[fpc-devel] Bit manipulation as optimization in FPC

[Florian Klaempfl]

Daniël Mantione schrieb:
> 
> Op Wed, 6 Sep 2006, schreef ik:
> 
>> Hello,
>>
>> I wonder if FPC optimize a code such as:
>>
>> if a < b then
>>  c := d
>> else
>> c := b;
>>
>> as :
>>
>> r := b + ((d - b) and -(a < b));
>>
>> If so, do you check the type of CPU (because as I understand, some CPU
>> will not execute as fast as other CPU's.
>>
>>
>> If not, then why, and how would you optimize such code ?
> 
> The construction above would be fastest with conditional moves; in the 
> above construction the optimal code generation would be:
> 
> mov   c,d
> cmp   a,b
> movge c,b
> 
> This would be much faster than any code generated for the construction you 
> propose.
> 
> Free Pascal won't generate the optimal code, but it can already use 
> conditional moves and I would expect that the compiler would use it in 
> this situation.

It does, if the reg. allocator starts correctly :)

> 
> Daniël
> 
> 
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