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[MBP] Avoid placing random blocks between loop preheader and header 

If we have a loop with a rarely taken path, we will prune that from the blocks which get added as part of the loop chain. The problem is that we weren't then recognizing the loop chain as schedulable when considering the preheader when forming the function chain. We'd then fall to various non-predecessors before finally scheduling the loop chain (as if the CFG was unnatural.) The net result was that there could be lots of garbage between a loop preheader and the loop, even though we could have directly fallen into the loop. It also meant we separated hot code with regions of colder code.

The particular reason for the rejection of the loop chain was that we were scanning predecessor of the header, seeing the backedge, believing that was a globally more important predecessor (true), but forgetting to account for the fact the backedge precessor was already part of the existing loop chain (oops!.

Differential Revision: http://reviews.llvm.org/D17830

llvm-svn: 262547
This commit is contained in:
Philip Reames 2016-03-03 00:01:42 +00:00
parent 1ef654024f
commit 23d933982a
2 changed files with 41 additions and 1 deletions
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3
llvm/lib/CodeGen/MachineBlockPlacement.cpp
View File

@ -466,7 +466,8 @@ MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
MBFI->getBlockFreq(BB) * RealSuccProb * HotProb.getCompl();
bool BadCFGConflict = false;
for (MachineBasicBlock *Pred : Succ->predecessors()) {
if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||
if (Pred == Succ || BlockToChain[Pred] == &SuccChain ||
(BlockFilter && !BlockFilter->count(Pred)) ||
BlockToChain[Pred] == &Chain)
continue;
BlockFrequency PredEdgeFreq =

39
llvm/test/CodeGen/X86/mbp-false-cfg-break.ll Normal file
View File

@ -0,0 +1,39 @@
; RUN: llc < %s -march=x86-64 | FileCheck %s
define void @test(i1 %cnd) !prof !{!"function_entry_count", i64 1024} {
; CHECK-LABEL: @test
; Using the assembly comments to indicate block order..
; CHECK: # %loop
; CHECK: # %backedge
; CHECK: # %exit
; CHECK: # %rare
; CHECK: # %rare.1
br i1 undef, label %rare.1, label %preheader, !prof !{!"branch_weights", i32 0, i32 1000}
rare.1:
call void @foo()
br label %preheader
preheader:
br label %loop
loop:
%iv = phi i32 [0, %preheader], [%iv.next, %backedge]
call void @foo()
br i1 %cnd, label %backedge, label %rare, !prof !{!"branch_weights", i32 1000000, i32 1}
rare:
call void @foo()
br label %backedge
backedge:
call void @foo()
%iv.next = add i32 %iv, 1
%cmp = icmp eq i32 %iv.next, 200
br i1 %cmp, label %loop, label %exit, !prof !{!"branch_weights", i32 1000, i32 1}
exit:
ret void
}
declare void @foo()