r203592 - PGO: Scale large counters down to 32-bits
Duncan P. N. Exon Smith
dexonsmith at apple.com
Tue Mar 11 11:18:10 PDT 2014
Author: dexonsmith
Date: Tue Mar 11 13:18:10 2014
New Revision: 203592
URL: http://llvm.org/viewvc/llvm-project?rev=203592&view=rev
Log:
PGO: Scale large counters down to 32-bits
PGO counters are 64-bit and branch weights are 32-bit. Scale them down
when necessary, instead of just taking the lower 32 bits.
<rdar://problem/16276448>
Added:
cfe/trunk/test/Profile/Inputs/c-counter-overflows.profdata
cfe/trunk/test/Profile/c-counter-overflows.c
Modified:
cfe/trunk/lib/CodeGen/CodeGenPGO.cpp
Modified: cfe/trunk/lib/CodeGen/CodeGenPGO.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CodeGenPGO.cpp?rev=203592&r1=203591&r2=203592&view=diff
==============================================================================
--- cfe/trunk/lib/CodeGen/CodeGenPGO.cpp (original)
+++ cfe/trunk/lib/CodeGen/CodeGenPGO.cpp Tue Mar 11 13:18:10 2014
@@ -872,29 +872,59 @@ void CodeGenPGO::destroyRegionCounters()
delete RegionCounts;
}
+/// \brief Calculate what to divide by to scale weights.
+///
+/// Given the maximum weight, calculate a divisor that will scale all the
+/// weights to strictly less than UINT32_MAX.
+static uint64_t calculateWeightScale(uint64_t MaxWeight) {
+ return MaxWeight < UINT32_MAX ? 1 : MaxWeight / UINT32_MAX + 1;
+}
+
+/// \brief Scale an individual branch weight (and add 1).
+///
+/// Scale a 64-bit weight down to 32-bits using \c Scale.
+///
+/// According to Laplace's Rule of Succession, it is better to compute the
+/// weight based on the count plus 1, so universally add 1 to the value.
+///
+/// \pre \c Scale was calculated by \a calculateWeightScale() with a weight no
+/// greater than \c Weight.
+static uint32_t scaleBranchWeight(uint64_t Weight, uint64_t Scale) {
+ assert(Scale && "scale by 0?");
+ uint64_t Scaled = Weight / Scale + 1;
+ assert(Scaled <= UINT32_MAX && "overflow 32-bits");
+ return Scaled;
+}
+
llvm::MDNode *CodeGenPGO::createBranchWeights(uint64_t TrueCount,
uint64_t FalseCount) {
+ // Check for empty weights.
if (!TrueCount && !FalseCount)
return 0;
+ // Calculate how to scale down to 32-bits.
+ uint64_t Scale = calculateWeightScale(std::max(TrueCount, FalseCount));
+
llvm::MDBuilder MDHelper(CGM.getLLVMContext());
- // TODO: need to scale down to 32-bits
- // According to Laplace's Rule of Succession, it is better to compute the
- // weight based on the count plus 1.
- return MDHelper.createBranchWeights(TrueCount + 1, FalseCount + 1);
+ return MDHelper.createBranchWeights(scaleBranchWeight(TrueCount, Scale),
+ scaleBranchWeight(FalseCount, Scale));
}
llvm::MDNode *CodeGenPGO::createBranchWeights(ArrayRef<uint64_t> Weights) {
- llvm::MDBuilder MDHelper(CGM.getLLVMContext());
- // TODO: need to scale down to 32-bits, instead of just truncating.
- // According to Laplace's Rule of Succession, it is better to compute the
- // weight based on the count plus 1.
+ // We need at least two elements to create meaningful weights.
+ if (Weights.size() < 2)
+ return 0;
+
+ // Calculate how to scale down to 32-bits.
+ uint64_t Scale = calculateWeightScale(*std::max_element(Weights.begin(),
+ Weights.end()));
+
SmallVector<uint32_t, 16> ScaledWeights;
ScaledWeights.reserve(Weights.size());
- for (ArrayRef<uint64_t>::iterator WI = Weights.begin(), WE = Weights.end();
- WI != WE; ++WI) {
- ScaledWeights.push_back(*WI + 1);
- }
+ for (uint64_t W : Weights)
+ ScaledWeights.push_back(scaleBranchWeight(W, Scale));
+
+ llvm::MDBuilder MDHelper(CGM.getLLVMContext());
return MDHelper.createBranchWeights(ScaledWeights);
}
Added: cfe/trunk/test/Profile/Inputs/c-counter-overflows.profdata
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Profile/Inputs/c-counter-overflows.profdata?rev=203592&view=auto
==============================================================================
--- cfe/trunk/test/Profile/Inputs/c-counter-overflows.profdata (added)
+++ cfe/trunk/test/Profile/Inputs/c-counter-overflows.profdata Tue Mar 11 13:18:10 2014
@@ -0,0 +1,10 @@
+main 8
+1
+68719476720
+64424509425
+68719476720
+21474836475
+21474836475
+21474836475
+4294967295
+
Added: cfe/trunk/test/Profile/c-counter-overflows.c
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Profile/c-counter-overflows.c?rev=203592&view=auto
==============================================================================
--- cfe/trunk/test/Profile/c-counter-overflows.c (added)
+++ cfe/trunk/test/Profile/c-counter-overflows.c Tue Mar 11 13:18:10 2014
@@ -0,0 +1,49 @@
+// Test that big branch weights get scaled down to 32-bits, rather than just
+// truncated.
+
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.9 -main-file-name c-counter-overflows.c %s -o - -emit-llvm -fprofile-instr-use=%S/Inputs/c-counter-overflows.profdata | FileCheck %s
+
+#include <stdint.h>
+
+// PGOGEN: @[[MAIN:__llvm_pgo_ctr[0-9]*]] = private global [2 x i64] zeroinitializer
+int main(int argc, const char *argv[]) {
+ // Need counts higher than 32-bits.
+ // CHECK: br {{.*}} !prof ![[FOR:[0-9]+]]
+ // max = 0xffffffff0
+ // scale = 0xffffffff0 / 0xffffffff + 1 = 17
+ // loop-body: 0xffffffff0 / 17 + 1 = 0xf0f0f0f0 + 1 = 4042322161 => -252645135
+ // loop-exit: 0x000000001 / 17 + 1 = 0x00000000 + 1 = 1 => 1
+ for (uint64_t I = 0; I < 0xffffffff0; ++I) {
+ // max = 0xffffffff * 15 = 0xefffffff1
+ // scale = 0xefffffff1 / 0xffffffff + 1 = 16
+ // CHECK: br {{.*}} !prof ![[IF:[0-9]+]]
+ if (I & 0xf) {
+ // 0xefffffff1 / 16 + 1 = 0xefffffff + 1 = 4026531840 => -268435456
+ } else {
+ // 0x0ffffffff / 16 + 1 = 0x0fffffff + 1 = 268435456 => 268435456
+ }
+
+ // max = 0xffffffff * 5 = 0x4fffffffb
+ // scale = 0x4fffffffb / 0xffffffff + 1 = 6
+ // CHECK: ], !prof ![[SWITCH:[0-9]+]]
+ switch ((I & 0xf) / 5) {
+ case 0:
+ // 0x4fffffffb / 6 = 0xd5555554 + 1 = 3579139413 => -715827883
+ break;
+ case 1:
+ // 0x4fffffffb / 6 = 0xd5555554 + 1 = 3579139413 => -715827883
+ break;
+ case 2:
+ // 0x4fffffffb / 6 = 0xd5555554 + 1 = 3579139413 => -715827883
+ break;
+ default:
+ // 0x0ffffffff / 6 = 0x2aaaaaaa + 1 = 715827883 => 715827883
+ break;
+ }
+ }
+ return 0;
+}
+
+// CHECK-DAG: ![[FOR]] = metadata !{metadata !"branch_weights", i32 -252645135, i32 1}
+// CHECK-DAG: ![[IF]] = metadata !{metadata !"branch_weights", i32 -268435456, i32 268435456}
+// CHECK-DAG: ![[SWITCH]] = metadata !{metadata !"branch_weights", i32 715827883, i32 -715827883, i32 -715827883, i32 -715827883}
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