------- Comment #4 from mikulas at artax dot karlin dot mff dot cuni dot cz 2009-08-09 21:16 ------- Regparm changed between gcc 3.x -> 4.x (I remember it too painfully, I had to rewrite some assembler files). In 3.x, all arguments were incrementing register count, even if they were on stack, if you had (float f1, int i2, int i3), f1 was on stack, i2 was in EDX and i3 was in ECX. In 4.x it changed so that f1 is on stack, i2 is in EAX and i3 is in EDX. Similarly, (double f1, int i2, int i3) was (stack, ECX, stack) in 3.x and (stack, EAX, EDX) in 4.x.
If we change it so that structures are always in registers, it will be the least painful thing, because they already are in registers almost always (except few pathological cases, like struct containing only float or double). So it won't likely hurt too much, because few programmers rely on regparm(3) for external ABI, few programmers pass structures directly and few programmers declare structure with only one member. And the programmer will be hurt only if he does all these three things. If you want to change it to be consistent with the documentation (not with existing implementation) and pass structures always on stack, I wouldn't object against it. Just don't change it later. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41017
