[PATCH] D84378: [PGO] Fix incorrect function entry count

[Rong Xu via Phabricator via llvm-commits]

xur created this revision.
xur added a reviewer: davidxl.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.

Function entry count might be zero after the profile counts
reset and before reentry to the function.

Zero profile entry count is very bad as the profile count from BFI
will be wrong.

A simple fix is to set the profile entry count to 1 if there are
non-zero profile counts in this function.


https://reviews.llvm.org/D84378

Files:
  llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
  llvm/test/Transforms/PGOProfile/Inputs/fix_entry_count.proftext
  llvm/test/Transforms/PGOProfile/fix_entry_count.ll


Index: llvm/test/Transforms/PGOProfile/fix_entry_count.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/PGOProfile/fix_entry_count.ll
@@ -0,0 +1,34 @@
+; RUN: llvm-profdata merge %S/Inputs/fix_entry_count.proftext -o %t.profdata
+; RUN: opt < %s -pgo-instr-use -pgo-instrument-entry=true -pgo-test-profile-file=%t.profdata -S | FileCheck %s --check-prefix=USE
+; RUN: opt < %s -passes=pgo-instr-use -pgo-instrument-entry=true -pgo-test-profile-file=%t.profdata -S | FileCheck %s --check-prefix=USE
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @test_simple_for(i32 %n) {
+; USE: define i32 @test_simple_for(i32 %n)
+; USE-SAME: !prof ![[ENTRY_COUNT:[0-9]*]]
+entry:
+  br label %for.cond
+
+for.cond:
+  %i = phi i32 [ 0, %entry ], [ %inc1, %for.inc ]
+  %sum = phi i32 [ 1, %entry ], [ %inc, %for.inc ]
+  %cmp = icmp slt i32 %i, %n
+  br i1 %cmp, label %for.body, label %for.end
+; USE: br i1 %cmp, label %for.body, label %for.end
+; USE-SAME: !prof ![[BW_FOR_COND:[0-9]+]]
+
+for.body:
+  %inc = add nsw i32 %sum, 1
+  br label %for.inc
+
+for.inc:
+  %inc1 = add nsw i32 %i, 1
+  br label %for.cond
+
+for.end:
+  ret i32 %sum
+}
+; USE: ![[ENTRY_COUNT]] = !{!"function_entry_count", i64 1}
+; USE: ![[BW_FOR_COND]] = !{!"branch_weights", i32 96, i32 1}
Index: llvm/test/Transforms/PGOProfile/Inputs/fix_entry_count.proftext
===================================================================
--- /dev/null
+++ llvm/test/Transforms/PGOProfile/Inputs/fix_entry_count.proftext
@@ -0,0 +1,8 @@
+:ir
+:entry_first
+test_simple_for
+34137660316
+2
+0
+96
+
Index: llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
===================================================================
--- llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
+++ llvm/lib/Transforms/Instrumentation/PGOInstrumentation.cpp
@@ -1131,11 +1131,18 @@
   if (NumCounters != CountFromProfile.size()) {
     return false;
   }
+  auto *FuncEntry = &*F.begin();
+
   // Set the profile count to the Instrumented BBs.
   uint32_t I = 0;
   for (BasicBlock *InstrBB : InstrumentBBs) {
     uint64_t CountValue = CountFromProfile[I++];
     UseBBInfo &Info = getBBInfo(InstrBB);
+    // If we reach here, we know that we have some nonzero count
+    // values in this function. The entry count should not be 0.
+    // Fix it if necessary.
+    if (InstrBB == FuncEntry && CountValue == 0)
+      CountValue = 1;
     Info.setBBInfoCount(CountValue);
   }
   ProfileCountSize = CountFromProfile.size();
@@ -1326,7 +1333,6 @@
   }
 #endif
   uint64_t FuncEntryCount = getBBInfo(&*F.begin()).CountValue;
-  F.setEntryCount(ProfileCount(FuncEntryCount, Function::PCT_Real));
   uint64_t FuncMaxCount = FuncEntryCount;
   for (auto &BB : F) {
     auto BI = findBBInfo(&BB);
@@ -1334,6 +1340,11 @@
       continue;
     FuncMaxCount = std::max(FuncMaxCount, BI->CountValue);
   }
+
+  // Fix the obviously inconsistent entry count.
+  if (FuncMaxCount > 0 && FuncEntryCount == 0)
+    FuncEntryCount = 1;
+  F.setEntryCount(ProfileCount(FuncEntryCount, Function::PCT_Real));
   markFunctionAttributes(FuncEntryCount, FuncMaxCount);
 
   // Now annotate select instructions


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