[Mesa-dev] [RFC PATCH 2/2] panfrost/midgard: Ignore imov/fmov distinction

[Alyssa Rosenzweig]

> Using nir_gather_ssa_types is wrong. In both Midgard and NIR, SSA
> values are just a bunch of bits with no int/float distinction, and
> therefore you shouldn't need to know how a register is used to compile
> the instruction producing it. The only distinction between imov and
> fmov, in both NIR and the Midgard ISA, is what modifiers they support.
> What you want to do is probably what you originally did, i.e. use fmov
> for NIR fmov as well as fabs and fneg, imov for imov (and if we delete
> fmov, just using it for fabs and fneg). If this fixes any bugs, it's
> just papering over bugs in your backend, and you should fix them
> instead. Note that later GLSL versions have intBitsToFloat() and
> floatBitsToInt() which blow away any assumption that the types will be
> consistent.

That's how my mental model was, but it doesn't look to be the case: the
blob is very consistent in emitting imov/fmov distinclty (and
icsel/fcsel), and a lot of bizarre bugs come up if you do it any other
way, even absent any modifiers. There's -some- difference, it's just not
obvious what. Although I'll admit playing with intBitsToFloat/etc allow
seemingly wrong shaders even with the blob..