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Re: [PATCH] widening_mul: Do cost check when propagating mult into plus/minus expressions


On 07/13/2011 04:34 PM, Richard Guenther wrote:
> On Wed, Jul 13, 2011 at 3:13 PM, Andreas Krebbel
> <krebbel@linux.vnet.ibm.com> wrote:
>> Hi,
>>
>> the widening_mul pass might increase the number of multiplications in
>> the code by transforming
>>
>> a = b * c
>> d = a + 2
>> e = a + 3
>>
>> into:
>>
>> d = b * c + 2
>> e = b * c + 3
>>
>> under the assumption that an FMA instruction is not more expensive
>> than a simple add.  This certainly isn't always true.  While e.g. on
>> s390 an fma is indeed not slower than an add execution-wise it has
>> disadvantages regarding instruction grouping.  It doesn't group with
>> any other instruction what has a major impact on the instruction
>> dispatch bandwidth.
>>
>> The following patch tries to figure out the costs for adds, mults and
>> fmas by building an RTX and asking the backends cost function in order
>> to estimate whether it is whorthwhile doing the transformation.
>>
>> With that patch the 436.cactus hotloop contains 28 less
>> multiplications than before increasing performance slightly (~2%).
>>
>> Bootstrapped and regtested on x86_64 and s390x.
> 
> Ick ;)

Just to defend myself I did not invent this. I basically copied it over from ivopts.

> Maybe this is finally the time to introduce target hook(s) to
> get us back costs for trees?  For this case we'd need two
> actually, or just one - dependent on what finegrained information
> we pass.  Choices:

Having another target hook for costs actually doesn't look that much better to me.
Duplicating the logic is not really practical for backend developers.  We rather should
have a generic interface for calculating tree costs based on the rtx_cost hook.

Bye,

-Andreas-


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