------- Comment #16 from mikulas at artax dot karlin dot mff dot cuni dot cz 2009-07-31 15:22 ------- H.J. Lu: No, you only have to align the stack in functions that do 16-byte SSE.
I mean this: there are two possible reasons for aligning the stack 1) improved performance (double, long double, MMX, 8-byte SSE) 2) avoid crashes (16-byte SSE) To solve the case 1), it is perfectly reasonable to not align the stack and simply make sure that every function subtracts the stack by multiple of 16-bytes. If, by chance, the code is called from some other code with unaligned stack (like in that obstack example), the user will only suffer lower performance, not crashes. It is legal to make floating point calculations from obstack allocation callback --- but it is not common. So we don't have to care about performance in this case. To solve the case 2), you need to manually realign the stack at function prologue. But there are few functions that use SSE in typical desktop/server environment, that's why I'm saying that it won't have big impact on performance. Anyway, if some scientist needs high 16-bit SSE performance, we can make a flag for him that avoids stack realign --- but don't compile typical desktop/server distribution with this flag, because there ARE cases where the stack is misaligned. That's what I'm proposing in comment #3: that every type has two alignments, preferred alignment and enforced alignment. Only "enforced alignment" will force stack realign. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838
