[fpc-devel] Do type-helpers block "regvar" opt?

[Martin Frb]

On 29/09/2025 14:17, Sven Barth via fpc-devel wrote:
> Martin Frb via fpc-devel <fpc-devel at lists.freepascal.org> schrieb am 
> Mo., 29. Sep. 2025, 12:30:
>
>     Tested with 3.3.1  d3ac07ad27
>     -O4
>     Win 64 bit
>
>     In the code below, compiled with -alr
>         .section .text.n_p$project1_$$_foo,"ax"
>         ....
>         # Var a located in register rbx
>     (and it stays in rbx, until the call to write is prepared)
>
>
>
>     But if I comment out the "write", and uncomment the "a.w"
>         # Var a located at rsp+32, size=OS_64
>
>     Even the initial 2 lines will then NOT use a register for "a"?
>
>
> The Self parameter of a type helper is a pointer, so the compiler 
> needs to be able to take the address which in turn prohibits the use 
> of the regvar optimization for that variable.
>

Ok, I see (well mostly....)

Does that need to affect (all the) code in which the pointer is used as 
just a pointer?

I mean, ok, (if) for the last statement (where the type helper is 
called) it needs to be on the stackframe, then the variable could be 
stored then. (and there after)

But for section of the code where it isn't using the type helper, a copy 
could be in  a register? => afaik if there are more vars than registers, 
then one register can be used for more than one var, just in different 
places? So then vars are swapped between frame and register?


Well, ok, the type helper could store a copy of self (pointer to the 
pointer) somewhere, and that could be accessed.
But then
- any code before the first call to the type helper should be safe to 
optimize
- code after would be tricky, as anything could be that copied pointer  
(maybe when "pure" functions exists, and the type helper method is pure)

It also raises the question (I could test that, but...) what happens if 
I take a pointer of any other local var => will the compiler see that, 
and prevent such a local var from being optimised to a register?


Anyway, not important to change. Just important for me to know (so I 
know I am not trying to get something to work, that can't work)
Thanks for the explanation.
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