[PATCH] D137184: [PGO] Add a threshold for number of critical edges in PGO

[Rong Xu via Phabricator via llvm-commits]

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For some auto-generated sources, we have a huge number of critical edges (like from switch statements).
We have seen instance of 183777 critical edges in one function.
After we split the critical edges in PGO instrumentation/profile-use pass, the CFG is so large that we
have compiler time issues in downstream passes (like in machine CSE and block placement).
Here I add a threshold to skip PGO if the number of critical edges are too large.

The threshold is large enough so that it will not affect the majority of PGO compilation.

Also sync the logic for skipping instrumentation and profile-use. I think
this is the correct thing to do.


https://reviews.llvm.org/D137184

Files:
  llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp


Index: llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
===================================================================
--- llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
+++ llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
@@ -302,12 +302,17 @@
 
 static cl::opt<unsigned> PGOFunctionSizeThreshold(
     "pgo-function-size-threshold", cl::Hidden,
-    cl::desc("Do not instrument functions smaller than this threshold"));
+    cl::desc("Do not instrument functions smaller than this threshold."));
 
 static cl::opt<bool> MatchMemProf(
     "pgo-match-memprof", cl::init(true), cl::Hidden,
     cl::desc("Perform matching and annotation of memprof profiles."));
 
+static cl::opt<unsigned> PGOFunctionCriticalEdgeThreshold(
+    "pgo-critical-edge-threshold", cl::init(20000), cl::Hidden,
+    cl::desc("Do not instrument functions with the number of critical edges "
+             " greater than this threshold."));
+
 namespace llvm {
 // Command line option to turn on CFG dot dump after profile annotation.
 // Defined in Analysis/BlockFrequencyInfo.cpp:  -pgo-view-counts
@@ -1846,6 +1851,38 @@
       ComdatMembers.insert(std::make_pair(C, &GA));
 }
 
+// Don't perform PGO instrumeatnion / profile-use.
+static bool skipPGO(const Function &F) {
+  if (F.isDeclaration())
+    return true;
+  if (F.hasFnAttribute(llvm::Attribute::NoProfile))
+    return true;
+  if (F.hasFnAttribute(llvm::Attribute::SkipProfile))
+    return true;
+  if (F.getInstructionCount() < PGOFunctionSizeThreshold)
+    return true;
+
+  // If there are too many critical edges, PGO might cause
+  // compiler time problem. Skip PGO if the number of
+  // critical edges execeed the threshold.
+  unsigned NumCriticalEdges = 0;
+  for (auto &BB : F) {
+    const Instruction *TI = BB.getTerminator();
+    for (unsigned I = 0, E = TI->getNumSuccessors(); I != E; ++I) {
+      if (isCriticalEdge(TI, I))
+        NumCriticalEdges++;
+    }
+  }
+  if (NumCriticalEdges > PGOFunctionCriticalEdgeThreshold) {
+    LLVM_DEBUG(dbgs() << "In func " << F.getName()
+                      << ", NumCriticalEdges=" << NumCriticalEdges
+                      << " exceed the threshold. Skip PGO.\n");
+    return true;
+  }
+
+  return false;
+}
+
 static bool InstrumentAllFunctions(
     Module &M, function_ref<TargetLibraryInfo &(Function &)> LookupTLI,
     function_ref<BranchProbabilityInfo *(Function &)> LookupBPI,
@@ -1858,13 +1895,7 @@
   collectComdatMembers(M, ComdatMembers);
 
   for (auto &F : M) {
-    if (F.isDeclaration())
-      continue;
-    if (F.hasFnAttribute(llvm::Attribute::NoProfile))
-      continue;
-    if (F.hasFnAttribute(llvm::Attribute::SkipProfile))
-      continue;
-    if (F.getInstructionCount() < PGOFunctionSizeThreshold)
+    if (skipPGO(F))
       continue;
     auto &TLI = LookupTLI(F);
     auto *BPI = LookupBPI(F);
@@ -2092,7 +2123,7 @@
   if (PGOInstrumentEntry.getNumOccurrences() > 0)
     InstrumentFuncEntry = PGOInstrumentEntry;
   for (auto &F : M) {
-    if (F.isDeclaration())
+    if (skipPGO(F))
       continue;
     auto &TLI = LookupTLI(F);
     auto *BPI = LookupBPI(F);


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