Error assembly code generated with Os and armv6-m option

[liusishuang]

toolchain:
arm-none-eabi-gcc (GNU Tools for Arm Embedded Processors 7-2018-q2-update) 7.3.1 20180622 (release) [ARM/embedded-7-branch revision 261907]
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

C code as follows(also in attachment):

struct time_stamp {
        unsigned int seconds : 16;
        unsigned int minutes : 16;
};

unsigned int rtc_read(void)
{
        unsigned int val = 0xffffffff;
        struct time_stamp *ts = (struct time_stamp *)&val;

        ts->minutes = 0;
        /* value come from register */
        ts->seconds = *(unsigned int *)0x20000000;

        return val;
}

Use command:
arm-none-eabi-gcc -S -mthumb -march=armv6-m -Os error_code.c

Assembly coed:
        .arch armv6-m
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 1
        .eabi_attribute 30, 4
        .eabi_attribute 34, 0
        .eabi_attribute 18, 4
        .file "error_code.c"
        .text
        .align 1
        .global rtc_read
        .syntax unified
        .code 16
        .thumb_func
        .fpu softvfp
        .type rtc_read, %function
rtc_read:
        @ args = 0, pretend = 0, frame = 8
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        movs r2, #0
        sub sp, sp, #8
        add r3, sp, #4
        strh r2, [r3, #2]
        movs r2, #128
        lsls r2, r2, #22
        ldr r2, [r2]
        ldr r0, [sp, #4]
        strh r2, [r3]
        add sp, sp, #8
        @ sp needed
        bx lr
        .size rtc_read, .-rtc_read
        .ident "GCC: (GNU Tools for Arm Embedded Processors 7-2018-q2-update) 7.3.1 20180622 (release) [ARM/embedded-7-branch revision 261907]"

In assembly code, r0 was assigned befor "strh r2. [r3]". As r0 is return value, it causee ts->sencods remain 0xffff when it supposed to be value in address 0x20000000.

If -march=armv6-m change to -march=armv7-m, or use last toolchain release version(gcc-arm-none-eabi-9-2019-q4-major), this problem won't happen again.