[LLVMdev] feedback on late machine combiner pass

[Daniil Troshkov]

Could you please provide a specific ll test and we will be able to consider
what happened in details using debugger


2014-06-03 2:41 GMT+04:00 Gerolf Hoflehner <ghoflehner at apple.com>:

> Hi,
>
> we noticed that combining instructions at the ISEL level can result in
> sub-optimal schedules, since an expensive instruction could be folded and
> lengthen the critical path. A simple illustrative example for this is
> folding mul-mul-add into mul-madd (fused multiply add), which serializes
> two multiplications instead of allowing them to execute in parallel.
> Unfortunately ISEL does not have the information necessary to avoid
> sub-optimial cases in all instances. The alternative is to post-pone
> instruction combining to a later pass and decide on whether the combined
> code sequence  performs better. This combiner pass would run at the machine
> IR level, use the machine trace model information,  and roughly implement
> the following architecture independent algorithm initially targeting
> mul-add/sub pattern:
>
> * Initial Instruction Combiner Algorithm
> 0. Recognize instruction patterns in a single basic block
> 1. For each add/sub check if it can be combined to madd/msub
> 2. For each pattern found, generate the alternative instruction sequence
> outside the basic block that contains mul - add/sub
> 3. Evaluate if the new pattern is more efficient:
> a) In Os substitute when the new pattern has fewer instructions
> b) Otherwise substitute when neither critical path nor resource length
> increases based on the machine trace model
>
> Do you have any concerns? Or is this the pass you always wanted? :-)
>
> Cheers
> Gerolf
>
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