On Wed, 5 Dec 2018 at 15:34, Roger Cuypers <rcuyp...@arcor.de> wrote:
>
> Greetings,
>
> I’m trying to have the Aarch64 gcc optimize a single function using the O3
> optimization in this manner:
>
> void __attribute__ ((optimize ("-O3", "-ftree-vectorize" )))
> However, when examining the assembler code, there is no trace of any
> optimization beyond the project default.
> The only way to successfully enforce the optimization is using -O3 in the gcc
> command line. Then the compiler produces ARM neon instructions.
>
> Do you know any issues regarding function level optimizations and the Aarch64
> gcc?
>
Hello,
Which version of GCC are you using?
I've just tried with gcc-arm-8.2, with this sample code:
==============================================
#define SIZE 100
int a[SIZE];
int b[SIZE];
//int myfunc() __attribute__ ((optimize ("-O3", "-ftree-vectorize"
)));
int myfunc()
{
int i;
int s=0;
for(i=0; i<SIZE; i++)
s+=a[i]+b[i];
return s;
}
==============================================
Compiled at -O2, the loop contains:
ldr w2, [x5, x1]
ldr w3, [x4, x1]
add x1, x1, 4
cmp x1, 400
add w2, w2, w3
add w0, w0, w2
If I un-comment the attribute line, I get:
ldr q0, [x2, x0]
ldr q2, [x1, x0]
add x0, x0, 16
cmp x0, 400
add v0.4s, v0.4s, v2.4s
add v1.4s, v1.4s, v0.4s
In addition, -ftree-vectorize is not needed, it's implied by -O3.
Thanks,
Christophe
> Regards
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