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. [1] https://www.felixcloutier.com/x86/fld Uros.
