[FuncSpec] Enable it only with -O3

[SjoerdMeijer]

Summary

Looks like function specialisation was running at all optimisation levels (if enabled on the command line, it is not on by default). I think that was an oversight and not something we want to do. Function specialisation duplicates functions when it triggers, so the backend is processing more functions/instructions resulting in compile-time increases, which seems more appropriate with -O3 and inline with GCC.

Please note that since function specialisation is not enabled by default, this didn't require updating any pass manager tests.

[ChuanqiXu]

llvm/lib/Passes/PassBuilderPipelines.cpp

1409

Do we need to consider SizeLevel here?

[SjoerdMeijer]

Ah, I never pressed the submit button... :-(
See my comment inline.

llvm/lib/Passes/PassBuilderPipelines.cpp

1409

I think this is fine as it is, because if the Speed level is 3, we know we are not optimising for size:

https://github.com/llvm/llvm-project/blob/a3c05982ac05637a8f40207db83c04a1710033d5/llvm/include/llvm/Passes/OptimizationLevel.h#L32

[aeubanks]

llvm/lib/Passes/PassBuilderPipelines.cpp

1409

isn't this the same as && Level == OptimizationLevel::O3? any reason why the two checks are slightly different? I think == OptimizationLevel::O3 is a bit more consistent with the other code nearby.

[ChuanqiXu]

LGTM with @aeubanks's the comment addressed.

[SjoerdMeijer]

llvm/lib/Passes/PassBuilderPipelines.cpp

1409

Oh yes, agreed. This was not very intentional, a copy-paste from another place. I will change/fix this before committing. Thanks for spotting this and reviewing.

llvm/lib/Passes/PassBuilderPipelines.cpp

PassBuilder::buildModuleSimplificationPipeline(OptimizationLevel Level,
   // tests in ICP sequences.
   if (Phase == ThinOrFullLTOPhase::ThinLTOPostLink)
     MPM.addPass(LowerTypeTestsPass(nullptr, nullptr, true));
 
   for (auto &C : PipelineEarlySimplificationEPCallbacks)
     C(MPM, Level);
 
   // Specialize functions with IPSCCP.
-  if (EnableFunctionSpecialization)
+  if (EnableFunctionSpecialization && Level == OptimizationLevel::O3)
     MPM.addPass(FunctionSpecializationPass());
 
   // Interprocedural constant propagation now that basic cleanup has occurred
   // and prior to optimizing globals.
   // FIXME: This position in the pipeline hasn't been carefully considered in
   // years, it should be re-analyzed.
   MPM.addPass(IPSCCPPass());
 
if (Level.getSpeedupLevel() > 1) {
 
     // Indirect call promotion. This should promote all the targets that are
     // left by the earlier promotion pass that promotes intra-module targets.
     // This two-step promotion is to save the compile time. For LTO, it should
     // produce the same result as if we only do promotion here.
     MPM.addPass(PGOIndirectCallPromotion(
         true /* InLTO */, PGOOpt && PGOOpt->Action == PGOOptions::SampleUse));
 
-    if (EnableFunctionSpecialization)
+    if (EnableFunctionSpecialization && Level == OptimizationLevel::O3)
       MPM.addPass(FunctionSpecializationPass());
     // Propagate constants at call sites into the functions they call.  This
     // opens opportunities for globalopt (and inlining) by substituting function
     // pointers passed as arguments to direct uses of functions.
     MPM.addPass(IPSCCPPass());
 
     // Attach metadata to indirect call sites indicating the set of functions
     // they may target at run-time. This should follow IPSCCP.

llvm/lib/Transforms/IPO/PassManagerBuilder.cpp

if (OptLevel > 1) {
     // Indirect call promotion. This should promote all the targets that are
     // left by the earlier promotion pass that promotes intra-module targets.
     // This two-step promotion is to save the compile time. For LTO, it should
     // produce the same result as if we only do promotion here.
     PM.add(
         createPGOIndirectCallPromotionLegacyPass(true, !PGOSampleUse.empty()));
 
     // Propage constant function arguments by specializing the functions.
-    if (EnableFunctionSpecialization)
+    if (EnableFunctionSpecialization && OptLevel > 2)
       PM.add(createFunctionSpecializationPass());
 
     // Propagate constants at call sites into the functions they call.  This
     // opens opportunities for globalopt (and inlining) by substituting function
     // pointers passed as arguments to direct uses of functions.
     PM.add(createIPSCCPPass());
 
     // Attach metadata to indirect call sites indicating the set of functions