* How to pack small type to big type without memory load/store ?
[not found] <d041b6dc-e39d-4b18-894a-35076cbaa4ba@Spark>
@ 2020-08-04 3:10 ` Jojo R
2020-08-04 6:23 ` Tadeus Prastowo
0 siblings, 1 reply; 2+ messages in thread
From: Jojo R @ 2020-08-04 3:10 UTC (permalink / raw)
To: gcc
Hi,
Form My ABI, float register is used by function call,
I want to pack float a and b into double v without any memory load/store,
The flowing is my demo:
Typedef union {
float ff[2];
double v;
} double_u;
Double pack_float (float a, float b) {
double_u tmp;
tmp.ff[0] = a;
tmp.ff[1] = b;
return tmp.v;
}
There is memory store in these statement:
tmp.ff[0] = a;
tmp.ff[1] = b;
Could someone give me some hints to avoid memory load/store ?
Jojo
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: How to pack small type to big type without memory load/store ?
2020-08-04 3:10 ` How to pack small type to big type without memory load/store ? Jojo R
@ 2020-08-04 6:23 ` Tadeus Prastowo
0 siblings, 0 replies; 2+ messages in thread
From: Tadeus Prastowo @ 2020-08-04 6:23 UTC (permalink / raw)
To: Jojo R; +Cc: gcc
On Tue, Aug 4, 2020 at 5:10 AM Jojo R <jiejie_rong@c-sky.com> wrote:
>
> Hi,
>
> Form My ABI, float register is used by function call,
> I want to pack float a and b into double v without any memory load/store,
>
> The flowing is my demo:
>
> Typedef union {
> float ff[2];
> double v;
> } double_u;
>
> Double pack_float (float a, float b) {
>
> double_u tmp;
>
> tmp.ff[0] = a;
> tmp.ff[1] = b;
>
> return tmp.v;
> }
>
> There is memory store in these statement:
>
> tmp.ff[0] = a;
> tmp.ff[1] = b;
>
> Could someone give me some hints to avoid memory load/store ?
Use inline assembly
(https://gcc.gnu.org/onlinedocs/gcc/Using-Assembly-Language-with-C.html)
because AFAIK, C/C++ cannot force its compiler to place a variable in
a register.
> Jojo
--
Best regards,
Tadeus
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[not found] <d041b6dc-e39d-4b18-894a-35076cbaa4ba@Spark>
2020-08-04 3:10 ` How to pack small type to big type without memory load/store ? Jojo R
2020-08-04 6:23 ` Tadeus Prastowo
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