[fpc-devel] Generated code of constant parameters of double type

[Luiz Americo Pereira Camara]

Florian Klaempfl escreveu:
> Luiz Americo Pereira Camara schrieb:
>> Florian Klaempfl escreveu:
>>> Luiz Americo Pereira Camara schrieb:
>>>> Florian Klaempfl escreveu:
>>>>> Luiz Americo Pereira Camara schrieb:
>>>>>> My point was that this could be a missing optimization 
>>>>>> opportunity: pass by reference a 8 bytes parameter when the 
>>>>>> pointer size is 4. 
>>>>>
>>>>> Don't forget that this makes an extra memory access.
>>>>
>>>> I will do my question in a simpler way:
>>>>
>>>> Using constant parameters (add const keyword in front of an 
>>>> argument) is beneficial for record types with size > pointer size 
>>>> and for string types but not for integer and pointer types.
>>>
>>> No, this is not necessarily true. The extra indirect can be also 
>>> slower.
>>
>> For string types it will avoid incrementing and decrementing the ref 
>> count. See the test i did at 
>> http://lazarusroad.blogspot.com/2008/11/effect-of-using-constant-parameter-for.html 
>> and the code diff at http://www.geocities.com/camara_luiz/coddiff.htm
>>
>> For record (TPoint) the constant version generate less code. See the 
>> attached diff.
>
> Please check also the code in the callee ;) 

I already checked: is equal.

Luiz

# Temps allocated between ebp+0 and ebp+0
# [33] begin
    pushl    %ebp
    movl    %esp,%ebp
    call    FPC_INITIALIZEUNITS
# [34] x.x := 1;
    movl    $1,U_P$ASMCONSTPARAMETERREC_X
# [35] x.y := 2;
    movl    $2,U_P$ASMCONSTPARAMETERREC_X+4
# [36] ByValue(X);
    movl    $U_P$ASMCONSTPARAMETERREC_X,%eax
    call    P$ASMCONSTPARAMETERREC_BYVALUE$POINT
# [37] ByReference(X);
    movl    $U_P$ASMCONSTPARAMETERREC_X,%eax
    call    P$ASMCONSTPARAMETERREC_BYREFERENCE$POINT
# [38] end.
    call    FPC_DO_EXIT
    leave
    ret