Re: [aarch64] union{float16_t;uint16}

[Mason <slash dot tmp at free dot fr>]

On 14/01/2017 02:22, Leo wrote:

> #include "arm_neon.h"
> #include <stdio.h>
> 
> typedef union {
>        float16_t m1;
>        unsigned short m2;
> } MYUNION;
> 
> int main(void)
> {
>        MYUNION U1;
>        U1.m1 = 5;
>        printf("m2 = 0x%x \n",U1.m2);
>        return 0;
> }

What representation for (float16_t)5 were you expecting?

Giving the code a spin...

$ arm-linux-gnueabihf-gcc foo.c 
In file included from foo.c:1:0:
/tmp/gcc-linaro-5.3-2016.02/lib/gcc/arm-linux-gnueabihf/5.3.1/include/arm_neon.h:31:2:
	error: #error You must enable NEON instructions (e.g. -mfloat-abi=softfp -mfpu=neon) to use arm_neon.h
 #error You must enable NEON instructions (e.g. -mfloat-abi=softfp -mfpu=neon) to use arm_neon.h
  ^
foo.c:5:8: error: unknown type name 'float16_t'
        float16_t m1;
        ^

$ arm-linux-gnueabihf-gcc -mfloat-abi=softfp -mfpu=neon foo.c 
In file included from /tmp/gcc-linaro-5.3-2016.02/arm-linux-gnueabihf/libc/usr/include/features.h:389:0,
                 from /tmp/gcc-linaro-5.3-2016.02/arm-linux-gnueabihf/libc/usr/include/stdint.h:25,
                 from /tmp/gcc-linaro-5.3-2016.02/lib/gcc/arm-linux-gnueabihf/5.3.1/include/stdint.h:9,
                 from /tmp/gcc-linaro-5.3-2016.02/lib/gcc/arm-linux-gnueabihf/5.3.1/include/arm_neon.h:38,
                 from foo.c:1:
/tmp/gcc-linaro-5.3-2016.02/arm-linux-gnueabihf/libc/usr/include/gnu/stubs.h:7:29:
	fatal error: gnu/stubs-soft.h: No such file or directory
compilation terminated.

Hmmm, looks like my toolchain doesn't support float16_t


https://gcc.gnu.org/onlinedocs/gcc/Half-Precision.html

Let's try __fp16

typedef union {
       __fp16 m1;
       unsigned short m2;
} MYUNION;

$ arm-linux-gnueabihf-gcc -S -O2 -mfp16-format=alternative foo.c

main:
	movw	r0, #:lower16:.LC0
	push	{r3, lr}
	mov	r1, #17664
	movt	r0, #:upper16:.LC0
	bl	printf
	movs	r0, #0
	pop	{r3, pc}

NB: I'm targeting armv7-a, while you're targeting armv8-a
(I missed the [aarch64] tag in the Subject. It's a good idea to repeat
important information in the body.)

Sending anyway FWIW.

Regards.