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Fix a defect in code-layout pass, improving Benchmarks/Olden/em3d/em3d by about 30%

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(4.58s vs 3.2s on an oldish Mac Tower). 

  The corresponding src is excerpted bellow. The lopp accounts for about 90% of execution time.
  --------------------
    cat -n test-suite/MultiSource/Benchmarks/Olden/em3d/make_graph.c
     90 
     91         for (k=0; k<j; k++)
     92           if (other_node == cur_node->to_nodes[k]) break;

  The defective layout is sketched bellow, where the two branches need to swap.
  ------------------------------------------------------------------------
      L:
         ...
      if (cond) goto out-of-loop
      goto L

  While this code sequence is defective, I don't understand why it incurs 1/3 of 
execution time. CPU-event-profiling indicates the poor laoyout dose not increase
in br-misprediction; it dosen't increase stall cycle at all, and it dosen't 
prevent the CPU detect the loop (i.e. Loop-Stream-Detector seems to be working fine
as well)... 

   The root cause of the problem is that the layout pass calls AnalyzeBranch() 
with basic-block which is not updated to reflect its current layout.

rdar://13966341


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183174 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Shuxin Yang 2013-06-04 01:00:57 +00:00
parent 4526d1cd4a
commit 45c7544339
1 changed files with 25 additions and 1 deletions
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26
lib/CodeGen/MachineBlockPlacement.cpp
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@ -991,6 +991,28 @@ void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
Cond.clear();
MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
// The "PrevBB" is not yet updated to reflect current code layout, so,
// o. it may fall-through to a block without explict "goto" instruction
// before layout, and no longer fall-through it after layout; or
// o. just opposite.
//
// AnalyzeBranch() may return erroneous value for FBB when these two
// situations take place. For the first scenario FBB is mistakenly set
// NULL; for the 2nd scenario, the FBB, which is expected to be NULL,
// is mistakenly pointing to "*BI".
//
bool needUpdateBr = true;
if (!Cond.empty() && (!FBB || FBB == *BI)) {
PrevBB->updateTerminator();
needUpdateBr = false;
Cond.clear();
TBB = FBB = 0;
if (TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
// FIXME: This should never take place.
TBB = FBB = 0;
}
}
// If PrevBB has a two-way branch, try to re-order the branches
// such that we branch to the successor with higher weight first.
if (TBB && !Cond.empty() && FBB &&
@ -1003,8 +1025,10 @@ void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
DebugLoc dl; // FIXME: this is nowhere
TII->RemoveBranch(*PrevBB);
TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
needUpdateBr = true;
}
PrevBB->updateTerminator();
if (needUpdateBr)
PrevBB->updateTerminator();
}
}