
+$test_single_bb_resolver.resolver = comdat any
+$test_multi_bb_resolver.resolver = comdat any
+$foo.resolver = comdat any
+$bar.resolver = comdat any
+
+@__aarch64_cpu_features = external local_unnamed_addr global { i64 }
+
+@test_single_bb_resolver.ifunc = weak_odr alias i32 (), ptr @test_single_bb_resolver
+@test_multi_bb_resolver.ifunc = weak_odr dso_local alias i32 (), ptr @test_multi_bb_resolver
+@foo.ifunc = weak_odr alias i32 (), ptr @foo
+@bar.ifunc = weak_odr dso_local alias i32 (), ptr @bar
+
+@test_single_bb_resolver = weak_odr ifunc i32 (), ptr @test_single_bb_resolver.resolver
+@test_multi_bb_resolver = weak_odr dso_local ifunc i32 (), ptr @test_multi_bb_resolver.resolver
+@foo = weak_odr ifunc i32 (), ptr @foo.resolver
+@bar = weak_odr dso_local ifunc i32 (), ptr @bar.resolver
+
+declare void @__init_cpu_features_resolver() local_unnamed_addr
+
+declare i32 @test_single_bb_resolver._Msve() #2
+
+declare i32 @test_single_bb_resolver._Msve2() #3
+
+define i32 @test_single_bb_resolver.default() #1 {
+; CHECK-LABEL: define i32 @test_single_bb_resolver.default(
+; CHECK-SAME: ) #[[ATTR2:[0-9]+]] {
+entry:
+  ret i32 0
+}
+
+define weak_odr ptr @test_single_bb_resolver.resolver() #0 comdat {
+; CHECK-LABEL: define weak_odr ptr @test_single_bb_resolver.resolver(
+; CHECK-SAME: ) #[[ATTR3:[0-9]+]] comdat {
+resolver_entry:
+  tail call void @__init_cpu_features_resolver()
+  %0 = load i64, ptr @__aarch64_cpu_features, align 8
+  %1 = and i64 %0, 68719476736
+  %.not = icmp eq i64 %1, 0
+  %2 = and i64 %0, 1073741824
+  %.not3 = icmp eq i64 %2, 0
+  %test_single_bb_resolver._Msve.test_single_bb_resolver.default = select i1 %.not3, ptr @test_single_bb_resolver.default, ptr @test_single_bb_resolver._Msve
+  %common.ret.op = select i1 %.not, ptr %test_single_bb_resolver._Msve.test_single_bb_resolver.default, ptr @test_single_bb_resolver._Msve2
+  ret ptr %common.ret.op
+}
+
+define i32 @foo._Msve() #2 {
+; CHECK-LABEL: define i32 @foo._Msve(
+; CHECK-SAME: ) #[[ATTR0:[0-9]+]] {
+; CHECK:    [[CALL:%.*]] = tail call i32 @test_single_bb_resolver._Msve()
+;
+entry:
+  %call = tail call i32 @test_single_bb_resolver()
+  %add = add nsw i32 %call, 30
+  ret i32 %add
+}
+
+define i32 @foo._Msve2() #3 {
+; CHECK-LABEL: define i32 @foo._Msve2(
+; CHECK-SAME: ) #[[ATTR1:[0-9]+]] {
+; CHECK:    [[CALL1:%.*]] = tail call i32 @test_single_bb_resolver._Msve2()
+; CHECK:    [[CALL2:%.*]] = tail call i32 @test_single_bb_resolver._Msve2()
+;
+entry:
+  %call1 = tail call i32 @test_single_bb_resolver()
+  %call2 = tail call i32 @test_single_bb_resolver()
+  %added = add nsw i32 %call1, %call2
+  %add = add nsw i32 %added, 20
+  ret i32 %add
+}
+
+define i32 @foo.default() #1 {
+; CHECK-LABEL: define i32 @foo.default(
+; CHECK-SAME: ) #[[ATTR2:[0-9]+]] {
+; CHECK:    [[CALL:%.*]] = tail call i32 @test_single_bb_resolver.default()
+;
+entry:
+  %call = tail call i32 @test_single_bb_resolver()
+  %add = add nsw i32 %call, 10
+  ret i32 %add
+}
+
+define weak_odr ptr @foo.resolver() #0 comdat {
+; CHECK-LABEL: define weak_odr ptr @foo.resolver(
+; CHECK-SAME: ) #[[ATTR3:[0-9]+]] comdat {
+resolver_entry:
+  tail call void @__init_cpu_features_resolver()
+  %0 = load i64, ptr @__aarch64_cpu_features, align 8
+  %1 = and i64 %0, 68719476736
+  %.not = icmp eq i64 %1, 0
+  %2 = ...
[truncated]

[andrewcarlotti]

The testcase and description look valid, although the description seems a little more conservative than necessary.

If you were to add a bar._Mmops version, would it's call be optimised to test_multi_bb_resolver._Mmops? And would that also enable the call from bar.default to be optimised to point to test_multi_bb_resolver.default?

[labrinea]

The testcase and description look valid, although the description seems a little more conservative than necessary.

"Subset" is perhaps not the right terminology. I meant to say the feature set is "less or equal to". In other words "implied".

If you were to add a bar._Mmops version, would it's call be optimised to test_multi_bb_resolver._Mmops?

Yes

And would that also enable the call from bar.default to be optimised to point to test_multi_bb_resolver.default?

No, because there is no corresponding caller to pick test_multi_bb_resolver._Msve2.

Ah, now I see your point. Routing bar.default through the resolver could not have picked test_multi_bb_resolver._Msve2, because if the caller doesn't have sve it can't have sve2!

[labrinea]

@jroelofs, aside from semantics which I'll clarify with Andrew, I am thinking that the constant folding part might be redundant and whether FeatureBitset is an alternative to the TargetParser (ignore the second part, bad idea).

[labrinea]

I've removed the constant folding since it was indeed unnecessary computation. Also I found a testcase which exposes a potential issue: a function version (for example AES) having the same target features as the default. In TargetParser we have ExtensionInfo->DependentFeatures to model this information (I believe @ilinpv introduced this change) and it seems that for some entries we don't add the corresponding backend feature (perhaps intentionally?). I will try to address this in a separate ticket.

[labrinea]

I've removed the constant folding since it was indeed unnecessary computation. Also I found a testcase which exposes a potential issue: a function version (for example AES) having the same target features as the default. In TargetParser we have ExtensionInfo->DependentFeatures to model this information (I believe @ilinpv introduced this change) and it seems that for some entries we don't add the corresponding backend feature (perhaps intentionally?). I will try to address this in a separate ticket.

@andrewcarlotti already has a patch for ACLE ARM-software/acle#315 which removes pmull since it is implied by aes, therefore we can add the missing +aes in ExtensionInfo->DependentFeatures for aes.

[labrinea]

Hi, @jroelofs. This patch has been quiet in a while and the reason is I have been collaborating with @tmatheson-arm in refactoring the TargetParser as a preliminary step. However the landscape doesn't seem to be changing much, it's mostly NFC. What I mean is that for this patch to work we need to be able to express every FMV feature in the LLVM IR attribute "target-features". This isn't the case for a few FMV features and I believe it won't be easy to change. For example the FMV feature AES in ExtensionInfo has no DependentFeatures because the SubtargetFeature for AES is fused with PMULL, meaning we can't link them (the fmv feature and the subtarget counterpart) together as they mean different things. I do not want to rely on the cleanup in ARM-software/acle#315 as features may diverge again between FMV and the AAch64 compiler backend. Spliting up backend features will break backwards compatibility in LLVM IR (because they will also have to be renamed), so maybe we need some other way to propagate FMV information from clang to LLVM (not via the target-features attribute). Is this worth an RFC, like introducing a new attribute for FMV in IR? Is this optimization still valuable to you? Shall we pursue this further or just give up?

[jroelofs]

Yes, this optimization is still interesting to me, but I’m not in a rush. We can take our time on it. I have a non-FMV project I need to focus on over the summer, but I expect to pick up work on things in this area in the fall.