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The logic for breaking the CFG in the presence of hot successors didn't

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properly account for the *global* probability of the edge being taken.
This manifested as a very large number of unconditional branches to
blocks being merged against the CFG even though they weren't
particularly hot within the CFG.

The fix is to check whether the edge being merged is both locally hot
relative to other successors for the source block, and globally hot
compared to other (unmerged) predecessors of the destination block.

This introduces a new crasher on GCC single-source, but it's currently
behind a flag, and Ben has offered to work on the reduction. =]

llvm-svn: 145010
This commit is contained in:
Chandler Carruth 2011-11-20 11:22:06 +00:00
parent f191cab95f
commit aac8e5082a
2 changed files with 65 additions and 3 deletions
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32
lib/CodeGen/MachineBlockPlacement.cpp
View File

@ -365,9 +365,35 @@ MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
// Only consider successors which are either "hot", or wouldn't violate
// any CFG constraints.
if (SuccChain.LoopPredecessors != 0 && SuccProb < HotProb) {
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
continue;
if (SuccChain.LoopPredecessors != 0) {
if (SuccProb < HotProb) {
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> CFG conflict\n");
continue;
}
// Make sure that a hot successor doesn't have a globally more important
// predecessor.
BlockFrequency CandidateEdgeFreq
= MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
bool BadCFGConflict = false;
for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
PE = (*SI)->pred_end();
PI != PE; ++PI) {
if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
BlockToChain[*PI] == &Chain)
continue;
BlockFrequency PredEdgeFreq
= MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
if (PredEdgeFreq >= CandidateEdgeFreq) {
BadCFGConflict = true;
break;
}
}
if (BadCFGConflict) {
DEBUG(dbgs() << " " << getBlockName(*SI)
<< " -> non-cold CFG conflict\n");
continue;
}
}
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb

36
test/CodeGen/X86/block-placement.ll
View File

@ -439,3 +439,39 @@ if.end:
}
!1 = metadata !{metadata !"branch_weights", i32 1000, i32 1}
declare i32 @f()
declare i32 @g()
declare i32 @h(i32 %x)
define i32 @test_global_cfg_break_profitability() {
; Check that our metrics for the profitability of a CFG break are global rather
; than local. A successor may be very hot, but if the current block isn't, it
; doesn't matter. Within this test the 'then' block is slightly warmer than the
; 'else' block, but not nearly enough to merit merging it with the exit block
; even though the probability of 'then' branching to the 'exit' block is very
; high.
; CHECK: test_global_cfg_break_profitability
; CHECK: calll f
; CHECK: calll g
; CHECK: calll h
; CHECK: ret
entry:
br i1 undef, label %then, label %else, !prof !2
then:
%then.result = call i32 @f()
br label %exit
else:
%else.result = call i32 @g()
br label %exit
exit:
%result = phi i32 [ %then.result, %then ], [ %else.result, %else ]
%result2 = call i32 @h(i32 %result)
ret i32 %result
}
!2 = metadata !{metadata !"branch_weights", i32 3, i32 1}