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.
Currently is not beneficial for double types and there's no way to
improve that. So when developing an interface that uses double types
does not matter using const or not. Now and in the future. Right?
It might matter for the optimizer in other situations.
I am developing an component that heavily uses double types. So for read
only arguments is better to use constant parameters?
For the record: Int64 has the same behavior as double regarding this issue.
Luiz
Title: WinMerge File Compare Report
| C:\temp\byvaluerec.txt |
| C:\temp\byrefrec.txt |
| .section .text.n_p$asmconstparameterrec_byvalue$point |
| .section .text.n_p$asmconstparameterrec_byreference$point |
| .balign 16,0x90 |
| .balign 16,0x90 |
| .globl P$ASMCONSTPARAMETERREC_BYVALUE$POINT |
| .globl P$ASMCONSTPARAMETERREC_BYREFERENCE$POINT |
| P$ASMCONSTPARAMETERREC_BYVALUE$POINT: |
| P$ASMCONSTPARAMETERREC_BYREFERENCE$POINT: |
| # Temps allocated between ebp-20 and ebp-12 |
| # Temps allocated between ebp-12 and ebp-12 |
| # [17] begin |
| # [25] begin |
| pushl %ebp |
| pushl %ebp |
| movl %esp,%ebp |
| movl %esp,%ebp |
| subl $20,%esp |
| subl $12,%esp |
| # Var V located at ebp-4 |
| # Var V located at ebp-4 |
| # Var i located at ebp-12 |
| # Var i located at ebp-12 |
| movl %eax,-4(%ebp) |
| movl %eax,-4(%ebp) |
| |
| # [26] i := V; |
| movl -4(%ebp),%edx |
| movl -4(%ebp),%edx |
| movl (%edx),%eax |
| movl (%edx),%eax |
| movl %eax,-20(%ebp) |
| |
| movl 4(%edx),%eax |
| |
| movl %eax,-16(%ebp) |
| |
| # [18] i := V; |
| |
| movl -20(%ebp),%eax |
| |
| movl %eax,-12(%ebp) |
| movl %eax,-12(%ebp) |
| movl -16(%ebp),%eax |
| movl 4(%edx),%eax |
| movl %eax,-8(%ebp) |
| movl %eax,-8(%ebp) |
| # [19] DoIt(i); |
| # [27] DoIt(i); |
| leal -12(%ebp),%eax |
| leal -12(%ebp),%eax |
| call P$ASMCONSTPARAMETERREC_DOIT$POINT |
| call P$ASMCONSTPARAMETERREC_DOIT$POINT |
| # [20] end; |
| # [28] end; |
| leave |
| leave |
| ret |
| ret |
_______________________________________________
fpc-devel maillist - [email protected]
http://lists.freepascal.org/mailman/listinfo/fpc-devel
