[llvm] r277187 - Codegen: MachineBlockPlacement Improve probability layout.

Philip Reames via llvm-commits llvm-commits at lists.llvm.org
Tue Aug 30 19:44:04 PDT 2016


Was there a phabricator review for this?  I want to catch up on the 
discussion.

On 07/29/2016 11:09 AM, Kyle Butt via llvm-commits wrote:
> Author: iteratee
> Date: Fri Jul 29 13:09:28 2016
> New Revision: 277187
>
> URL: http://llvm.org/viewvc/llvm-project?rev=277187&view=rev
> Log:
> Codegen: MachineBlockPlacement Improve probability layout.
>
> The following pattern was being layed out poorly:
>
>                A
>               / \
>              B   C
>             / \ / \
>            D   E   ? (Doesn't matter)
>
> Where A->B is far more likely than A->C, and prob(B->D) = prob(B->E)
>
> The current algorithm gives:
> A,B,C,E (D goes on worklist)
>
> It does this even if C has a frequency count of 0. This patch
> adjusts the layout calculation so that if freq(B->E) >> freq(C->E)
> then we go ahead and layout E rather than C. Fallthrough half the time
> is better than fallthrough never, or fallthrough very rarely. The
> resulting layout is:
>
> A,B,E, (C and D are in a worklist)
>
> Modified:
>      llvm/trunk/lib/CodeGen/MachineBlockPlacement.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=277187&r1=277186&r2=277187&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
> +++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Fri Jul 29 13:09:28 2016
> @@ -631,18 +631,46 @@ bool MachineBlockPlacement::hasBetterLay
>     // BB->Succ. This is equivalent to looking the CFG backward with backward
>     // edge: Prob(Succ->BB) needs to >= HotProb in order to be selected (without
>     // profile data).
> -
> +  // --------------------------------------------------------------------------
> +  // Case 3: forked diamond
> +  //       S
> +  //      / \
> +  //     /   \
> +  //   BB    Pred
> +  //   | \   / |
> +  //   |  \ /  |
> +  //   |   X   |
> +  //   |  / \  |
> +  //   | /   \ |
> +  //   S1     S2
> +  //
> +  // The current block is BB and edge BB->S1 is now being evaluated.
> +  // As above S->BB was already selected because
> +  // prob(S->BB) > prob(S->Pred). Assume that prob(BB->S1) >= prob(BB->S2).
> +  //
> +  // topo-order:
> +  //
> +  //     S-------|                     ---S
> +  //     |       |                     |  |
> +  //  ---BB      |                     |  BB
> +  //  |          |                     |  |
> +  //  |  Pred----|                     |  S1----
> +  //  |  |                             |       |
> +  //  --(S1 or S2)                     ---Pred--
> +  //
> +  // topo-cost = freq(S->Pred) + freq(BB->S1) + freq(BB->S2)
> +  //    + min(freq(Pred->S1), freq(Pred->S2))
> +  // Non-topo-order cost:
> +  // In the worst case, S2 will not get laid out after Pred.
> +  // non-topo-cost = 2 * freq(S->Pred) + freq(BB->S2).
> +  // To be conservative, we can assume that min(freq(Pred->S1), freq(Pred->S2))
> +  // is 0. Then the non topo layout is better when
> +  // freq(S->Pred) < freq(BB->S1).
> +  // This is exactly what is checked below.
> +  // Note there are other shapes that apply (Pred may not be a single block,
> +  // but they all fit this general pattern.)
>     BranchProbability HotProb = getLayoutSuccessorProbThreshold(BB);
>   
> -  // Forward checking. For case 2, SuccProb will be 1.
> -  if (SuccProb < HotProb) {
> -    DEBUG(dbgs() << "    Not a candidate: " << getBlockName(Succ) << " "
> -                 << "Respecting topological ordering because "
> -                 << "probability is less than prob treshold: "
> -                 << SuccProb << "\n");
> -    return true;
> -  }
> -
>     // Make sure that a hot successor doesn't have a globally more
>     // important predecessor.
>     BlockFrequency CandidateEdgeFreq = MBFI->getBlockFreq(BB) * RealSuccProb;
> @@ -653,11 +681,11 @@ bool MachineBlockPlacement::hasBetterLay
>           (BlockFilter && !BlockFilter->count(Pred)) ||
>           BlockToChain[Pred] == &Chain)
>         continue;
> -    // Do backward checking. For case 1, it is actually redundant check. For
> -    // case 2 above, we need a backward checking to filter out edges that are
> -    // not 'strongly' biased. With profile data available, the check is mostly
> -    // redundant too (when threshold prob is set at 50%) unless S has more than
> -    // two successors.
> +    // Do backward checking.
> +    // For all cases above, we need a backward checking to filter out edges that
> +    // are not 'strongly' biased. With profile data available, the check is
> +    // mostly redundant for case 2 (when threshold prob is set at 50%) unless S
> +    // has more than two successors.
>       // BB  Pred
>       //  \ /
>       //  Succ
> @@ -666,6 +694,8 @@ bool MachineBlockPlacement::hasBetterLay
>       //      i.e. freq(BB->Succ) > freq(BB->Succ) * HotProb + freq(Pred->Succ) *
>       //      HotProb
>       //      i.e. freq((BB->Succ) * (1 - HotProb) > freq(Pred->Succ) * HotProb
> +    // Case 1 is covered too, because the first equation reduces to:
> +    // prob(BB->Succ) > HotProb. (freq(Succ) = freq(BB) for a triangle)
>       BlockFrequency PredEdgeFreq =
>           MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
>       if (PredEdgeFreq * HotProb >= CandidateEdgeFreq * HotProb.getCompl()) {
>
> 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=277187&r1=277186&r2=277187&view=diff
> ==============================================================================
> --- llvm/trunk/test/CodeGen/X86/block-placement.ll (original)
> +++ llvm/trunk/test/CodeGen/X86/block-placement.ll Fri Jul 29 13:09:28 2016
> @@ -1283,6 +1283,174 @@ exit:
>     ret void
>   }
>   
> +declare void @a()
> +declare void @b()
> +
> +define void @test_forked_hot_diamond(i32* %a) {
> +; Test that a hot-branch with probability > 80% followed by a 50/50 branch
> +; will not place the cold predecessor if the probability for the fallthrough
> +; remains above 80%
> +; CHECK-LABEL: test_forked_hot_diamond
> +; CHECK: %entry
> +; CHECK: %then
> +; CHECK: %fork1
> +; CHECK: %else
> +; CHECK: %fork2
> +; 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 %then, label %else, !prof !5
> +
> +then:
> +  call void @hot_function()
> +  %gep2 = getelementptr i32, i32* %a, i32 2
> +  %val2 = load i32, i32* %gep2
> +  %cond2 = icmp ugt i32 %val2, 2
> +  br i1 %cond2, label %fork1, label %fork2, !prof !8
> +
> +else:
> +  call void @cold_function()
> +  %gep3 = getelementptr i32, i32* %a, i32 3
> +  %val3 = load i32, i32* %gep3
> +  %cond3 = icmp ugt i32 %val3, 3
> +  br i1 %cond3, label %fork1, label %fork2, !prof !8
> +
> +fork1:
> +  call void @a()
> +  br label %exit
> +
> +fork2:
> +  call void @b()
> +  br label %exit
> +
> +exit:
> +  call void @hot_function()
> +  ret void
> +}
> +
> +define void @test_forked_hot_diamond_gets_cold(i32* %a) {
> +; Test that a hot-branch with probability > 80% followed by a 50/50 branch
> +; will place the cold predecessor if the probability for the fallthrough
> +; falls below 80%
> +; The probability for both branches is 85%. For then2 vs else1
> +; this results in a compounded probability of 83%.
> +; Neither then2->fork1 nor then2->fork2 has a large enough relative
> +; probability to break the CFG.
> +; Relative probs:
> +; then2 -> fork1 vs else1 -> fork1 = 71%
> +; then2 -> fork2 vs else2 -> fork2 = 74%
> +; CHECK-LABEL: test_forked_hot_diamond_gets_cold
> +; CHECK: %entry
> +; CHECK: %then1
> +; CHECK: %then2
> +; CHECK: %else1
> +; CHECK: %fork1
> +; CHECK: %else2
> +; CHECK: %fork2
> +; 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 %then1, label %else1, !prof !9
> +
> +then1:
> +  call void @hot_function()
> +  %gep2 = getelementptr i32, i32* %a, i32 2
> +  %val2 = load i32, i32* %gep2
> +  %cond2 = icmp ugt i32 %val2, 2
> +  br i1 %cond2, label %then2, label %else2, !prof !9
> +
> +else1:
> +  call void @cold_function()
> +  br label %fork1
> +
> +then2:
> +  call void @hot_function()
> +  %gep3 = getelementptr i32, i32* %a, i32 3
> +  %val3 = load i32, i32* %gep2
> +  %cond3 = icmp ugt i32 %val2, 3
> +  br i1 %cond3, label %fork1, label %fork2, !prof !8
> +
> +else2:
> +  call void @cold_function()
> +  br label %fork2
> +
> +fork1:
> +  call void @a()
> +  br label %exit
> +
> +fork2:
> +  call void @b()
> +  br label %exit
> +
> +exit:
> +  call void @hot_function()
> +  ret void
> +}
> +
> +define void @test_forked_hot_diamond_stays_hot(i32* %a) {
> +; Test that a hot-branch with probability > 88.88% (1:8) followed by a 50/50
> +; branch will not place the cold predecessor as the probability for the
> +; fallthrough stays above 80%
> +; (1:8) followed by (1:1) is still (1:4)
> +; Here we use 90% probability because two in a row
> +; have a 89 % probability vs the original branch.
> +; CHECK-LABEL: test_forked_hot_diamond_stays_hot
> +; CHECK: %entry
> +; CHECK: %then1
> +; CHECK: %then2
> +; CHECK: %fork1
> +; CHECK: %else1
> +; CHECK: %else2
> +; CHECK: %fork2
> +; 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 %then1, label %else1, !prof !10
> +
> +then1:
> +  call void @hot_function()
> +  %gep2 = getelementptr i32, i32* %a, i32 2
> +  %val2 = load i32, i32* %gep2
> +  %cond2 = icmp ugt i32 %val2, 2
> +  br i1 %cond2, label %then2, label %else2, !prof !10
> +
> +else1:
> +  call void @cold_function()
> +  br label %fork1
> +
> +then2:
> +  call void @hot_function()
> +  %gep3 = getelementptr i32, i32* %a, i32 3
> +  %val3 = load i32, i32* %gep2
> +  %cond3 = icmp ugt i32 %val2, 3
> +  br i1 %cond3, label %fork1, label %fork2, !prof !8
> +
> +else2:
> +  call void @cold_function()
> +  br label %fork2
> +
> +fork1:
> +  call void @a()
> +  br label %exit
> +
> +fork2:
> +  call void @b()
> +  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}
> +!8 = !{!"branch_weights", i32 5001, i32 4999}
> +!9 = !{!"branch_weights", i32 85, i32 15}
> +!10 = !{!"branch_weights", i32 90, i32 10}
>
>
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