我在c中编写了一个非常简单的memset，它在-O2之前都可以正常工作，但在-O3下却不能工作...
记忆集：
void * memset(void * blk, int c, size_t n)
{
unsigned char * dst = blk;
while (n-- > 0)
*dst++ = (unsigned char)c;
return blk;
}
...在使用-O2时编译为该程序集：
20000430 <memset>:
20000430: e3520000 cmp r2, #0 @ compare param 'n' with zero
20000434: 012fff1e bxeq lr @ if equal return to caller
20000438: e6ef1071 uxtb r1, r1 @ else zero extend (extract byte from) param 'c'
2000043c: e0802002 add r2, r0, r2 @ add pointer 'blk' to 'n'
20000440: e1a03000 mov r3, r0 @ move pointer 'blk' to r3
20000444: e4c31001 strb r1, [r3], #1 @ store value of 'c' to address of r3, increment r3 for next pass
20000448: e1530002 cmp r3, r2 @ compare current store address to calculated max address
2000044c: 1afffffc bne 20000444 <memset+0x14> @ if not equal store next byte
20000450: e12fff1e bx lr @ else back to caller
这对我来说很有意义。我注释了这里发生的事情。
当我用-O3编译时，程序崩溃。我的内存集反复调用自己，直到它吞噬了整个堆栈：
200005e4 <memset>:
200005e4: e3520000 cmp r2, #0 @ compare param 'n' with zero
200005e8: e92d4010 push {r4, lr} @ ? (1)
200005ec: e1a04000 mov r4, r0 @ move pointer 'blk' to r4 (temp to hold return value)
200005f0: 0a000001 beq 200005fc <memset+0x18> @ if equal (first line compare) jump to epilogue
200005f4: e6ef1071 uxtb r1, r1 @ zero extend (extract byte from) param 'c'
200005f8: ebfffff9 bl 200005e4 <memset> @ call myself ? (2)
200005fc: e1a00004 mov r0, r4 @ epilogue start. move return value to r0
20000600: e8bd8010 pop {r4, pc} @ restore r4 and back to caller
I can't figure out how this optimised version is supposed to work without any strb or similar. It doesn't matter if I try to set the memory to '0' or something else so the function is not only called on .bss (zero initialised) variables.
（1）这是一个问题。当函数由于n为零而没有提前退出时，此推送会不断重复，而没有匹配的弹出消息，如（2）所称。我用uart打印验证了这一点。同样，r2从未被触及过，那么为什么零比较会变成真实？
请帮助我了解这里的情况。编译器是否假设我可能不满足的先决条件？
背景：我在裸机项目中使用需要memset的外部代码，所以我自己滚动了代码。它仅在启动时使用一次，对性能没有要求。
