Thanks for your detailedly reply & info Mason
1.I have missed a important infomation : big-endian, sorry ...
For little-endian,it's OK; but for big-endian ,it's still a random value.
2. For big-endian,my expecting value is 17664 (0x4500)
Leo
------------------ Original ------------------
From: "Mason";<slash.tmp@xxxxxxx>;
Date: Mon, Jan 16, 2017 05:36 PM
To: "GCC help"<gcc-help@xxxxxxxxxxx>;
Cc: "Leo"<81905678@xxxxxx>;
Subject: Re: [aarch64] union{float16_t;uint16}
On 16/01/2017 10:29, Mason wrote:
> 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.)
On https://godbolt.org/
Picking ARM64 gcc 5.4 -O2
Your code is translated to
main:
stp x29, x30, [sp, -16]!
adrp x1, .LC0
add x1, x1, :lo12:.LC0
mov w2, 17664
add x29, sp, 0
mov w0, 1
bl __printf_chk
mov w0, 0
ldp x29, x30, [sp], 16
ret
.LC0:
.string "m2 = 0x%x \n"
Same 17664 i.e. 0x4500
Regards.
