On Wed, Aug 31, 2022 at 12:00:14PM -0500, Peter Bergner wrote: > On 8/31/22 11:05 AM, Segher Boessenkool wrote: > > On Wed, Aug 31, 2022 at 10:48:26AM -0500, Peter Bergner wrote: > >> Ditto for -msoft-float better disable any -maltivec and -mvsx, etc. > > > > Oh? Why should it disable -maltivec? -mvsx makes a little sense on > > one hand, but totally none on the other either. > > VSX has to be disabled, since VSX replies on the FP registers existing.
It doesn't? On a CPU supporting VSX the FP registers are overlaid on the VSX registers, not the other way around. GCC says cc1: warning: '-mvsx' requires hardware floating point and that's okay with me of course, but it doesn't say why it is required. Implementation convenience coupled with lack of a use case is my best guess :-) OTOH VMX and hard float are completely orthogonal (the VMX FP things do not even use the fpscr for example). > As for Altivec, I'm pretty sure there are some inherent dependencies > there, probably both in hardware and our GCC backend implementation. > I could be wrong, but my guess is things will fall over the ground > if as allow -maltivec along with -msoft-float. Does the linux kernel > only build with -msoft-float assuming it disables altivec and vsx? > Or does it explicitly always also add -mno-altivec? No. Instead, it just works! Try this: === typedef float vf __attribute__((vector_size(16))); vf f(float x) { x *= 42; return (vf){x, x, x, x}; } === with -maltivec -msoft-float. It does not use the FPRs, and it does use the VMX registers and VMX instructions. > So in linux*.h, we have the following which came from a 2004 commit from Alan: > > linux64.h:#define OS_MISSING_POWERPC64 !TARGET_64BIT That macro returns 1 on OSes that do not properly support -mpowerpc64. > ...so I think there was no real reason, other than old 64-bit linux kernels > didn't > save the upper register state in 32-bit mode binaries. And it is still not saved in 32-bit user mode (setjmp/longjmp and getcontext/setcontext). Most bigger programs will fail, and most smaller programs (including everything in the GCC testsuite) work fine. But we should not enable -mpowerpc64 on 32-bit Linux by default. Segher