Re: Tr : [redundency elimination, code motion, commun expression elimination] GCC optimizations

[Jeff Law <law at redhat dot com>]

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On 09/23/11 09:53, Jeff Kenton wrote:
> On 09/23/2011 11:16 AM, Ian Lance Taylor wrote:
>> David Brown<david@westcontrol.com>  writes:
>> 
>>>> It's the same for
>>>> 
>>>> if (x == 1)      { a=b; c=d; e=f; foo(); } else if (x == 2) {
>>>> a=b; c=d; e=f; foo(); }
>>>> 
>>>> GCC don't factor out the common part
>>>> 
>>> You are right - which is odd, since there is no reason why it
>>> could not (unlike the original case where there is no "else").
>>> Surely this would count as a significant missed optimisation,
>>> especially for big switch() code.
>> I don't think it is a significant missed optimization, as people
>> rarely write code like that.  They normally write
>> 
>> if (x == 1 || x == 2) { a=b; c=d; e=f; foo(); }
>> 
>> The proposed optimization only applies when somebody has
>> laboriously written out the exact same sequence of code twice.
>> 
>> I'm not opposed to such an optimization if it works reliably and
>> is not too expensive.  I just don't think it will make much
>> difference on ordinary code.
>> 
>> Ian
> 
> People don't write that kind of code (we hope), but machine
> generated code can have all kinds of weirdness.  Still not a big
> deal, but it might be more common than you think.
The tail merging code in the RTL optimizers might pick this stuff up
and Tom's work on block merging might as well.  It'd be worth looking
into why they aren't triggering.

jeff
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