Question

I have an inline AT&T style assembly block, which works with XMM registers and there are no problems in Release configuration of my XCode project, however I’ve stumbled upon this strange error (which is supposedly a GCC bug) in Debug configuration… Can I fix it somehow? There is nothing special in assembly code, but I am using a lot of memory constraints (12 constraints), can this cause this problem?

Answer 1

Not a complete answer, sorry, but the comments section is too short for this …

Can you post a sample asm("..." :::) line that demonstrates the problem ?

The use of XMM registers is not the issue, the error message indicates that GCC wanted to create code like, say:

movdqa (%rax),%xmm0

i.e. memory loads/stores through pointers held in general registers, and you specified more memory locations than available general-purpose regs (it’s probably 12 in debug mode because because RBP, RSP are used for frame/stackpointer and likely RBX for the global offset table and RAX reserved for returns) without realizing register re-use potential.

You might be able to eek things out by doing something like:

void *all_mem_args_tbl[16] = { memarg1, memarg2, ... };
void *trashme;

asm ("movq (%0), %1\n\t"
     "movdqa (%1), %xmm0\n\t"
     "movq 8(%0), %1\n\t"
     "movdqa (%1), %xmm1\n\t"
     ...
     : "r"all_mem_args_tbl : "r"(trashme) : ...);

i.e. put all the mem locations into a table that you pass as operand, and then manage the actual general-purpose register use on your own. It might be two pointer accesses through the indirection table, but whether that makes a difference is hard to say without knowing your complete assembler code piece.