How can I indicate that the memory *pointed* to by an inline ASM argument may be used?

0 votes
asked Jun 3, 2019 by beeonrope

Consider the following small function:

void foo(int* iptr) { iptr[10] = 1; __asm__ volatile ("nop"::"r"(iptr):); iptr[10] = 2;
}

Using gcc, this compiles to:

foo: nop mov DWORD PTR [rdi+40], 2 ret

Note in particular, that the first write to iptr, iptr[10] = 1 doesn't occur at all: the inline asm nop is the first thing in the function, and only the final write of 2 appears (after the ASM call). Apparently the compiler decides that it only needs to provide an up-to-date version of the value of iptr itself, but not the memory it points to.

I can tell the compiler that memory must be up to date with a memory clobber, like so:

void foo(int* iptr) { iptr[10] = 1; __asm__ volatile ("nop"::"r"(iptr):"memory"); iptr[10] = 2;
}

which results in the expected code:

foo: mov DWORD PTR [rdi+40], 1 nop mov DWORD PTR [rdi+40], 2 ret

However, this is too strong of a condition, since it tells the compiler all memory has to be written. For example, in the following function:

void foo2(int* iptr, long* lptr) { iptr[10] = 1; lptr[20] = 100; __asm__ volatile ("nop"::"r"(iptr):); iptr[10] = 2; lptr[20] = 200;
}

The desired behavior is to let the compiler optimize away the first write to lptr[20], but not the first write to iptr[10]. The "memory" clobber cannot achieve this because it means both writes have to occur:

foo2: mov DWORD PTR [rdi+40], 1 mov QWORD PTR [rsi+160], 100 ; lptr[10] written unecessarily nop mov DWORD PTR [rdi+40], 2 mov QWORD PTR [rsi+160], 200 ret

Is there some way to tell compilers accepting gcc extended asm syntax that the input to the asm includes the pointer and anything it can point to?

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