[llvm] r272729 - Set machine block placement hot prob threshold for both static and runtime profile.

Tue Jun 14 20:12:53 PDT 2016

Added documentation.

I actually overlooked an error in the code review. The patch produces the
right threshold by default, but when parameter is set to be non-default,
the result is not correct. I have corrected it in this patch.

David

On Tue, Jun 14, 2016 at 7:11 PM, Sean Silva via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

>
>
> On Tue, Jun 14, 2016 at 3:27 PM, Dehao Chen via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> Author: dehao
>> Date: Tue Jun 14 17:27:17 2016
>> New Revision: 272729
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=272729&view=rev
>> Log:
>> Set machine block placement hot prob threshold for both static and
>> runtime profile.
>>
>> Summary: With runtime profile, we have more confidence in branch
>> probability, thus during basic block layout, we set a lower hot prob
>> threshold so that blocks can be layouted optimally.
>>
>> Reviewers: djasper, davidxl
>>
>> Subscribers: llvm-commits
>>
>> Differential Revision: http://reviews.llvm.org/D20991
>>
>> Modified:
>>     llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
>>     llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp
>>     llvm/trunk/test/CodeGen/X86/block-placement.ll
>>
>> Modified: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp?rev=272729&r1=272728&r2=272729&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
>> +++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Tue Jun 14 17:27:17
>> 2016
>> @@ -125,6 +125,7 @@ BranchFoldPlacement("branch-fold-placeme
>>                cl::init(true), cl::Hidden);
>>
>>  extern cl::opt<unsigned> StaticLikelyProb;
>> +extern cl::opt<unsigned> ProfileLikelyProb;
>>
>>  namespace {
>>  class BlockChain;
>> @@ -520,13 +521,20 @@ bool MachineBlockPlacement::shouldPredBl
>>      return false;
>>  }
>>
>> -// FIXME (PGO handling)
>> -// For now this method just returns a fixed threshold. It needs to be
>> enhanced
>> -// such that BB and Succ is passed in so that CFG shapes are examined
>> such that
>> -// the threshold is computed with more precise cost model when PGO is on.
>> -static BranchProbability getLayoutSuccessorProbThreshold() {
>> -  BranchProbability HotProb(StaticLikelyProb, 100);
>> -  return HotProb;
>> +// When profile is not present, return the StaticLikelyProb.
>> +// When profile is available, we need to handle the triangle-shape CFG.
>> +static BranchProbability getLayoutSuccessorProbThreshold(
>> +      MachineBasicBlock *BB) {
>> +  if (!BB->getParent()->getFunction()->getEntryCount())
>> +    return BranchProbability(StaticLikelyProb, 100);
>> +  if (BB->succ_size() == 2) {
>> +    const MachineBasicBlock *Succ1 = *BB->succ_begin();
>> +    const MachineBasicBlock *Succ2 = *(BB->succ_begin() + 1);
>> +    if (Succ1->isSuccessor(Succ2) || Succ2->isSuccessor(Succ1))
>> +      return BranchProbability(
>> +          200 - 2 * ProfileLikelyProb, 200 - ProfileLikelyProb);
>>
>
> Please comment on how these numbers were derived.
>
>
>> +  }
>> +  return BranchProbability(ProfileLikelyProb, 100);
>>  }
>>
>>  /// Checks to see if the layout candidate block \p Succ has a better
>> layout
>> @@ -593,7 +601,7 @@ bool MachineBlockPlacement::hasBetterLay
>>    // edge: Prob(Succ->BB) needs to >= HotProb in order to be selected
>> (without
>>    // profile data).
>>
>> -  BranchProbability HotProb = getLayoutSuccessorProbThreshold();
>> +  BranchProbability HotProb = getLayoutSuccessorProbThreshold(BB);
>>
>>    // Forward checking. For case 2, SuccProb will be 1.
>>    if (SuccProb < HotProb) {
>>
>> Modified: llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp?rev=272729&r1=272728&r2=272729&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp (original)
>> +++ llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp Tue Jun 14
>> 17:27:17 2016
>> @@ -29,6 +29,12 @@ cl::opt<unsigned> StaticLikelyProb(
>>      cl::desc("branch probability threshold to be considered very
>> likely"),
>>      cl::init(80), cl::Hidden);
>>
>> +cl::opt<unsigned> ProfileLikelyProb(
>> +    "profile-likely-prob",
>> +    cl::desc("branch probability threshold to be considered very likely "
>> +             "when profile is available"),
>> +    cl::init(51), cl::Hidden);
>>
>
> Please mention in the description that this is a percentage.
>
> -- Sean Silva
>
>
>> +
>>  char MachineBranchProbabilityInfo::ID = 0;
>>
>>  void MachineBranchProbabilityInfo::anchor() {}
>>
>> Modified: llvm/trunk/test/CodeGen/X86/block-placement.ll
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/block-placement.ll?rev=272729&r1=272728&r2=272729&view=diff
>>
>> ==============================================================================
>> --- llvm/trunk/test/CodeGen/X86/block-placement.ll (original)
>> +++ llvm/trunk/test/CodeGen/X86/block-placement.ll Tue Jun 14 17:27:17
>> 2016
>> @@ -1207,4 +1207,82 @@ exit:
>>    ret void
>>  }
>>
>> +define void @test_hot_branch_profile(i32* %a) !prof !6 {
>> +; Test that a hot branch that has a probability a little larger than 60%
>> will
>> +; break CFG constrains when doing block placement when profile is
>> available.
>> +; CHECK-LABEL: test_hot_branch_profile:
>> +; CHECK: %entry
>> +; CHECK: %then
>> +; CHECK: %exit
>> +; CHECK: %else
>> +
>> +entry:
>> +  %gep1 = getelementptr i32, i32* %a, i32 1
>> +  %val1 = load i32, i32* %gep1
>> +  %cond1 = icmp ugt i32 %val1, 1
>> +  br i1 %cond1, label %then, label %else, !prof !5
>> +
>> +then:
>> +  call void @hot_function()
>> +  br label %exit
>> +
>> +else:
>> +  call void @cold_function()
>> +  br label %exit
>> +
>> +exit:
>> +  call void @hot_function()
>> +  ret void
>> +}
>> +
>> +define void @test_hot_branch_triangle_profile(i32* %a) !prof !6 {
>> +; Test that a hot branch that has a probability a little larger than 80%
>> will
>> +; break triangle shaped CFG constrains when doing block placement if
>> profile
>> +; is present.
>> +; CHECK-LABEL: test_hot_branch_triangle_profile:
>> +; CHECK: %entry
>> +; CHECK: %exit
>> +; CHECK: %then
>> +
>> +entry:
>> +  %gep1 = getelementptr i32, i32* %a, i32 1
>> +  %val1 = load i32, i32* %gep1
>> +  %cond1 = icmp ugt i32 %val1, 1
>> +  br i1 %cond1, label %exit, label %then, !prof !5
>> +
>> +then:
>> +  call void @hot_function()
>> +  br label %exit
>> +
>> +exit:
>> +  call void @hot_function()
>> +  ret void
>> +}
>> +
>> +define void @test_hot_branch_triangle_profile_topology(i32* %a) !prof !6
>> {
>> +; Test that a hot branch that has a probability between 50% and 66% will
>> not
>> +; break triangle shaped CFG constrains when doing block placement if
>> profile
>> +; is present.
>> +; CHECK-LABEL: test_hot_branch_triangle_profile_topology:
>> +; CHECK: %entry
>> +; CHECK: %then
>> +; CHECK: %exit
>> +
>> +entry:
>> +  %gep1 = getelementptr i32, i32* %a, i32 1
>> +  %val1 = load i32, i32* %gep1
>> +  %cond1 = icmp ugt i32 %val1, 1
>> +  br i1 %cond1, label %exit, label %then, !prof !7
>> +
>> +then:
>> +  call void @hot_function()
>> +  br label %exit
>> +
>> +exit:
>> +  call void @hot_function()
>> +  ret void
>> +}
>> +
>>  !5 = !{!"branch_weights", i32 84, i32 16}
>> +!6 = !{!"function_entry_count", i32 10}
>> +!7 = !{!"branch_weights", i32 60, i32 40}
>>
>>
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>> llvm-commits at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>
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