<html> <head> <base href="https://bugs.freedesktop.org/" /> </head> <body> <div> <a class="bz_bug_link bz_status_NEW " title="NEW - Add FP64 support to the i965 shader backends" href="https://bugs.freedesktop.org/show_bug.cgi?id=92760#c31">Comment # 31</a> on <a class="bz_bug_link bz_status_NEW " title="NEW - Add FP64 support to the i965 shader backends" href="https://bugs.freedesktop.org/show_bug.cgi?id=92760">bug 92760</a> from <a class="email" href="mailto:itoral@igalia.com" title="Iago Toral <itoral@igalia.com>"> Iago Toral</a> <pre>(In reply to Jason Ekstrand from <a href="show_bug.cgi?id=92760#c30">comment #30</a>) > Created <a href="attachment.cgi?id=120957" name="attach_120957" title="NIR indirect lowering pass">attachment 120957</a> <a href="attachment.cgi?id=120957&action=edit" title="NIR indirect lowering pass">[details]</a> > NIR indirect lowering pass > > (In reply to Iago Toral from <a href="show_bug.cgi?id=92760#c29">comment #29</a>) > > Hey Jason/Connor, > > > > the lowering of trunc for doubles has some code that looks like this > > (pseudo-code): > > > > if (exponent < 0) { > > mask = 0x0 > > } else if (exponent > 52) { > > mask = 0x7fffffffffffffff; > > } else { > > /* This is a 64-bit integer op, needs to be split into hi/lo 32-bit ops */ > > mask = (1LL << frac_bits) - 1; > > } > > > > The current implementation I have works fine using bcsel. It looks something > > like this (again, pseudo-code): > > > > mask = bcsel(exponent < 0, > > 0x7fffffffffffffff, > > bcsel(exponent > 52, > > 0x0000000000000000, > > (1LL << frac_bits) -1)) > > > > My problem with this is that "(1LL << frac_bits) - 1" is a 64-bit integer > > operation that we have to implement in terms of hi/lo 32-bit integer > > operations (at least until we support 64-bit integers), so it is really a > > bunch of instructions. Because I use bcsel, it means that we generate code > > for that even if exponent is not in [1..51], which is not ideal. > > Right. I would encourage you not to use if's too much because branching may > be more expensive than bcsel depending on what paths different invocations > take. However, if one side of the if is overwhelmingly more likely than the > other, then control-flow is probably a good idea. Yeah, in this case exponents in the range 0..52 would be a lot more common than anything else. > > I was thinking about rewriting this as an if/else ladder instead, however, I > > noticed that because this occurs in SSA mode I would have to deal with the > > phi nodes etc manually and I don't see any other case where we do something > > like that outside the NIR to SSA pass, so I wonder if this is actually a > > good idea at all. What do you think? > > > > If you think the if/else ladder is the way to go, is there any documentation > > or > > code references I can look at to have an idea as to how that should be > > implemented for a lowering pass in SSA mode? > > I attached a pass that I've written recently (not yet sent out for review, > but it does work) that does exactly this. It replaces indirect load/store > operations with if-ladders and phi nodes (if needed). Most of it comes down > to using nir_insert_cf_node to insert it at the builder's cursor and then > making sure you set the cursor to something reasonable when you're done. Awesome, thanks a lot Jason!</pre> </div> <hr> You are receiving this mail because: <ul> <li>You are the QA Contact for the bug.</li> </ul> </body> </html>