On Wed, Jul 31, 2024 at 3:17 PM Uros Bizjak <ubiz...@gmail.com> wrote: > > On Wed, Jul 31, 2024 at 9:11 AM Hongtao Liu <crazy...@gmail.com> wrote: > > > > On Wed, Jul 31, 2024 at 1:06 AM Uros Bizjak <ubiz...@gmail.com> wrote: > > > > > > On Tue, Jul 30, 2024 at 3:00 PM Richard Biener <rguent...@suse.de> wrote: > > > > > > > > On Tue, 30 Jul 2024, Alexander Monakov wrote: > > > > > > > > > > > > > > On Tue, 30 Jul 2024, Richard Biener wrote: > > > > > > > > > > > > Oh, and please add a small comment why we don't use XFmode here. > > > > > > > > > > > > Will do. > > > > > > > > > > > > /* Do not enable XFmode, there is padding in it and it > > > > > > suffers > > > > > > from normalization upon load like SFmode and DFmode when > > > > > > not using SSE. */ > > > > > > > > > > Is it really true? I have no evidence of FLDT performing normalization > > > > > (as mentioned in PR 114659, if it did, there would be no way to > > > > > spill/reload > > > > > x87 registers). > > > > > > > > What mangling fld performs depends on the contents of the FP control > > > > word which is awkward. IIRC there's at least a bugreport that it > > > > turns sNaN into a qNaN, it seems I was wrong about denormals > > > > (when DM is not masked). And yes, IIRC x87 instability is also > > > > related to spills (IIRC we spill in the actual mode of the reg, not in > > > > XFmode), but -fexcess-precision=standard should hopefully avoid that. > > > > It's also not clear whether all implementations conformed to the > > > > specs wrt extended-precision format loads. > > > > > > FYI, FLDT does not mangle long-double values and does not generate > > > exceptions. Please see [1], but ignore shadowed text and instead read > > > the "Floating-Point Exceptions" section. So, as far as hardware is > > > concerned, it *can* be used to transfer 10-byte values, but I don't > > > want to judge from the compiler PoV if this is the way to go. We can > > > enable it, perhaps temporarily to experiment a bit - it is easy to > > > disable if it causes problems. > > > > > > Let's CC Intel folks for their opinion, if it is worth using an aging > > > x87 to transfer 80-bit data. > > I prefer not, in another hook ix86_can_change_mode_class, we have > > > > 20372 /* x87 registers can't do subreg at all, as all values are > > reformatted > > 20373 to extended precision. */ > > 20374 if (MAYBE_FLOAT_CLASS_P (regclass)) > > 20375 return false; > > No, the above applies to SFmode subreg of XFmode value, which is a > no-go. My question refers to the plain XFmode (80-bit) moves, where > x87 is used simply to: > > fldt mem1 > ... > fstp mem2 > > where x87 is used to perform a move from one 80-bit location to the other. > > > I guess it eventually needs reload for XFmode. > > There are no reloads, as we would like to perform bit-exact 80-bit > move, e.g. array of 10 chars. Oh, It's memory copy. I suspect that the hardware doesn't enable memory renaming for x87 instructions. So I prefer not. > > Uros.
-- BR, Hongtao
