MatzeB created this revision.
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- Add getLikelyBranchWeight() function returning the value of the
`-likely-branch-weight` option defaulting to 2000.
- Remove `-unlikely-branch-weight` option and just use 1.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D158668
Files:
clang/test/CodeGenCXX/attr-likelihood-if-branch-weights.cpp
llvm/docs/MisExpect.rst
llvm/include/llvm/IR/ProfDataUtils.h
llvm/lib/IR/ProfDataUtils.cpp
llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
llvm/test/Transforms/LowerExpectIntrinsic/phi_unexpect.ll
llvm/test/Transforms/PGOProfile/misexpect-branch-overflow.ll
Index: llvm/test/Transforms/PGOProfile/misexpect-branch-overflow.ll
===================================================================
--- llvm/test/Transforms/PGOProfile/misexpect-branch-overflow.ll
+++ llvm/test/Transforms/PGOProfile/misexpect-branch-overflow.ll
@@ -2,7 +2,7 @@
; This can happen when the sum of all counters exceeds the max size of uint32_t
; RUN: llvm-profdata merge %S/Inputs/misexpect-branch-overflow.proftext -o %t.profdata
-; RUN: opt < %s -passes="function(lower-expect),pgo-instr-use" -pgo-test-profile-file=%t.profdata -pgo-warn-misexpect -pass-remarks=misexpect -unlikely-branch-weight=2147483648 -likely-branch-weight=2147483648 -S 2>&1 | FileCheck %s --check-prefix=OVERFLOW
+; RUN: opt < %s -passes=pgo-instr-use -pgo-test-profile-file=%t.profdata -pgo-warn-misexpect -pass-remarks=misexpect -S 2>&1 | FileCheck %s --check-prefix=OVERFLOW
; OVERFLOW-NOT: warning: misexpect-branch.c:22:0: 50.00%
; OVERFLOW-NOT: remark: misexpect-branch.c:22:0: Potential performance regression from use of the llvm.expect intrinsic: Annotation was correct on 50.00% (2147483648 / 4294967296) of profiled executions.
@@ -32,9 +32,8 @@
%lnot1 = xor i1 %lnot, true, !dbg !15
%lnot.ext = zext i1 %lnot1 to i32, !dbg !15
%conv = sext i32 %lnot.ext to i64, !dbg !15
- %expval = call i64 @llvm.expect.i64(i64 %conv, i64 1), !dbg !15
- %tobool = icmp ne i64 %expval, 0, !dbg !15
- br i1 %tobool, label %if.then, label %if.else, !dbg !15
+ %tobool = icmp ne i64 %conv, 0, !dbg !15
+ br i1 %tobool, label %if.then, label %if.else, !dbg !15, !prof !21
if.then: ; preds = %entry
%1 = load i32, ptr %rando, align 4, !dbg !16, !tbaa !10
@@ -100,3 +99,4 @@
!18 = !DILocation(line: 25, scope: !6)
!19 = !DILocation(line: 27, scope: !6)
!20 = !DILocation(line: 28, scope: !6)
+!21 = !{!"branch_weights", i32 2147483648, i32 2147483648}
Index: llvm/test/Transforms/LowerExpectIntrinsic/phi_unexpect.ll
===================================================================
--- llvm/test/Transforms/LowerExpectIntrinsic/phi_unexpect.ll
+++ llvm/test/Transforms/LowerExpectIntrinsic/phi_unexpect.ll
@@ -1,4 +1,4 @@
-; RUN: opt -S -passes='function(lower-expect),strip-dead-prototypes' -likely-branch-weight=2147483647 -unlikely-branch-weight=1 < %s | FileCheck %s
+; RUN: opt -S -passes='function(lower-expect),strip-dead-prototypes' -likely-branch-weight=2147483647 < %s | FileCheck %s
; The C case
; if (__builtin_expect_with_probability(((a0 == 1) || (a1 == 1) || (a2 == 1)), 1, 0))
Index: llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
+++ llvm/lib/Transforms/Scalar/LowerExpectIntrinsic.cpp
@@ -21,6 +21,7 @@
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/MDBuilder.h"
+#include "llvm/IR/ProfDataUtils.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
@@ -36,31 +37,11 @@
STATISTIC(ExpectIntrinsicsHandled,
"Number of 'expect' intrinsic instructions handled");
-// These default values are chosen to represent an extremely skewed outcome for
-// a condition, but they leave some room for interpretation by later passes.
-//
-// If the documentation for __builtin_expect() was made explicit that it should
-// only be used in extreme cases, we could make this ratio higher. As it stands,
-// programmers may be using __builtin_expect() / llvm.expect to annotate that a
-// branch is likely or unlikely to be taken.
-
-// WARNING: these values are internal implementation detail of the pass.
-// They should not be exposed to the outside of the pass, front-end codegen
-// should emit @llvm.expect intrinsics instead of using these weights directly.
-// Transforms should use TargetTransformInfo's getPredictableBranchThreshold().
-static cl::opt<uint32_t> LikelyBranchWeight(
- "likely-branch-weight", cl::Hidden, cl::init(2000),
- cl::desc("Weight of the branch likely to be taken (default = 2000)"));
-static cl::opt<uint32_t> UnlikelyBranchWeight(
- "unlikely-branch-weight", cl::Hidden, cl::init(1),
- cl::desc("Weight of the branch unlikely to be taken (default = 1)"));
-
static std::tuple<uint32_t, uint32_t>
getBranchWeight(Intrinsic::ID IntrinsicID, CallInst *CI, int BranchCount) {
if (IntrinsicID == Intrinsic::expect) {
// __builtin_expect
- return std::make_tuple(LikelyBranchWeight.getValue(),
- UnlikelyBranchWeight.getValue());
+ return std::make_tuple(getLikelyBranchWeight(), 1);
} else {
// __builtin_expect_with_probability
assert(CI->getNumOperands() >= 3 &&
Index: llvm/lib/IR/ProfDataUtils.cpp
===================================================================
--- llvm/lib/IR/ProfDataUtils.cpp
+++ llvm/lib/IR/ProfDataUtils.cpp
@@ -23,6 +23,18 @@
using namespace llvm;
+// These default values are chosen to represent an extremely skewed outcome for
+// a condition, but they leave some room for interpretation by later passes.
+//
+// If the documentation for __builtin_expect() was made explicit that it should
+// only be used in extreme cases, we could make this ratio higher. As it stands,
+// programmers may be using __builtin_expect() / llvm.expect to annotate that a
+// branch is likely or unlikely to be taken.
+
+static cl::opt<uint32_t> LikelyBranchWeight(
+ "likely-branch-weight", cl::Hidden, cl::init(2000),
+ cl::desc("Weight of the branch likely to be taken (default = 2000)"));
+
namespace {
// MD_prof nodes have the following layout
@@ -184,4 +196,6 @@
return extractProfTotalWeight(I.getMetadata(LLVMContext::MD_prof), TotalVal);
}
+uint32_t getLikelyBranchWeight() { return LikelyBranchWeight; }
+
} // namespace llvm
Index: llvm/include/llvm/IR/ProfDataUtils.h
===================================================================
--- llvm/include/llvm/IR/ProfDataUtils.h
+++ llvm/include/llvm/IR/ProfDataUtils.h
@@ -99,5 +99,12 @@
/// metadata was found.
bool extractProfTotalWeight(const Instruction &I, uint64_t &TotalWeights);
+/// Return the weight for a "likely branch". A 1/LikelyBranchWeight ratio
+/// corresponds to the probability of `llvm.expect` matching the expected
+/// value.
+/// Note: There is also `TargetTransformInfo::getPredictableBranchThreshold`
+/// for a value matching the current target.
+uint32_t getLikelyBranchWeight();
+
} // namespace llvm
#endif
Index: llvm/docs/MisExpect.rst
===================================================================
--- llvm/docs/MisExpect.rst
+++ llvm/docs/MisExpect.rst
@@ -34,12 +34,12 @@
We calculate the threshold for emitting MisExpect related diagnostics
based on the values the compiler assigns to ``llvm.expect`` intrinsics,
-which can be set through the ``-likely-branch-weight`` and
-``-unlikely-branch-weight`` LLVM options. During verification, if the
-profile weights mismatch the calculated threshold, then we will emit a
-remark or warning detailing a potential performance regression. The
-diagnostic also reports the percentage of the time the annotation was
-correct during profiling to help developers reason about how to proceed.
+which can be set through the ``-likely-branch-weight`` LLVM option.
+During verification, if the profile weights mismatch the calculated threshold,
+then we will emit a remark or warning detailing a potential performance
+regression. The diagnostic also reports the percentage of the time the
+annotation was correct during profiling to help developers reason about how
+to proceed.
The diagnostics are also available in the form of optimization remarks,
which can be serialized and processed through the ``opt-viewer.py``
Index: clang/test/CodeGenCXX/attr-likelihood-if-branch-weights.cpp
===================================================================
--- clang/test/CodeGenCXX/attr-likelihood-if-branch-weights.cpp
+++ clang/test/CodeGenCXX/attr-likelihood-if-branch-weights.cpp
@@ -1,5 +1,5 @@
// RUN: %clang_cc1 -O1 -emit-llvm %s -o - -triple=x86_64-linux-gnu | FileCheck -DLIKELY=2000 -DUNLIKELY=1 %s
-// RUN: %clang_cc1 -O1 -emit-llvm %s -triple=x86_64-linux-gnu -mllvm -likely-branch-weight=99 -mllvm -unlikely-branch-weight=42 -o - | FileCheck -DLIKELY=99 -DUNLIKELY=42 %s
+// RUN: %clang_cc1 -O1 -emit-llvm %s -triple=x86_64-linux-gnu -mllvm -likely-branch-weight=99 -o - | FileCheck -DLIKELY=99 %s
extern volatile bool b;
extern volatile int i;
@@ -144,5 +144,5 @@
}
}
-// CHECK: ![[PROF_LIKELY]] = !{!"branch_weights", i32 [[UNLIKELY]], i32 [[LIKELY]]}
-// CHECK: ![[PROF_UNLIKELY]] = !{!"branch_weights", i32 [[LIKELY]], i32 [[UNLIKELY]]}
+// CHECK: ![[PROF_LIKELY]] = !{!"branch_weights", i32 1, i32 [[LIKELY]]}
+// CHECK: ![[PROF_UNLIKELY]] = !{!"branch_weights", i32 [[LIKELY]], i32 1
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