https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98519
--- Comment #21 from Michael Meissner <meissner at gcc dot gnu.org> --- I have patches that fix the problem in the hook. My original idea of not allowing prefixed insns in the hook doesn't work because when the hook is called, it only sees pseudo registers. So what my patch does instead is to change the constraints. It makes 3 new constraints: em -- Like m, but do not allow prefixed memory eo -- Like o, but do not allow prefixed memory ep -- Only accept prefixed memory Now, 'ep' isn't needed, but I added it to give people a way to write asm using prefixed instructions. If 'eo' and 'em' are ok, but we don't want to add 'ep', that is fine, it is simple to remove. The hook then goes through the constraints, and changes 'm' to 'em', and 'o' to 'eo'. That way old memory asm's won't see prefixed addresses. If the register allocator sees a prefixed address, it takes the address using 'pla' and substitutes a base register. In know in general, the preference is not to add new constraints, but I feel by adding the constraints, it makes it to be a simpler problem that writing a new RTL pass or adding new target hooks.