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[PGO] Add simplified branch weights for Objective-C for-collection loops. 

Conceptually one of these loops is just a while-loop, but the actual code-gen
is more complicated. We don't instrument all the different control flow edges
to get accurate counts for each conditional branch, nor do I think it makes
sense to do so. Instead, make the simplifying assumption that the loop behaves
like a while-loop. Use the same branch weights for the first check for an
empty collection as would be used for the back-edge of a while loop, and use
that same weighting for the innermost loop, ignoring the possibility that there
may be some extra code to go fetch more elements.

llvm-svn: 204767
This commit is contained in:
Bob Wilson 2014-03-25 23:26:31 +00:00
parent 3a485fba1f
commit 0ed74d9634
2 changed files with 15 additions and 9 deletions
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19
clang/lib/CodeGen/CGObjC.cpp
View File

@ -1509,9 +1509,13 @@ void CodeGenFunction::EmitObjCForCollectionStmt(const ObjCForCollectionStmt &S){
llvm::Value *zero = llvm::Constant::getNullValue(UnsignedLongLTy);
// If the limit pointer was zero to begin with, the collection is
// empty; skip all this.
// empty; skip all this. Set the branch weight assuming this has the same
// probability of exiting the loop as any other loop exit.
uint64_t EntryCount = PGO.getCurrentRegionCount();
RegionCounter Cnt = getPGORegionCounter(&S);
Builder.CreateCondBr(Builder.CreateICmpEQ(initialBufferLimit, zero, "iszero"),
EmptyBB, LoopInitBB);
EmptyBB, LoopInitBB,
PGO.createBranchWeights(EntryCount, Cnt.getCount()));
// Otherwise, initialize the loop.
EmitBlock(LoopInitBB);
@ -1540,7 +1544,6 @@ void CodeGenFunction::EmitObjCForCollectionStmt(const ObjCForCollectionStmt &S){
llvm::PHINode *count = Builder.CreatePHI(UnsignedLongLTy, 3, "forcoll.count");
count->addIncoming(initialBufferLimit, LoopInitBB);
RegionCounter Cnt = getPGORegionCounter(&S);
Cnt.beginRegion(Builder);
// Check whether the mutations value has changed from where it was
@ -1649,10 +1652,13 @@ void CodeGenFunction::EmitObjCForCollectionStmt(const ObjCForCollectionStmt &S){
llvm::Value *indexPlusOne
= Builder.CreateAdd(index, llvm::ConstantInt::get(UnsignedLongLTy, 1));
// TODO: We should probably model this as a "continue" for PGO
// If we haven't overrun the buffer yet, we can continue.
// Set the branch weights based on the simplifying assumption that this is
// like a while-loop, i.e., ignoring that the false branch fetches more
// elements and then returns to the loop.
Builder.CreateCondBr(Builder.CreateICmpULT(indexPlusOne, count),
LoopBodyBB, FetchMoreBB);
LoopBodyBB, FetchMoreBB,
PGO.createBranchWeights(Cnt.getCount(), EntryCount));
index->addIncoming(indexPlusOne, AfterBody.getBlock());
count->addIncoming(count, AfterBody.getBlock());
@ -1673,8 +1679,6 @@ void CodeGenFunction::EmitObjCForCollectionStmt(const ObjCForCollectionStmt &S){
index->addIncoming(zero, Builder.GetInsertBlock());
count->addIncoming(refetchCount, Builder.GetInsertBlock());
// TODO: We should be applying PGO weights here, but this needs to handle the
// branch before FetchMoreBB or we risk getting the numbers wrong.
Builder.CreateCondBr(Builder.CreateICmpEQ(refetchCount, zero),
EmptyBB, LoopBodyBB);
@ -1697,7 +1701,6 @@ void CodeGenFunction::EmitObjCForCollectionStmt(const ObjCForCollectionStmt &S){
PopCleanupBlock();
EmitBlock(LoopEnd.getBlock());
// TODO: Once we calculate PGO weights above, set the region count here
}
void CodeGenFunction::EmitObjCAtTryStmt(const ObjCAtTryStmt &S) {

5
clang/test/Profile/objc-general.m
View File

@ -45,7 +45,8 @@ struct NSFastEnumerationState;
{
__block id result;
// PGOGEN: store {{.*}} @[[FRC]], i64 0, i64 1
// FIXME: We don't emit branch weights for this yet.
// PGOUSE: br {{.*}} !prof ![[FR1:[0-9]+]]
// PGOUSE: br {{.*}} !prof ![[FR2:[0-9]+]]
for (id x in array) {
// PGOGEN: define {{.*}}_block_invoke
// PGOUSE: define {{.*}}_block_invoke
@ -60,6 +61,8 @@ struct NSFastEnumerationState;
}
@end
// PGOUSE-DAG: ![[FR1]] = metadata !{metadata !"branch_weights", i32 2, i32 3}
// PGOUSE-DAG: ![[FR2]] = metadata !{metadata !"branch_weights", i32 3, i32 2}
// PGOUSE-DAG: ![[BL1]] = metadata !{metadata !"branch_weights", i32 2, i32 2}
int main(int argc, const char *argv[]) {