[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 created this revision.
congh added reviewers: djasper, davidxl.
congh added a subscriber: llvm-commits.

Consider the following diamond CFG:

```
  A
 / \
 B C
  \/
  D
```

Suppose A->B and A->C have probabilities 81% and 19%. In block-placement, A->B is called a hot edge and the final placement should be ABDC. However, the current implementation outputs ABCD. This is because when choosing the next block of B, it checks if Freq(C->D) > Freq(B->D) * 20%, which is true (if Freq(A) = 100, then Freq(B->D) = 81, Freq(C->D) = 19, and 19 > 81*20%=16.2). Actually, we should use 25% instead of 20% as the probability here, so that we have 19 < 81*25%=20.25, and the desired ABDC layout will be generated.



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
@@ -415,8 +415,14 @@
 
       // 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.
+      const BranchProbability ColdProb(1, 4); // 25%
       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)) ||


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D12778.34496.patch
Type: text/x-patch
Size: 2602 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150910/667fdf54/attachment.bin>