Index: lib/CodeGen/MachineBlockPlacement.cpp =================================================================== --- lib/CodeGen/MachineBlockPlacement.cpp +++ lib/CodeGen/MachineBlockPlacement.cpp @@ -631,18 +631,46 @@ // 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 @@ (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 @@ // 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()) { Index: test/CodeGen/X86/block-placement.ll =================================================================== --- test/CodeGen/X86/block-placement.ll +++ test/CodeGen/X86/block-placement.ll @@ -1283,6 +1283,167 @@ 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% +; 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 89.9% probability because two in a row +; have a 88.89 % 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 897, i32 103} +!10 = !{!"branch_weights", i32 899, i32 101}