[clang] 1113fbf - [CodeGen] Improve likelihood branch weights
Mark de Wever via cfe-commits
cfe-commits at lists.llvm.org
Sun Oct 4 05:24:47 PDT 2020
Author: Mark de Wever
Date: 2020-10-04T14:24:27+02:00
New Revision: 1113fbf44c2250621548e278d2a1e11ab2b2d63d
URL: https://github.com/llvm/llvm-project/commit/1113fbf44c2250621548e278d2a1e11ab2b2d63d
DIFF: https://github.com/llvm/llvm-project/commit/1113fbf44c2250621548e278d2a1e11ab2b2d63d.diff
LOG: [CodeGen] Improve likelihood branch weights
Bruno De Fraine discovered some issues with D85091. The branch weights
generated for `logical not` and `ternary conditional` were wrong. The
`logical and` and `logical or` differed from the code generated of
`__builtin_predict`.
Adjusted the generated code for the likelihood to match
`__builtin_predict`. The patch is based on Bruno's suggestions.
Differential Revision: https://reviews.llvm.org/D88363
Added:
clang/test/CodeGenCXX/attr-likelihood-if-vs-builtin-expect.cpp
Modified:
clang/lib/CodeGen/CGStmt.cpp
clang/lib/CodeGen/CodeGenFunction.cpp
clang/lib/CodeGen/CodeGenFunction.h
Removed:
################################################################################
diff --git a/clang/lib/CodeGen/CGStmt.cpp b/clang/lib/CodeGen/CGStmt.cpp
index 83dd1be31633..c9e6ce2df2c0 100644
--- a/clang/lib/CodeGen/CGStmt.cpp
+++ b/clang/lib/CodeGen/CGStmt.cpp
@@ -27,7 +27,6 @@
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/Support/SaveAndRestore.h"
-#include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h"
using namespace clang;
using namespace CodeGen;
@@ -652,20 +651,6 @@ void CodeGenFunction::EmitIndirectGotoStmt(const IndirectGotoStmt &S) {
EmitBranch(IndGotoBB);
}
-static Optional<std::pair<uint32_t, uint32_t>>
-getLikelihoodWeights(const IfStmt &If) {
- switch (Stmt::getLikelihood(If.getThen(), If.getElse())) {
- case Stmt::LH_Unlikely:
- return std::pair<uint32_t, uint32_t>(llvm::UnlikelyBranchWeight,
- llvm::LikelyBranchWeight);
- case Stmt::LH_None:
- return None;
- case Stmt::LH_Likely:
- return std::pair<uint32_t, uint32_t>(llvm::LikelyBranchWeight,
- llvm::UnlikelyBranchWeight);
- }
- llvm_unreachable("Unknown Likelihood");
-}
void CodeGenFunction::EmitIfStmt(const IfStmt &S) {
// C99 6.8.4.1: The first substatement is executed if the expression compares
@@ -713,17 +698,11 @@ void CodeGenFunction::EmitIfStmt(const IfStmt &S) {
// Prefer the PGO based weights over the likelihood attribute.
// When the build isn't optimized the metadata isn't used, so don't generate
// it.
- llvm::MDNode *Weights = nullptr;
+ Stmt::Likelihood LH = Stmt::LH_None;
uint64_t Count = getProfileCount(S.getThen());
- if (!Count && CGM.getCodeGenOpts().OptimizationLevel) {
- Optional<std::pair<uint32_t, uint32_t>> LHW = getLikelihoodWeights(S);
- if (LHW) {
- llvm::MDBuilder MDHelper(CGM.getLLVMContext());
- Weights = MDHelper.createBranchWeights(LHW->first, LHW->second);
- }
- }
-
- EmitBranchOnBoolExpr(S.getCond(), ThenBlock, ElseBlock, Count, Weights);
+ if (!Count && CGM.getCodeGenOpts().OptimizationLevel)
+ LH = Stmt::getLikelihood(S.getThen(), S.getElse());
+ EmitBranchOnBoolExpr(S.getCond(), ThenBlock, ElseBlock, Count, LH);
// Emit the 'then' code.
EmitBlock(ThenBlock);
diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 47ef5c830723..363b418dc198 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -42,6 +42,7 @@
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Operator.h"
#include "llvm/Support/CRC.h"
+#include "llvm/Transforms/Scalar/LowerExpectIntrinsic.h"
#include "llvm/Transforms/Utils/PromoteMemToReg.h"
using namespace clang;
using namespace CodeGen;
@@ -1477,15 +1478,30 @@ bool CodeGenFunction::ConstantFoldsToSimpleInteger(const Expr *Cond,
return true;
}
+static Optional<std::pair<uint32_t, uint32_t>>
+getLikelihoodWeights(Stmt::Likelihood LH) {
+ switch (LH) {
+ case Stmt::LH_Unlikely:
+ return std::pair<uint32_t, uint32_t>(llvm::UnlikelyBranchWeight,
+ llvm::LikelyBranchWeight);
+ case Stmt::LH_None:
+ return None;
+ case Stmt::LH_Likely:
+ return std::pair<uint32_t, uint32_t>(llvm::LikelyBranchWeight,
+ llvm::UnlikelyBranchWeight);
+ }
+ llvm_unreachable("Unknown Likelihood");
+}
+
/// EmitBranchOnBoolExpr - Emit a branch on a boolean condition (e.g. for an if
/// statement) to the specified blocks. Based on the condition, this might try
/// to simplify the codegen of the conditional based on the branch.
-/// \param Weights The weights determined by the likelihood attributes.
+/// \param LH The value of the likelihood attribute on the True branch.
void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
llvm::BasicBlock *TrueBlock,
llvm::BasicBlock *FalseBlock,
uint64_t TrueCount,
- llvm::MDNode *Weights) {
+ Stmt::Likelihood LH) {
Cond = Cond->IgnoreParens();
if (const BinaryOperator *CondBOp = dyn_cast<BinaryOperator>(Cond)) {
@@ -1500,7 +1516,7 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
// br(1 && X) -> br(X).
incrementProfileCounter(CondBOp);
return EmitBranchOnBoolExpr(CondBOp->getRHS(), TrueBlock, FalseBlock,
- TrueCount, Weights);
+ TrueCount, LH);
}
// If we have "X && 1", simplify the code to use an uncond branch.
@@ -1509,7 +1525,7 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
ConstantBool) {
// br(X && 1) -> br(X).
return EmitBranchOnBoolExpr(CondBOp->getLHS(), TrueBlock, FalseBlock,
- TrueCount, Weights);
+ TrueCount, LH);
}
// Emit the LHS as a conditional. If the LHS conditional is false, we
@@ -1522,8 +1538,11 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
ConditionalEvaluation eval(*this);
{
ApplyDebugLocation DL(*this, Cond);
+ // Propagate the likelihood attribute like __builtin_expect
+ // __builtin_expect(X && Y, 1) -> X and Y are likely
+ // __builtin_expect(X && Y, 0) -> only Y is unlikely
EmitBranchOnBoolExpr(CondBOp->getLHS(), LHSTrue, FalseBlock, RHSCount,
- Weights);
+ LH == Stmt::LH_Unlikely ? Stmt::LH_None : LH);
EmitBlock(LHSTrue);
}
@@ -1533,7 +1552,7 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
// Any temporaries created here are conditional.
eval.begin(*this);
EmitBranchOnBoolExpr(CondBOp->getRHS(), TrueBlock, FalseBlock, TrueCount,
- Weights);
+ LH);
eval.end(*this);
return;
@@ -1548,7 +1567,7 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
// br(0 || X) -> br(X).
incrementProfileCounter(CondBOp);
return EmitBranchOnBoolExpr(CondBOp->getRHS(), TrueBlock, FalseBlock,
- TrueCount, Weights);
+ TrueCount, LH);
}
// If we have "X || 0", simplify the code to use an uncond branch.
@@ -1557,7 +1576,7 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
!ConstantBool) {
// br(X || 0) -> br(X).
return EmitBranchOnBoolExpr(CondBOp->getLHS(), TrueBlock, FalseBlock,
- TrueCount, Weights);
+ TrueCount, LH);
}
// Emit the LHS as a conditional. If the LHS conditional is true, we
@@ -1572,9 +1591,12 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
ConditionalEvaluation eval(*this);
{
+ // Propagate the likelihood attribute like __builtin_expect
+ // __builtin_expect(X || Y, 1) -> only Y is likely
+ // __builtin_expect(X || Y, 0) -> both X and Y are unlikely
ApplyDebugLocation DL(*this, Cond);
EmitBranchOnBoolExpr(CondBOp->getLHS(), TrueBlock, LHSFalse, LHSCount,
- Weights);
+ LH == Stmt::LH_Likely ? Stmt::LH_None : LH);
EmitBlock(LHSFalse);
}
@@ -1584,7 +1606,7 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
// Any temporaries created here are conditional.
eval.begin(*this);
EmitBranchOnBoolExpr(CondBOp->getRHS(), TrueBlock, FalseBlock, RHSCount,
- Weights);
+ LH);
eval.end(*this);
@@ -1597,9 +1619,11 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
if (CondUOp->getOpcode() == UO_LNot) {
// Negate the count.
uint64_t FalseCount = getCurrentProfileCount() - TrueCount;
+ // The values of the enum are chosen to make this negation possible.
+ LH = static_cast<Stmt::Likelihood>(-LH);
// Negate the condition and swap the destination blocks.
return EmitBranchOnBoolExpr(CondUOp->getSubExpr(), FalseBlock, TrueBlock,
- FalseCount, Weights);
+ FalseCount, LH);
}
}
@@ -1608,9 +1632,11 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
llvm::BasicBlock *LHSBlock = createBasicBlock("cond.true");
llvm::BasicBlock *RHSBlock = createBasicBlock("cond.false");
+ // The ConditionalOperator itself has no likelihood information for its
+ // true and false branches. This matches the behavior of __builtin_expect.
ConditionalEvaluation cond(*this);
EmitBranchOnBoolExpr(CondOp->getCond(), LHSBlock, RHSBlock,
- getProfileCount(CondOp), Weights);
+ getProfileCount(CondOp), Stmt::LH_None);
// When computing PGO branch weights, we only know the overall count for
// the true block. This code is essentially doing tail duplication of the
@@ -1630,14 +1656,14 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
{
ApplyDebugLocation DL(*this, Cond);
EmitBranchOnBoolExpr(CondOp->getLHS(), TrueBlock, FalseBlock,
- LHSScaledTrueCount, Weights);
+ LHSScaledTrueCount, LH);
}
cond.end(*this);
cond.begin(*this);
EmitBlock(RHSBlock);
EmitBranchOnBoolExpr(CondOp->getRHS(), TrueBlock, FalseBlock,
- TrueCount - LHSScaledTrueCount, Weights);
+ TrueCount - LHSScaledTrueCount, LH);
cond.end(*this);
return;
@@ -1666,8 +1692,12 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
}
}
- // Create branch weights based on the number of times we get here and the
- // number of times the condition should be true.
+ llvm::MDNode *Weights = nullptr;
+ Optional<std::pair<uint32_t, uint32_t>> LHW = getLikelihoodWeights(LH);
+ if (LHW) {
+ llvm::MDBuilder MDHelper(CGM.getLLVMContext());
+ Weights = MDHelper.createBranchWeights(LHW->first, LHW->second);
+ }
if (!Weights) {
uint64_t CurrentCount = std::max(getCurrentProfileCount(), TrueCount);
Weights = createProfileWeights(TrueCount, CurrentCount - TrueCount);
diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
index 16656de4e8f7..a9fb47864700 100644
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -4365,7 +4365,7 @@ class CodeGenFunction : public CodeGenTypeCache {
/// evaluate to true based on PGO data.
void EmitBranchOnBoolExpr(const Expr *Cond, llvm::BasicBlock *TrueBlock,
llvm::BasicBlock *FalseBlock, uint64_t TrueCount,
- llvm::MDNode *Weights = nullptr);
+ Stmt::Likelihood LH = Stmt::LH_None);
/// Given an assignment `*LHS = RHS`, emit a test that checks if \p RHS is
/// nonnull, if \p LHS is marked _Nonnull.
diff --git a/clang/test/CodeGenCXX/attr-likelihood-if-vs-builtin-expect.cpp b/clang/test/CodeGenCXX/attr-likelihood-if-vs-builtin-expect.cpp
new file mode 100644
index 000000000000..5e73cd096742
--- /dev/null
+++ b/clang/test/CodeGenCXX/attr-likelihood-if-vs-builtin-expect.cpp
@@ -0,0 +1,223 @@
+// RUN: %clang_cc1 -O1 -emit-llvm %s -o - -triple=x86_64-linux-gnu | FileCheck %s
+
+// Verifies the output of __builtin_expect versus the output of the likelihood
+// attributes. They should generate the same probabilities for the branches.
+
+extern bool a();
+extern bool b();
+extern bool c();
+
+void ab1(int &i) {
+ // CHECK-LABEL: define{{.*}}ab1
+ // CHECK: br {{.*}} !prof !2
+ // CHECK: br {{.*}} !prof !2
+ // CHECK: br {{.*}} !prof !2
+ if (__builtin_expect(a() && b() && a(), 1)) {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void al(int &i) {
+ // CHECK-LABEL: define{{.*}}al
+ // CHECK: br {{.*}} !prof !2
+ // CHECK: br {{.*}} !prof !2
+ // CHECK: br {{.*}} !prof !2
+ if (a() && b() && c()) [[likely]] {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void ab0(int &i) {
+ // CHECK-LABEL: define{{.*}}ab0
+ // CHECK: br {{.*}}else{{$}}
+ // CHECK: br {{.*}}else{{$}}
+ // CHECK: br {{.*}} !prof !8
+ if (__builtin_expect(a() && b() && c(), 0)) {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void au(int &i) {
+ // CHECK-LABEL: define{{.*}}au
+ // CHECK: br {{.*}}else{{$}}
+ // CHECK: br {{.*}}else{{$}}
+ // CHECK: br {{.*}} !prof !8
+ if (a() && b() && c()) [[unlikely]] {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void ob1(int &i) {
+ // CHECK-LABEL: define{{.*}}ob1
+ // CHECK: br {{.*}}false{{$}}
+ // CHECK: br {{.*}}rhs{{$}}
+ // CHECK: br {{.*}} !prof !2
+ if (__builtin_expect(a() || b() || a(), 1)) {
+ i = 0;
+ } else {
+ --i;
+ }
+}
+
+void ol(int &i) {
+ // CHECK-LABEL: define{{.*}}ol
+ // CHECK: br {{.*}}false{{$}}
+ // CHECK: br {{.*}}false2{{$}}
+ // CHECK: br {{.*}} !prof !2
+ if (a() || b() || c()) [[likely]] {
+ i = 0;
+ } else {
+ --i;
+ }
+}
+
+void ob0(int &i) {
+ // CHECK-LABEL: define{{.*}}ob0
+ // CHECK: br {{.*}} !prof !8
+ // CHECK: br {{.*}} !prof !8
+ // CHECK: br {{.*}} !prof !8
+ if (__builtin_expect(a() || b() || c(), 0)) {
+ i = 0;
+ } else {
+ --i;
+ }
+}
+
+void ou(int &i) {
+ // CHECK-LABEL: define{{.*}}ou
+ // CHECK: br {{.*}} !prof !8
+ // CHECK: br {{.*}} !prof !8
+ // CHECK: br {{.*}} !prof !8
+ if (a() || b() || c()) [[unlikely]] {
+ i = 0;
+ } else {
+ --i;
+ }
+}
+
+void nb1(int &i) {
+ // CHECK-LABEL: define{{.*}}nb1
+ // CHECK: storemerge{{.*}} !prof !8
+ if (__builtin_expect(!a(), 1)) {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void nl(int &i) {
+ // CHECK-LABEL: define{{.*}}nl
+ // CHECK: storemerge{{.*}} !prof !8
+ if (!a()) [[likely]] {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void nb0(int &i) {
+ // CHECK-LABEL: define{{.*}}nb0
+ // CHECK: storemerge{{.*}} !prof !2
+ if (__builtin_expect(!a(), 0)) {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void nu(int &i) {
+ // CHECK-LABEL: define{{.*}}nu
+ // CHECK: storemerge{{.*}} !prof !2
+ if (!a()) [[unlikely]] {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void tb1(int &i) {
+ // CHECK-LABEL: define{{.*}}tb1
+ // CHECK: br {{.*}}false{{$}}
+ // CHECK: br {{.*}}end{{$}}
+ // CHECK: br {{.*}}end{{$}}
+ // CHECK: storemerge{{.*}} !prof !2
+ if (__builtin_expect(a() ? b() : c(), 1)) {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void tl(int &i) {
+ // CHECK-LABEL: define{{.*}}tl
+ // CHECK: br {{.*}}false{{$}}
+ // CHECK: br {{.*}}end{{$}}
+ // CHECK: br {{.*}}end{{$}}
+ // CHECK: storemerge{{.*}} !prof !2
+ if (bool d = a() ? b() : c()) [[likely]] {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void tl2(int &i) {
+ // CHECK-LABEL: define{{.*}}tl
+ // CHECK: br {{.*}}false{{$}}
+ // CHECK: br {{.*}} !prof !2
+ // CHECK: br {{.*}} !prof !2
+ if (a() ? b() : c()) [[likely]] {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void tb0(int &i) {
+ // CHECK-LABEL: define{{.*}}tb0
+ // CHECK: br {{.*}}false{{$}}
+ // CHECK: br {{.*}}end{{$}}
+ // CHECK: br {{.*}}end{{$}}
+ // CHECK: storemerge{{.*}} !prof !8
+ if (__builtin_expect(a() ? b() : c(), 0)) {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void tu(int &i) {
+ // CHECK-LABEL: define{{.*}}tu
+ // CHECK: br {{.*}}false{{$}}
+ // CHECK: br {{.*}}end{{$}}
+ // CHECK: br {{.*}}end{{$}}
+ // CHECK: storemerge{{.*}} !prof !8
+ if (bool d = a() ? b() : c()) [[unlikely]] {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+void tu2(int &i) {
+ // CHECK-LABEL: define{{.*}}tu
+ // CHECK: br {{.*}}false{{$}}
+ // CHECK: br {{.*}} !prof !8
+ // CHECK: br {{.*}} !prof !8
+ if (a() ? b() : c()) [[unlikely]] {
+ ++i;
+ } else {
+ --i;
+ }
+}
+
+// CHECK: !2 = !{!"branch_weights", i32 2000, i32 1}
+// CHECK: !8 = !{!"branch_weights", i32 1, i32 2000}
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