On 07/31/2017 08:04 AM, Florian Weimer wrote: > Fedora is considering getting rid of the i686 kernel. If that happens, > all i686 installations will be in fact x86-64, so they have SSE2. > > There has been an uncoordinated, breaking ABI change for i386 when the > stack alignment requirements were changed. A lot of software uses > 4-byte alignment, perhaps based on this recommendation from the GCC manual: Well, I think it's also the case that 4 byte alignment was baked into the x86 ABI eons ago -- getting everyone to update has been tough. We've certainly seen problems with other compilers leaving the stack in a mis-aligned state. > > '-mincoming-stack-boundary=NUM' > Assume the incoming stack is aligned to a 2 raised to NUM byte > boundary. If '-mincoming-stack-boundary' is not specified, the one > specified by '-mpreferred-stack-boundary' is used. > > On Pentium and Pentium Pro, 'double' and 'long double' values > should be aligned to an 8-byte boundary (see '-malign-double') or > suffer significant run time performance penalties. On Pentium III, > the Streaming SIMD Extension (SSE) data type '__m128' may not work > properly if it is not 16-byte aligned. > … > This extra alignment does consume extra stack space, and generally > increases code size. Code that is sensitive to stack space usage, > such as embedded systems and operating system kernels, may want to > reduce the preferred alignment to '-mpreferred-stack-boundary=2'. I doubt many folks actively use this -- however, it only takes a couple of mis-guided library developers using this option to create a world of pain. > > If we start compiling system libraries with SSE2 support enabled, we > must make sure that they do not assume the stack is aligned by than 4 > bytes. Would -mincoming-stack-boundary=2 do that? Uros would know for sure. > > Will GCC still maintain stack alignment if such code is called with a > properly aligned stack? (This is important so that callbacks can still > use SSE2 with the default ABI.) It's supposed to. My concern would be that most of the time a mis-aligned stack just works -- it's only when we see those key SSE2 instructions that it'll fault. So bugs in this support could stay latent for a long time. jeff
