[llvm] r200262 - PGO branch weight: keep halving the weights until they can fit into

[Manman Ren]

Author: mren
Date: Mon Jan 27 17:39:03 2014
New Revision: 200262

URL: http://llvm.org/viewvc/llvm-project?rev=200262&view=rev
Log:
PGO branch weight: keep halving the weights until they can fit into
uint32.

When folding branches to common destination, the updated branch weights
can exceed uint32 by more than factor of 2. We should keep halving the
weights until they can fit into uint32.

Modified:
    llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
    llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll

Modified: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=200262&r1=200261&r2=200262&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp Mon Jan 27 17:39:03 2014
@@ -748,21 +748,22 @@ static void GetBranchWeights(TerminatorI
   }
 }
 
-/// Sees if any of the weights are too big for a uint32_t, and halves all the
-/// weights if any are.
+/// Keep halving the weights until all can fit in uint32_t.
 static void FitWeights(MutableArrayRef<uint64_t> Weights) {
-  bool Halve = false;
-  for (unsigned i = 0; i < Weights.size(); ++i)
-    if (Weights[i] > UINT_MAX) {
-      Halve = true;
-      break;
-    }
+  while (true) {
+    bool Halve = false;
+    for (unsigned i = 0; i < Weights.size(); ++i)
+      if (Weights[i] > UINT_MAX) {
+        Halve = true;
+        break;
+      }
 
-  if (! Halve)
-    return;
+    if (! Halve)
+      return;
 
-  for (unsigned i = 0; i < Weights.size(); ++i)
-    Weights[i] /= 2;
+    for (unsigned i = 0; i < Weights.size(); ++i)
+      Weights[i] /= 2;
+  }
 }
 
 /// FoldValueComparisonIntoPredecessors - The specified terminator is a value

Modified: llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll?rev=200262&r1=200261&r2=200262&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll (original)
+++ llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll Mon Jan 27 17:39:03 2014
@@ -338,6 +338,32 @@ c:
  ret void
 }
 
+;; When folding branches to common destination, the updated branch weights
+;; can exceed uint32 by more than factor of 2. We should keep halving the
+;; weights until they can fit into uint32.
+ at max_regno = common global i32 0, align 4
+define void @test14(i32* %old, i32 %final) {
+; CHECK-LABEL: @test14
+; CHECK: br i1 %or.cond, label %for.exit, label %for.inc, !prof !10
+for.cond:
+  br label %for.cond2
+for.cond2:
+  %i.1 = phi i32 [ %inc19, %for.inc ], [ 0, %for.cond ]
+  %bit.0 = phi i32 [ %shl, %for.inc ], [ 1, %for.cond ]
+  %tobool = icmp eq i32 %bit.0, 0
+  br i1 %tobool, label %for.exit, label %for.body3, !prof !10
+for.body3:
+  %v3 = load i32* @max_regno, align 4
+  %cmp4 = icmp eq i32 %i.1, %v3
+  br i1 %cmp4, label %for.exit, label %for.inc, !prof !11
+for.inc:
+  %shl = shl i32 %bit.0, 1
+  %inc19 = add nsw i32 %i.1, 1
+  br label %for.cond2
+for.exit:
+  ret void
+}
+
 !0 = metadata !{metadata !"branch_weights", i32 3, i32 5}
 !1 = metadata !{metadata !"branch_weights", i32 1, i32 1}
 !2 = metadata !{metadata !"branch_weights", i32 1, i32 2}
@@ -348,6 +374,8 @@ c:
 !7 = metadata !{metadata !"branch_weights", i32 33, i32 9, i32 8, i32 7}
 !8 = metadata !{metadata !"branch_weights", i32 33, i32 9, i32 8}
 !9 = metadata !{metadata !"branch_weights", i32 7, i32 6}
+!10 = metadata !{metadata !"branch_weights", i32 672646, i32 21604207}
+!11 = metadata !{metadata !"branch_weights", i32 6960, i32 21597248}
 
 ; CHECK: !0 = metadata !{metadata !"branch_weights", i32 5, i32 11}
 ; CHECK: !1 = metadata !{metadata !"branch_weights", i32 1, i32 5}
@@ -359,4 +387,6 @@ c:
 ; CHECK: !7 = metadata !{metadata !"branch_weights", i32 17, i32 9, i32 8, i32 7, i32 17}
 ; CHECK: !8 = metadata !{metadata !"branch_weights", i32 24, i32 33}
 ; CHECK: !9 = metadata !{metadata !"branch_weights", i32 8, i32 33}
-; CHECK-NOT: !9
+;; The false weight prints out as a negative integer here, but inside llvm, we
+;; treat the weight as an unsigned integer.
+; CHECK: !10 = metadata !{metadata !"branch_weights", i32 112017436, i32 -735157296}