On Fri, Jun 6, 2008 at 4:28 PM, H.J. Lu <[EMAIL PROTECTED]> wrote: > On Fri, Jun 06, 2008 at 06:50:26AM -0700, H.J. Lu wrote: >> On Fri, Jun 06, 2008 at 10:28:34AM +0200, Jan Hubicka wrote: >> > > >> > > ymm0 and xmm0 are the same register. xmm0 is the lower 128bit >> > > of xmm0. I am not sure if we need separate XMM registers from >> > > YMM registers. >> > >> > >> > Yes, I know that xmm0 is lower part of ymm0. I still think we ought to >> > be able to support varargs that do save ymm0 registers only when ymm >> > values are passed same way as we touch SSE only when SSE values are >> > passed via EAX hint. >> >> Which register do you propose for hint? The current psABI uses RAX >> for XMM registers. We can't change it to AL and AH for YMM without >> breaking backward compatibility. >> >> > This way we will be able to support e.g. printf that has YMM printing % >> > construct but don't need YMM enabled hardware when those are not used. >> > >> > This is why I think extending EAX to contain information about amount of >> > XMM values to save and in addition YMM values to save is sane. Then old >> > non-YMM aware varargs prologues will crash when YMM values are passed, >> > but all other combinations will work. >> >> I don't think it is necessary since -mavx will enable AVX code >> generation for all SSE codes. Unless the function only uses integer, >> it will crash on non-YMM aware hardware. That is if there is one >> SSE register is used, which is hinted in RAX, varargs prologue will >> use AVX instructions to save it. We don't need another hint for AVX >> instructions. >> >> > > >> > > > >> > > > I personally don't have much preferences over 1. or 2.. 1. seems >> > > > relatively easy to implement too, or is packaging two 128bit values to >> > > > single 256bit difficult in va_arg expansion? >> > > > >> > > >> > > Access to 256bit register as lower and upper 128bits needs 2 >> > > instructions. For store >> > > >> > > vmovaps %xmm7, -143(%rax) >> > > vextractf128 $1, %ymm7, -15(%rax) >> > > >> > > For load >> > > >> > > vmovaps -143(%rax),%xmm7 >> > > vinsert128 $1, -15(%rax),%ymm7,%ymm7 >> > > >> > > If we go beyond 256bit, we need more instructions to access >> > > the full register. For 512bit, it will be split into lower 128bit, >> > > middle 128bit and upper 256bit. 1024bit will have 4 parts. >> > > >> > > For #2, only one instruction will be needed for 256bit and >> > > beyond. >> > >> > Yes, but we will still save half of stack space. Well, I don't have >> > much preferences here. If it seems saner to simply save whole thing >> > saving lower part twice, I am fine with that. >> >> I was told that it wasn't very easy to get decent performance with >> split access. I extended my proposal to include a 16bit bitmask to >> indicate which YMM regisetrs should be saved. If the bit is 0, >> we should only save the the lower 128bit in the original register >> save area. Otherwise, we should only save the same whole YMM register. >> > > My second thought. How useful is such a bitmask? Do we really > need it? Is that accepetable to save the lower 128bit twice?
Why do we need to save the lower 128bit at all if a ymm reg is passed? Can't we assume "type-correctness"? Richard.
