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

[Alastair Murray]

Hello,

This is still being worked on (the complicated case is exactly that!).

Regards,
Alastair.

On 17/08/12 22:55, Manman Ren wrote:
>
> 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>
>
>