Mordante created this revision.
Mordante added reviewers: aaron.ballman, rjmccall, rsmith.
Mordante added a project: clang.
Herald added a subscriber: dexonsmith.
Mordante requested review of this revision.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D89899
Files:
clang/include/clang/Basic/AttrDocs.td
clang/lib/CodeGen/CGStmt.cpp
clang/lib/CodeGen/CodeGenFunction.cpp
clang/lib/CodeGen/CodeGenFunction.h
clang/test/CodeGenCXX/attr-likelihood-if-branch-weights.cpp
clang/test/CodeGenCXX/attr-likelihood-iteration-stmt.cpp
Index: clang/test/CodeGenCXX/attr-likelihood-iteration-stmt.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/attr-likelihood-iteration-stmt.cpp
@@ -0,0 +1,75 @@
+// RUN: %clang_cc1 -O1 -disable-llvm-passes -emit-llvm %s -o - -triple=x86_64-linux-gnu | FileCheck %s
+
+void wl(int e){
+ // CHECK-LABEL: define{{.*}}wl
+ // CHECK: br {{.*}} !prof !6
+ while(e) [[likely]] ++e;
+}
+
+void wu(int e){
+ // CHECK-LABEL: define{{.*}}wu
+ // CHECK: br {{.*}} !prof !9
+ while(e) [[unlikely]] ++e;
+}
+
+void w_branch_elided(unsigned e){
+ // CHECK-LABEL: define{{.*}}w_branch_elided
+ // CHECK-NOT: br {{.*}} !prof
+ while(1) [[likely]] ++e;
+}
+
+void dl(int e){
+ // CHECK-LABEL: define{{.*}}dl
+ // CHECK: br {{.*}} !prof !6
+ do [[likely]] ++e; while(e);
+}
+
+void du(int e){
+ // CHECK-LABEL: define{{.*}}du
+ // CHECK: br {{.*}} !prof !9
+ do [[unlikely]] ++e; while(e);
+}
+
+void d_branch_elided(unsigned e){
+ // CHECK-LABEL: define{{.*}}d_branch_elided
+ // CHECK-NOT: br {{.*}} !prof
+ do [[unlikely]] ++e; while(0);
+}
+
+void fl(unsigned e)
+{
+ // CHECK-LABEL: define{{.*}}fl
+ // CHECK: br {{.*}} !prof !6
+ for(int i = 0; i != e; ++e) [[likely]];
+}
+
+void fu(int e)
+{
+ // CHECK-LABEL: define{{.*}}fu
+ // CHECK: br {{.*}} !prof !9
+ for(int i = 0; i != e; ++e) [[unlikely]];
+}
+
+void f_branch_elided()
+{
+ // CHECK-LABEL: define{{.*}}f_branch_elided
+ // CHECK-NOT: br {{.*}} !prof
+ for(;;) [[likely]];
+}
+
+void frl(int (&&e) [4])
+{
+ // CHECK-LABEL: define{{.*}}frl
+ // CHECK: br {{.*}} !prof !6
+ for(int i : e) [[likely]];
+}
+
+void fru(int (&&e) [4])
+{
+ // CHECK-LABEL: define{{.*}}fru
+ // CHECK: br {{.*}} !prof !9
+ for(int i : e) [[unlikely]];
+}
+
+// CHECK: !6 = !{!"branch_weights", i32 2000, i32 1}
+// CHECK: !9 = !{!"branch_weights", i32 1, i32 2000}
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
@@ -69,6 +69,7 @@
// CHECK-NOT: br {{.*}} %if.end{{.*}} !prof
if (b)
+ // CHECK: br {{.*}} !prof !7
while (B())
[[unlikely]] { b = false; }
}
@@ -83,6 +84,7 @@
// CHECK-NOT: br {{.*}} %if.end{{.*}} !prof
if (b)
+ // CHECK: br {{.*}} !prof !7
do
[[unlikely]] {}
while (B());
@@ -96,6 +98,7 @@
// CHECK-NOT: br {{.*}} %if.end{{.*}} !prof
if (b)
+ // CHECK: br {{.*}} !prof !7
for (; B();)
[[unlikely]] {}
}
Index: clang/lib/CodeGen/CodeGenFunction.h
===================================================================
--- clang/lib/CodeGen/CodeGenFunction.h
+++ clang/lib/CodeGen/CodeGenFunction.h
@@ -1407,6 +1407,13 @@
llvm::MDNode *createProfileWeightsForLoop(const Stmt *Cond,
uint64_t LoopCount) const;
+ /// Calculate the branch weight for PGO data or the likelihood attribe.
+ /// The function tries to get the weight of \ref createProfileWeightsForLoop.
+ /// If that fails it gets the weight of \ref createBranchWeights.
+ llvm::MDNode *createProfileOrBranchWeightsForLoop(const Stmt *Cond,
+ uint64_t LoopCount,
+ const Stmt *Body) const;
+
public:
/// Increment the profiler's counter for the given statement by \p StepV.
/// If \p StepV is null, the default increment is 1.
Index: clang/lib/CodeGen/CodeGenFunction.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenFunction.cpp
+++ clang/lib/CodeGen/CodeGenFunction.cpp
@@ -2573,3 +2573,12 @@
llvm::MDBuilder MDHelper(CGM.getLLVMContext());
return MDHelper.createBranchWeights(LHW->first, LHW->second);
}
+
+llvm::MDNode *CodeGenFunction::createProfileOrBranchWeightsForLoop(
+ const Stmt *Cond, uint64_t LoopCount, const Stmt *Body) const {
+ llvm::MDNode *Weights = createProfileWeightsForLoop(Cond, LoopCount);
+ if (!Weights && CGM.getCodeGenOpts().OptimizationLevel)
+ Weights = createBranchWeights(Stmt::getLikelihood(Body));
+
+ return Weights;
+}
Index: clang/lib/CodeGen/CGStmt.cpp
===================================================================
--- clang/lib/CodeGen/CGStmt.cpp
+++ clang/lib/CodeGen/CGStmt.cpp
@@ -805,9 +805,9 @@
llvm::BasicBlock *ExitBlock = LoopExit.getBlock();
if (ConditionScope.requiresCleanups())
ExitBlock = createBasicBlock("while.exit");
- Builder.CreateCondBr(
- BoolCondVal, LoopBody, ExitBlock,
- createProfileWeightsForLoop(S.getCond(), getProfileCount(S.getBody())));
+ llvm::MDNode *Weights = createProfileOrBranchWeightsForLoop(
+ S.getCond(), getProfileCount(S.getBody()), S.getBody());
+ Builder.CreateCondBr(BoolCondVal, LoopBody, ExitBlock, Weights);
if (ExitBlock != LoopExit.getBlock()) {
EmitBlock(ExitBlock);
@@ -890,9 +890,9 @@
// As long as the condition is true, iterate the loop.
if (EmitBoolCondBranch) {
uint64_t BackedgeCount = getProfileCount(S.getBody()) - ParentCount;
- Builder.CreateCondBr(
- BoolCondVal, LoopBody, LoopExit.getBlock(),
- createProfileWeightsForLoop(S.getCond(), BackedgeCount));
+ llvm::MDNode *Weights = createProfileOrBranchWeightsForLoop(
+ S.getCond(), BackedgeCount, S.getBody());
+ Builder.CreateCondBr(BoolCondVal, LoopBody, LoopExit.getBlock(), Weights);
}
LoopStack.pop();
@@ -960,9 +960,9 @@
// C99 6.8.5p2/p4: The first substatement is executed if the expression
// compares unequal to 0. The condition must be a scalar type.
llvm::Value *BoolCondVal = EvaluateExprAsBool(S.getCond());
- Builder.CreateCondBr(
- BoolCondVal, ForBody, ExitBlock,
- createProfileWeightsForLoop(S.getCond(), getProfileCount(S.getBody())));
+ llvm::MDNode *Weights = createProfileOrBranchWeightsForLoop(
+ S.getCond(), getProfileCount(S.getBody()), S.getBody());
+ Builder.CreateCondBr(BoolCondVal, ForBody, ExitBlock, Weights);
if (ExitBlock != LoopExit.getBlock()) {
EmitBlock(ExitBlock);
@@ -1041,9 +1041,9 @@
// The body is executed if the expression, contextually converted
// to bool, is true.
llvm::Value *BoolCondVal = EvaluateExprAsBool(S.getCond());
- Builder.CreateCondBr(
- BoolCondVal, ForBody, ExitBlock,
- createProfileWeightsForLoop(S.getCond(), getProfileCount(S.getBody())));
+ llvm::MDNode *Weights = createProfileOrBranchWeightsForLoop(
+ S.getCond(), getProfileCount(S.getBody()), S.getBody());
+ Builder.CreateCondBr(BoolCondVal, ForBody, ExitBlock, Weights);
if (ExitBlock != LoopExit.getBlock()) {
EmitBlock(ExitBlock);
Index: clang/include/clang/Basic/AttrDocs.td
===================================================================
--- clang/include/clang/Basic/AttrDocs.td
+++ clang/include/clang/Basic/AttrDocs.td
@@ -1738,7 +1738,8 @@
In Clang, the attributes will be ignored if they're not placed on
* the ``case`` or ``default`` label of a ``switch`` statement,
-* or on the substatement of an ``if`` or ``else`` statement.
+* or on the substatement of an ``if`` or ``else`` statement,
+* or on the substatement of an ``for``, ``do``, or ``while`` statement.
The C++ Standard recommends to honor them on every statement in the
path of execution, but that can be confusing:
@@ -1769,8 +1770,7 @@
}
-At the moment the attributes only have effect when used in an ``if``, ``else``,
-or ``switch`` statement.
+Below some usage examples likelihood attributes and their effect:
.. code-block:: c++
@@ -1850,6 +1850,26 @@
break;
}
+ for(int i = 0; i != size; ++i) [[likely]] {
+ ... // The loop the likely path of execution
+ }
+
+ while(i != size) [[unlikely]] {
+ ... // The loop is the unlikely path of execution
+ } // The generated code will optimize to skip the loop body
+
+ do [[unlikely]] { // The loop will always iterate once
+ ... // The likelihood attribute affects the likelihood of the
+ } while(i != size); // while at the end of the ``do while`` statement
+
+ while(true) [[unlikely]] {
+ ... // The attribute has no effect
+ } // Clang elides comparison and generates an infinite loop
+
+ do [[likely]] {
+ ... // The attribute has no effect
+ } while(0); // Clang elides comparison and generates no loop
+
}];
}
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