Functions that are noreturn will needlessly preserve LR and needlessly return

[Gary Fuehrer]

GCC ARM embedded is generating poor assembly for this use-case:

/* Declaration 'int main(void)' is built-into ansi C compilers */

void Reset_Handler() {
    <any not-need-LR code and calls>
    main();
 G: goto G; /* or call any noreturn method */
}

And this should generate exactly the same assembly:

void __attribute__((noreturn)) Reset_Handler() {
    <any not-need-LR code and calls>
    main();
    __asm ("0: b.n 0b"); /* or halt or whatever */
}

Specifically it should generate:

<Reset_Handler>:
    <any not-need-LR code and calls>
    bl <main>
0: b.n 0b

Instead, the generated assembly is preserving LR:

<Reset_Handler>:
    push {r3, lr}
    <any not-need-LR code and calls>
    bl <main>
0: b.n 0b

And, in the latter case, it is also emitting a return:

<Reset_Handler>:
    push {r3, lr}
    <any not-need-LR code and calls>
    bl <main>
0: b.n 0b
    pop {r3, pc}