[FuncSpec] Consider small noinline functions for specialisation

[chill]

Summary

Small functions with size under a given threshold are not
considered for specialisaion on the presumption that they
are easy to inline. This does not apply to noinline
functions, though.

[ChuanqiXu]

LGTM. Thanks!

llvm/test/Transforms/FunctionSpecialization/func-spec-noinline.ll

36

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llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

private:
   InstructionCost getSpecializationCost(Function *F) {
     CodeMetrics &Metrics = analyzeFunction(F);
     // If the code metrics reveal that we shouldn't duplicate the function, we
     // shouldn't specialize it. Set the specialization cost to Invalid.
     // Or if the lines of codes implies that this function is easy to get
     // inlined so that we shouldn't specialize it.
     if (Metrics.notDuplicatable || !Metrics.NumInsts.isValid() ||
         (!ForceFunctionSpecialization &&
-         Metrics.NumInsts < SmallFunctionThreshold)) {
-      InstructionCost C{};
-      C.setInvalid();
-      return C;
-    }
+         !F->hasFnAttribute(Attribute::NoInline) &&
+         Metrics.NumInsts < SmallFunctionThreshold))
+      return InstructionCost::getInvalid();
 
     // Otherwise, set the specialization cost to be the cost of all the
     // instructions in the function and penalty for specializing more functions.
     unsigned Penalty = NbFunctionsSpecialized + 1;
     return Metrics.NumInsts * InlineConstants::getInstrCost() * Penalty;
   }
 
   InstructionCost getUserBonus(User *U, llvm::TargetTransformInfo &TTI,

llvm/test/Transforms/FunctionSpecialization/func-spec-noinline.ll

+; RUN: opt -S --passes=function-specialization < %s | FileCheck %s
+define dso_local i32 @p0(i32 noundef %x) {
+entry:
+  %add = add nsw i32 %x, 1
+  ret i32 %add
+}
+
+define dso_local i32 @p1(i32 noundef %x) {
+entry:
+  %sub = add nsw i32 %x, -1
+  ret i32 %sub
+}
+
+define internal fastcc i32 @f(i32 noundef %x, ptr nocapture noundef readonly %p) noinline {
+entry:
+  %call = tail call i32 %p(i32 noundef %x)
+  %add = add nsw i32 %call, %x
+  ret i32 %add
+}
+
+define dso_local i32 @g0(i32 noundef %x) {
+entry:
+  %call = tail call fastcc i32 @f(i32 noundef %x, ptr noundef nonnull @p0)
+  ret i32 %call
+}
+
+define dso_local i32 @g1(i32 noundef %x) {
+entry:
+  %call = tail call fastcc i32 @f(i32 noundef %x, ptr noundef nonnull @p1)
+  ret i32 %call
+}
+
+; Check that a noinline function is specialized, even if it's small.
+; CHECK: @f.1
+; CHECK: @f.2
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