[PGO] Fix incorrect count threshold calculation when 0% cutoff

DefaultCutoffsData does not have an entry for the 0th percentile.  As a
result, when the getEntryForPercentile method is called with a
percentile argument of 0, it returns a ProfileSummaryEntry for the 1st
percentile instead.  This behavior affects the threshold calculations,
such as getHotCountThreshold, causing them to incorrectly identify some
sample profile counts as hot when they should not be.

This patch addresses the issue by handling the 0th percentile case in
the getEntryForPercentile method.  This ensures that when the
-profile-summary-cutoff-hot (or -cold) option is set to 0, samples are
not incorrectly recognized as hot or cold.

Author: ken-matsui

llvm/lib/ProfileData/ProfileSummaryBuilder.cpp

@@ -77,9 +77,15 @@ static const uint32_t DefaultCutoffsData[] = {
 const ArrayRef<uint32_t> ProfileSummaryBuilder::DefaultCutoffs =
     DefaultCutoffsData;
 
+static const ProfileSummaryEntry ZeroCutoffEntry = {0, UINT64_MAX, 0};
+
 const ProfileSummaryEntry &
 ProfileSummaryBuilder::getEntryForPercentile(const SummaryEntryVector &DS,
                                              uint64_t Percentile) {
+  if (Percentile == 0) {
+    return ZeroCutoffEntry;
+  }
+
   auto It = partition_point(DS, [=](const ProfileSummaryEntry &Entry) {
     return Entry.Cutoff < Percentile;
   });

llvm/test/Analysis/ProfileSummary/basic.ll

@@ -2,12 +2,16 @@
 ; RUN: opt < %s -disable-output -profile-summary-hot-count=500 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=OVERRIDE-HOT
 ; RUN: opt < %s -disable-output -profile-summary-cold-count=0 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=OVERRIDE-COLD
 ; RUN: opt < %s -disable-output -profile-summary-cold-count=200 -profile-summary-hot-count=1000 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=OVERRIDE-BOTH
+; RUN: opt < %s -disable-output -profile-summary-cutoff-hot=0 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=HOT-CUTOFF-0
+; RUN: opt < %s -disable-output -profile-summary-cutoff-cold=0 -profile-summary-hot-count=18446744073709551615 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=COLD-CUTOFF-0
 
 define void @f1() !prof !20 {
 ; CHECK-LABEL: f1 :hot
 ; OVERRIDE-HOT-LABEL: f1
 ; OVERRIDE-COLD-LABEL: f1 :hot
 ; OVERRIDE-BOTH-LABEL: f1
+; HOT-CUTOFF-0-LABEL: f1{{$}}
+; COLD-CUTOFF-0-LABEL: f1 :cold
 
   ret void
 }
@@ -17,6 +21,8 @@ define void @f2() !prof !21 {
 ; OVERRIDE-HOT-LABEL: f2 :cold
 ; OVERRIDE-COLD-LABEL: f2
 ; OVERRIDE-BOTH-LABEL: f2
+; HOT-CUTOFF-0-LABEL: f2 :cold
+; COLD-CUTOFF-0-LABEL: f2 :cold
 
   ret void
 }
@@ -26,6 +32,8 @@ define void @f3() !prof !22 {
 ; OVERRIDE-HOT-LABEL: f3
 ; OVERRIDE-COLD-LABEL: f3
 ; OVERRIDE-BOTH-LABEL: f3
+; HOT-CUTOFF-0-LABEL: f3{{$}}
+; COLD-CUTOFF-0-LABEL: f3 :cold
 
   ret void
 }