[PGO] Do not do counter promotion for loops without exit blocks ('infinite loops')

[davidxl]

Summary

For those loops, promotion counter can lead to loss of profile as there is no place to sync the counter value back to counter memory.

Test case is included in another patch.

[vsk]

With an IR-based test case, D40663 shouldn't be necessary --

; RUN opt -instrprof -S -o - <%s -do-counter-promotion=1 | FileCheck
; CHECK: store

target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.12.0"

@__profn_foo = private constant [3 x i8] c"foo"

define void @foo() {
entry:
  br label %while.body

while.body:                                       ; preds = %entry, %while.body
  call void @llvm.instrprof.increment(i8* getelementptr inbounds ([3 x i8], [3 x i8]* @__profn_foo, i32 0, i32 0), i64 14813359968, i32 1, i32 0)
  call void (...) @bar() #2
  br label %while.body
}

declare void @bar(...)

declare void @llvm.instrprof.increment(i8*, i64, i32, i32) #0

attributes #0 = { nounwind }

!llvm.module.flags = !{!0, !1}
!llvm.ident = !{!2}

!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{i32 7, !"PIC Level", i32 2}
!2 = !{!"clang version 6.0.0 (trunk 319373) (llvm/trunk 319391)"}

lib/Transforms/Instrumentation/InstrProfiling.cpp

       if (BlockSet.insert(ExitBlock).second) {
         ExitBlocks.push_back(ExitBlock);
         InsertPts.push_back(&*ExitBlock->getFirstInsertionPt());
       }
     }
   }
 
   bool run(int64_t *NumPromoted) {
+    // Skip 'infinite' loops:
+    if (ExitBlocks.size() == 0)
+      return false;
     unsigned MaxProm = getMaxNumOfPromotionsInLoop(&L);
     if (MaxProm == 0)
       return false;
 
     unsigned Promoted = 0;
     for (auto &Cand : LoopToCandidates[&L]) {
 
       SmallVector<PHINode *, 4> NewPHIs;