与GCC内联组装的奇怪行为

Question

当在GCC中内联装配时，我发现自己定期添加空ASM块，以便在早期块中保持变量，例如：

asm("rcr $1,%[borrow];"
    "movq 0(%[b_],%[i],8),%%rax;"
    "adcq %%rax,0(%[r_top],%[i],8);"
    "rcl $1,%[borrow];"
    : [borrow]"+r"(borrow)
    : [i]"r"(i),[b_]"r"(b_.data),[r_top]"r"(r_top.data)
    : "%rax","%rdx");

asm("" : : "r"(borrow) : ); // work-around to keep borrow alive ...

.

奇怪的另一个例子是下面的代码在没有优化的情况下重大工作，但是使用-o3它seg-faults：

ulong carry = 0,hi = 0,qh = s.data[1],ql = s.data[0];
asm("movq 0(%[b]),%%rax;"
    "mulq %[ql];"
    "movq %%rax,0(%[sb]);"
    "movq %%rdx,%[hi];"
    : [hi]"=r"(hi)
    : [ql]"r"(ql),[b]"r"(b.data),[sb]"r"(sb.data)
    : "%rax","%rdx","memory");
for (long i = 1; i < b.size; i++)
{
    asm("movq 0(%[b],%[i],8),%%rax;"
        "mulq %[ql];"
        "xorq %%r10,%%r10;"
        "addq %%rax,%[hi];"
        "adcq %%rdx,%[carry];"
        "adcq $0,%%r10;"
        "movq -8(%[b],%[i],8),%%rax;"
        "mulq %[qh];"
        "addq %%rax,%[hi];"
        "adcq %%rdx,%[carry];"
        "adcq $0,%%r10;"
        "movq %[hi],0(%[sb],%[i],8);"
        "movq %[carry],%[hi];"
        "movq %%r10,%[carry];"
        : [carry]"+r"(carry),[hi]"+r"(hi)
        : [i]"r"(i),[ql]"r"(ql),[qh]"r"(qh),[b]"r"(b.data),[sb]"r"(sb.data)
        : "%rax","%rdx","%r10","memory");
}
asm("movq -8(%[b],%[i],8),%%rax;"
    "mulq %[qh];"
    "addq %%rax,%[hi];"
    "adcq %%rdx,%[carry];"
    "movq %[hi],0(%[sb],%[i],8);"
    "movq %[carry],8(%[sb],%[i],8);"
    : [hi]"+r"(hi),[carry]"+r"(carry)
    : [i]"r"(long(b.size)),[qh]"r"(qh),[b]"r"(b.data),[sb]"r"(sb.data)
    : "%rax","%rdx","memory");

.

我认为它必须符合它使用这么多寄存器的事实。在这里有一些我缺少的东西，或者是刚刚用gcc内联组装的寄存器分配吗？

Answer 1

What you're missing is that GCC's optimiser will assume that the only side-effect of an asm block is to change the output operands. If those operands are not subsequently used, it may assume that the asm block in unnecessary and can be deleted.

e.g. in your first example, if borrow is not subsequently used, it is free to assume that there is no point in including the asm block at all, because its only side-effect is to update a variable that is never used again. And in the second example, if hi and carry are not used again after the code you've shown, it will probably infer that it can delete pretty much everything!

You can tell GCC that your inline assembly blocks should not be deleted by writing asm volatile(...) instead of just asm(...).

For more detail about this, see http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html (about half-way down the page).