[PATCH] D19674: [SimplifyCFG] propagate branch metadata when creating select

[Sanjay Patel via llvm-commits]

spatel added inline comments.

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Comment at: test/Transforms/SimplifyCFG/preserve-branchweights.ll:422
@@ -421,3 +421,3 @@
 ; CHECK-NEXT:    [[BRMERGE:%.*]] = or i1 %cmpb, %cmpa
-; CHECK-NEXT:    [[DOTMUX:%.*]] = select i1 %cmpb, i32 0, i32 2
+; CHECK-NEXT:    [[DOTMUX:%.*]] = select i1 %cmpb, i32 0, i32 2, !prof !11
 ; CHECK-NEXT:    [[OUTVAL:%.*]] = select i1 [[BRMERGE]], i32 [[DOTMUX]], i32 1, !prof !12
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davidxl wrote:
> spatel wrote:
> > davidxl wrote:
> > > If I read the code correctly, should the weights for select be 9:5 ? !11 does not look correct.
> > I thought the weights for the select would be equal to the weights of the original branch because they have the same comparison condition (%cmpb), 3:5. How do you calculate 9:5?
> Should it be 3 : 5 * (1/3)  which 9 : 5?  When block 2 is entered, there is only 1/3 probability it reaches block3.
Aha - yes. Although it's the same comparison operand, we have changed the program structure - we are speculatively executing the select, so the original branch weights no longer apply.

This brings up my question from the original summary then - do you know if there's a reason not to use the BranchProbability class for calculating the new weights? 



http://reviews.llvm.org/D19674