[SamplePGO] Add switch to honor zero count on block level as accurate

[wenlei]

Summary

Add a new LLVM switch -profile-sample-block-accurate to trust zero block counts for branches. Currently we leave out such zero counts when annotating branch weight metadata, which would lead to weights being considered as unknown.

[wenlei]

This turns out to improve HHVM (https://github.com/facebook/hhvm) RPS by 0.1%, while reducing .text size by 4%, using AutoFDO. I hope to get it in first so we can do more evaluation. Eventually either add a new driver switch just like fprofile-sample-accurate, or merge it into fprofile-sample-accurate if it turns out to be helpful in general.

[hoy]

lgtm.

llvm/lib/Transforms/IPO/SampleProfile.cpp

static cl::opt<std::string> SampleProfileRemappingFile(
     cl::desc("Profile remapping file loaded by -sample-profile"), cl::Hidden);
 
 static cl::opt<bool> ProfileSampleAccurate(
     "profile-sample-accurate", cl::Hidden, cl::init(false),
     cl::desc("If the sample profile is accurate, we will mark all un-sampled "
              "callsite and function as having 0 samples. Otherwise, treat "
              "un-sampled callsites and functions conservatively as unknown. "));
 
+static cl::opt<bool> ProfileSampleBlockAccurate(
+    "profile-sample-block-accurate", cl::Hidden, cl::init(false),
+    cl::desc("If the sample profile is accurate, we will mark all un-sampled "
+             "branches and calls as having 0 samples. Otherwise, treat "
+             "them conservatively as unknown. "));
+
 static cl::opt<bool> ProfileAccurateForSymsInList(
     "profile-accurate-for-symsinlist", cl::Hidden, cl::ZeroOrMore,
     cl::init(true),
     cl::desc("For symbols in profile symbol list, regard their profiles to "
              "be accurate. It may be overriden by profile-sample-accurate. "));
 
 static cl::opt<bool> ProfileMergeInlinee(
     "sample-profile-merge-inlinee", cl::Hidden, cl::init(true),
if (BlockWeights[BB]) {
           else if (OverwriteExistingWeights)
             I.setMetadata(LLVMContext::MD_prof, nullptr);
         } else if (!isa<IntrinsicInst>(&I)) {
           I.setMetadata(LLVMContext::MD_prof,
                         MDB.createBranchWeights(
                             {static_cast<uint32_t>(BlockWeights[BB])}));
         }
       }
-    } else if (OverwriteExistingWeights) {
+    } else if (OverwriteExistingWeights || ProfileSampleBlockAccurate) {
       // Set profile metadata (possibly annotated by LTO prelink) to zero or
       // clear it for cold code.
       for (auto &I : BB->getInstList()) {
         if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
           if (cast<CallBase>(I).isIndirectCall())
             I.setMetadata(LLVMContext::MD_prof, nullptr);
           else
             I.setMetadata(LLVMContext::MD_prof, MDB.createBranchWeights(0));