在GCC内联汇编中使用数组元素作为输出存储约束时，为什么需要指定“内存”破坏者？

Question

I'm trying to do some asm level (MIPS with some DSP extentions) optimizations of an audio codec. There is some DSP processing involved after which I'm coming to the point the result needs to be stored back into an array. Here's a code I thought it should do it:

asm(
    eDSP_MFLO(8, 1) // move the accumulated result to $8
    "sw $8, %0\n"   // result => array
    : "=m"(s[i])
    :: "$8"
);

The problem is this code works or not (I get junk in the array when it doesn't), depending on its surrounding code, unless I add "memory" to the clobber list:

asm(
    eDSP_MFLO(8, 1) // move the accumulated result to $8
    "sw $8, %0\n"   // result => array element s[i]
    : "=m"(s[i])
    :: "$8", "memory"
);

I'm having hard times understanding why it is necessary. I wouldn't question it if I calculated the offset into array in the asm block myself so that the compiler didn't know which memory addresses have been changed, but since GCC is doing these steps by itself why does it require the extra "memory" clobber?

Answer 1

Take a look at the generated assembly code for the whole function both with and without the "memory" constraint (use gcc -S ... ). It looks like gcc has a copy of s[i] in a register that was loaded from memory before the asm() statement, and it doesn't realize that that register contains out of date information after the asm() statement unless you add the "memory" constraint.