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[ThinLTO] Represent relative BF using a scaled representation .

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Summary:
The current integer representation of relative block frequency prevents
representing relative block frequencies below 1. This change uses a 8 of
the 29 bits to represent the decimal part by using a fixed scale of -8.

Reviewers: tejohnson, davidxl

Subscribers: mehdi_amini, inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D43520

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325823 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Easwaran Raman 2018-02-22 19:44:08 +00:00
parent 1e23b92b96
commit 95d34a2e59
3 changed files with 23 additions and 16 deletions
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23
include/llvm/IR/ModuleSummaryIndex.h
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@ -26,6 +26,7 @@
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/ScaledNumber.h"
#include <algorithm>
#include <array>
#include <cassert>
@ -59,7 +60,11 @@ struct CalleeInfo {
// The size of the bit-field might need to be adjusted if more values are
// added to HotnessType enum.
uint32_t Hotness : 3;
/// The value stored in RelBlockFreq has to be interpreted as the digits of
/// a scaled number with a scale of \p -ScaleShift.
uint32_t RelBlockFreq : 29;
static constexpr int32_t ScaleShift = 8;
static constexpr uint64_t MaxRelBlockFreq = (1 << 29) - 1;
CalleeInfo()
@ -73,10 +78,20 @@ struct CalleeInfo {
HotnessType getHotness() const { return HotnessType(Hotness); }
// When there are multiple edges between the same (caller, callee) pair, the
// relative block frequencies are summed up.
void updateRelBlockFreq(uint64_t RBF) {
uint64_t Sum = SaturatingAdd<uint64_t>(RelBlockFreq, RBF);
/// Update \p RelBlockFreq from \p BlockFreq and \p EntryFreq
///
/// BlockFreq is divided by EntryFreq and added to RelBlockFreq. To represent
/// fractional values, the result is represented as a fixed point number with
/// scale of -ScaleShift.
void updateRelBlockFreq(uint64_t BlockFreq, uint64_t EntryFreq) {
if (EntryFreq == 0)
return;
using Scaled64 = ScaledNumber<uint64_t>;
Scaled64 Temp(BlockFreq, ScaleShift);
Temp /= Scaled64::get(EntryFreq);
uint64_t Sum =
SaturatingAdd<uint64_t>(Temp.toInt<uint64_t>(), RelBlockFreq);
Sum = std::min(Sum, uint64_t(MaxRelBlockFreq));
RelBlockFreq = static_cast<uint32_t>(Sum);
}

14
lib/Analysis/ModuleSummaryAnalysis.cpp
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@ -279,17 +279,9 @@ computeFunctionSummary(ModuleSummaryIndex &Index, const Module &M,
// Add the relative block frequency to CalleeInfo if there is no profile
// information.
if (BFI != nullptr && Hotness == CalleeInfo::HotnessType::Unknown) {
auto BBFreq = BFI->getBlockFreq(&BB).getFrequency();
// FIXME: This might need some scaling to prevent BBFreq values from
// being rounded down to 0.
auto EntryFreq = BFI->getEntryFreq();
// Block frequencies can be directly set for a block and so we need to
// handle the case of entry frequency being 0.
if (EntryFreq)
BBFreq /= EntryFreq;
else
BBFreq = 0;
ValueInfo.updateRelBlockFreq(BBFreq);
uint64_t BBFreq = BFI->getBlockFreq(&BB).getFrequency();
uint64_t EntryFreq = BFI->getEntryFreq();
ValueInfo.updateRelBlockFreq(BBFreq, EntryFreq);
}
} else {
// Skip inline assembly calls.

2
test/Bitcode/thinlto-function-summary-callgraph-relbf.ll
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@ -11,7 +11,7 @@
; CHECK: <GLOBALVAL_SUMMARY_BLOCK
; CHECK-NEXT: <VERSION
; See if the call to func is registered.
; CHECK-NEXT: <PERMODULE_RELBF {{.*}} op4=1 {{.*}} op7=1
; CHECK-NEXT: <PERMODULE_RELBF {{.*}} op4=1 {{.*}} op7=256
; CHECK-NEXT: </GLOBALVAL_SUMMARY_BLOCK>
; CHECK: <STRTAB_BLOCK
; CHECK-NEXT: blob data = 'undefinedglobmainfunc{{.*}}'