evaluate whether DR should use gcc's switch to 16-byte stack alignment

From bruen...@google.com on July 17, 2012 11:57:04

gcc 4.5+ have violated the sysv x86 ABI that linux followed which mandates only 4-byte stack alignment, in order to use aligned SSE instrs. xref http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838 , http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38496 , http://www.uclibc.org/docs/psABI-i386.pdf .

this adds extra code and extra stack space. it's not clear such alignment is a win for DR: we should measure whether aligned SSE instrs outweigh increased code and stack. one factor is that we want binary compatibility w/ older machines, so we can't use really recent ISA instrs. if it's not a win we'd pass -mpreferred-stack-boundary=2.

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=847