https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89334
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> --- If you are using inline asm, you need to know what you are doing. https://gcc.gnu.org/onlinedocs/gcc-8.2.0/gcc/Simple-Constraints.html#Simple-Constraints ‘r’ A register operand is allowed provided that it is in a general register. while q is: https://gcc.gnu.org/onlinedocs/gcc-8.2.0/gcc/Machine-Constraints.html#Machine-Constraints ‘q’ Any register accessible as rl. In 32-bit mode, a, b, c, and d; in 64-bit mode, any integer register. By using r constraint, you tell the compiler it is ok to allocate that value in any gpr register, so for 32-bit mode eax, ebx, ecx, edx, esi, edi, ebp (or, in theory esp, though that is fixed register). That would be ok if the assembly pattern used e.g. %k1 instead of %1. But as you want to use the %?l in there, you need to tell the compiler that it may only use the selected registers, otherwise it is a lottery. It can compile fine, if you are lucky and the compiler chooses the eax/ebx/ecx/edx registers, or it can fail the way it failed for you.
