------- Comment #48 from mikulas at artax dot karlin dot mff dot cuni dot cz 2009-09-26 04:25 ------- It can be seen from the patch. I don't know how to detect that a structure contains an array with required SSE align, so I realign the stack for all types BLKmode with alignment >= 16. That may also catch structures containing long double with -m128bit-long-double and do unneeded align for them.
Another point where it aligns and doesn't have to is: there are some SSE variables used --- that trigger frame generation to prepare for a possible align --- after register allocation, they are not spilled, so the alignment is not needed --- but there are some other aligned types on the stack (for example floating point; they do not need enforced alignment). Then, my patch simply realigns the stack. I think the patch is a good hack that may be added to 4.4 so that Gentoo people stop whining that gcc -O3 is unstable. But for 4.5, the stack realign needs to be redesigned. There are other cases ( PR/40667 ) where it triggers stack alignment that is not needed. As I said in comment #3: introduce preferred and enforced alignment for all types will do the right thing. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838
