[PATCH] D12778: Change the probability to check if a hot successor has a more important predecessor in block-placement.

[Cong Hou via llvm-commits]

congh updated this revision to Diff 37162.
congh added a comment.

Update the patch according to Daniel's comment.


http://reviews.llvm.org/D12778

Files:
  lib/CodeGen/MachineBlockPlacement.cpp
  test/CodeGen/AArch64/fast-isel-branch-cond-split.ll
  test/CodeGen/X86/block-placement.ll

Index: test/CodeGen/X86/block-placement.ll
===================================================================
--- test/CodeGen/X86/block-placement.ll
+++ test/CodeGen/X86/block-placement.ll
@@ -1083,3 +1083,35 @@
   %ret = phi i32 [ %val1, %then ], [ %val2, %else ]
   ret i32 %ret
 }
+
+declare void @hot_function()
+
+define void @test_hot_branch(i32* %a) {
+; Test that a hot branch that has a probability a little larger than 80% will
+; break CFG constrains when doing block placement.
+; CHECK-LABEL: test_hot_branch:
+; 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 !3
+
+then:
+  call void @hot_function()
+  br label %exit
+
+else:
+  call void @cold_function()
+  br label %exit
+
+exit:
+  call void @hot_function()
+  ret void
+}
+
+!3 = !{!"branch_weights", i32 84, i32 16}
Index: test/CodeGen/AArch64/fast-isel-branch-cond-split.ll
===================================================================
--- test/CodeGen/AArch64/fast-isel-branch-cond-split.ll
+++ test/CodeGen/AArch64/fast-isel-branch-cond-split.ll
@@ -19,8 +19,8 @@
 }
 
 ; CHECK-LABEL: test_and
-; CHECK:       cbz w0, {{LBB[0-9]+_2}}
-; CHECK:       cbnz w1, {{LBB[0-9]+_3}}
+; CHECK:       cbnz w0, {{LBB[0-9]+_2}}
+; CHECK:       cbz w1, {{LBB[0-9]+_1}}
 define i64 @test_and(i32 %a, i32 %b) {
 bb1:
   %0 = icmp ne i32 %a, 0
Index: lib/CodeGen/MachineBlockPlacement.cpp
===================================================================
--- lib/CodeGen/MachineBlockPlacement.cpp
+++ lib/CodeGen/MachineBlockPlacement.cpp
@@ -413,10 +413,19 @@
         continue;
       }
 
-      // Make sure that a hot successor doesn't have a globally more
-      // important predecessor.
+      // Make sure that a hot successor doesn't have a globally more important
+      // predecessor.
+      //
+      // We need to make sure a hot successor will be chosen, which has at least
+      // 80% probability. Considering a diamond CFG, and suppose two branches
+      // have 80% and 20% probabilities respectively. The ratio between
+      // probabilities of the cold edge and hot edge is 20/80, which is 25%.
+      // This is why we use 25% instead of 20% as the cold probability below.
+      assert(HotProb.getDenominator() == HotProb.getCompl().getDenominator());
+      const BranchProbability ColdProb(HotProb.getCompl().getNumerator(),
+                                       HotProb.getNumerator());
       BlockFrequency CandidateEdgeFreq =
-          MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
+          MBFI->getBlockFreq(BB) * SuccProb * ColdProb;
       bool BadCFGConflict = false;
       for (MachineBasicBlock *Pred : Succ->predecessors()) {
         if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||


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