[llvm-commits] [PATCH] Preserve branch weight metadata when creating switches in SimplifyCFG

[Manman Ren]

Hi Alastair,

Thanks for the patch. 

Some of the logic is not clear to me. 
For example, in FoldValueComparisionIntoPredecessors, if PredDefault is not equal to BB:
P --> 1 --> S1                           (weightP_1)
          2 --> BB                           (weightP_2)
          3 --> BB                           (weightP_3)
          default -> PredDefault  (weightP_0)
BB -> 2 --> S2                       (weightB_1)
           4 --> S3                       (wegihtB_2)
           default -> BBDefault (weightB_0)

We will create a new switch for P:
P --> 1 --> S1                              (weightP_1)
          2 --> S2                              (weightP_2)
          3 --> default                      (weightP_3)
          default -> PredDefault    (weightP_0)
         
But from you patch, I think we will get
P --> 1 --> S1                              (weightP_1)
          2 --> S2                              (weightB_2)
          3 --> default                      (0)
          default -> PredDefault    (not sure, it seems that you are updating DefaultWeight inside a loop)

The case where PredDefault is equal to BB is more complicated:
P --> 1 --> BB                           (weightP_1)
          2 --> S1                           (weightP_2)
          3 --> S2                           (weightP_3)
          default -> BB                 (weightP_0)
BB -> 2 --> S3                       (weightB_1)
           4 --> S4                       (weightB_2)
           default -> BBDefault (weightB_0)
We will create:
P --> 2 --> S1                       (weightP_2*(weightB_0 + weightB_2))
          3 --> S2                       (weightP_3*(weightB_0 + weightB_2))
          4 --> S4                       ((weightP_1+weightP_0)*weightB_2)
          default -> BBDefault    ((weightP_1+weightP_0)*weightB_0)

I may be wrong in what the weight should be, since it is just complicated :)
You can look at how we update the metadata in FoldBranchToCommonDest if you have not done so.

I also think we should have some utilities for fixed-point arithmetic	such as (APInt with a scale factor) to avoid handling overflow all the time.

Thanks,
Manman

On Aug 17, 2012, at 11:45 AM, Alastair Murray wrote:

> Hello all,
> 
> Evan Cheng is CC'ed because he originally suggested I look at this issue, Manman Ren is CC'ed because he has expressed interest in this topic.
> 
> Please review the attached patch. Note: I don't have commit rights.
> 
> The patch preserves branch weight metadata in SimplifyCFG in various situations where switch instructions are inserted (converting linked if-statements or icmp's to a switch instruction, merging switch instructions, merging an if-statement into a switch instruction).
> 
> As metadata can not be assumed to be consistent with the current form of the IR the added code only preserves metadata if it seems consistent.
> 
> It has been tested with both profiling data (over projects/test-suite) and __builtin_expect.  In the case of __builtin_expect the branch weights are scaled when creating switch instructions to account for the different probability model.  if (C1) { A } else if (C2) { B} means P(B) = P(C2 | !C1), but a switch statement just models the probability as P(C2).
> 
> Regards,
> Alastair Murray
> <preserve_switch_branch_metadata.patch>