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Fix for ignoring fall-through profile data when jump is followed by no-op 

Summary:
When a conditional jump is followed by one or more no-ops, the
destination of fall-through branch was recorded as the first no-op in
FuncBranchInfo. However the fall-through basic block after the jump
starts after the no-ops, so the profile data could not match the CFG
and was ignored.

(cherry picked from FBD3496084)
This commit is contained in:
Theodoros Kasampalis 2016-06-27 14:51:38 -07:00 committed by Maksim Panchenko
parent d09b00ebff
commit 287fa51324
1 changed files with 21 additions and 4 deletions
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25
bolt/BinaryFunction.cpp
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@ -775,11 +775,28 @@ bool BinaryFunction::buildCFG() {
<< Twine::utohexstr(Branch.second) << "]\n");
BinaryBasicBlock *FromBB = getBasicBlockContainingOffset(Branch.first);
assert(FromBB && "cannot find BB containing FROM branch");
// Try to find the destination basic block. If the jump instruction was
// followed by a no-op then the destination offset recorded in FTBranches
// will point to that no-op but the destination basic block will start
// after the no-op due to ingoring no-ops when creating basic blocks.
// So we have to skip any no-ops when trying to find the destination
// basic block.
BinaryBasicBlock *ToBB = getBasicBlockAtOffset(Branch.second);
// We have a fall-through that does not point to another BB, ignore it as
// it may happen in cases where we have a BB finished by two branches.
if (ToBB == nullptr)
continue;
if (ToBB == nullptr) {
auto I = Instructions.find(Branch.second), E = Instructions.end();
while (ToBB == nullptr && I != E && MIA->isNoop(I->second)) {
++I;
if (I == E)
break;
ToBB = getBasicBlockAtOffset(I->first);
}
if (ToBB == nullptr) {
// We have a fall-through that does not point to another BB, ignore it
// as it may happen in cases where we have a BB finished by two
// branches.
continue;
}
}
// Does not add a successor if we can't find profile data, leave it to the
// inference pass to guess its frequency