[Mesa-dev] [PATCH 22/59] compiler/nir: add lowering for 16-bit ldexp

[Iago Toral]

On Wed, 2018-12-05 at 13:20 +0200, Pohjolainen, Topi wrote:
> On Wed, Dec 05, 2018 at 11:53:44AM +0100, Iago Toral wrote:
> > On Wed, 2018-12-05 at 11:39 +0200, Pohjolainen, Topi wrote:
> > > I remember people preferring to order things 16, 32, 64 before.
> > > Should
> > > we follow that here as well?
> > 
> > Yes, it makes sense. I'll change that.
> > 
> > > On Tue, Dec 04, 2018 at 08:16:46AM +0100, Iago Toral Quiroga
> > > wrote:
> > > > ---
> > > >  src/compiler/nir/nir_opt_algebraic.py | 5 +++++
> > > >  1 file changed, 5 insertions(+)
> > > > 
> > > > diff --git a/src/compiler/nir/nir_opt_algebraic.py
> > > > b/src/compiler/nir/nir_opt_algebraic.py
> > > > index 6c3b77c9b6e..747f1751086 100644
> > > > --- a/src/compiler/nir/nir_opt_algebraic.py
> > > > +++ b/src/compiler/nir/nir_opt_algebraic.py
> > > > @@ -778,6 +778,8 @@ def fexp2i(exp, bits):
> > > >        return ('ishl', ('iadd', exp, 127), 23)
> > > >     elif bits == 64:
> > > >        return ('pack_64_2x32_split', 0, ('ishl', ('iadd', exp,
> > > > 1023), 20))
> > > > +   elif bits == 16:
> > > > +      return ('i2i16', ('ishl', ('iadd', exp, 15), 10))
> > > >     else:
> > > >        assert False
> > > >  
> > > > @@ -796,6 +798,8 @@ def ldexp(f, exp, bits):
> > > >        exp = ('imin', ('imax', exp, -252), 254)
> > > >     elif bits == 64:
> > > >        exp = ('imin', ('imax', exp, -2044), 2046)
> > > > +   elif bits == 16:
> > > > +      exp = ('imin', ('imax', exp, -30), 30)
> > > 
> > > I expected this to be:
> > > 
> > >          exp = ('imin', ('imax', exp, -29), 30)
> > 
> > Actually, I think this should be -28, since the minimum exponent
> > value
> > is -14.
> 
> I kept wondering about. The offset is 15 and -14 - 15 yields -29. But
> -28
> in turn would be more in line with the 32- and 64-bit cases.

I think the idea is to have this be 2x the minimum (and maximum)
exponents we can represent, since below we are dividing it by two and
emitting two exponentials, each with half that exponent. That way we
ensure that when we divide the exponent by 2 we still produce a
representable exponent for the bit-size.

Iago

> > 
> > > >     else:
> > > >        assert False
> > > >  
> > > > @@ -814,6 +818,7 @@ def ldexp(f, exp, bits):
> > > >  optimizations += [
> > > >     (('ldexp at 32', 'x', 'exp'), ldexp('x', 'exp', 32), 'options-
> > > > > lower_ldexp'),
> > > > 
> > > >     (('ldexp at 64', 'x', 'exp'), ldexp('x', 'exp', 64), 'options-
> > > > > lower_ldexp'),
> > > > 
> > > > +   (('ldexp at 16', 'x', 'exp'), ldexp('x', 'exp', 16), 'options-
> > > > > lower_ldexp'),
> > > > 
> > > >  ]
> > > >  
> > > >  # Unreal Engine 4 demo applications open-codes
> > > > bitfieldReverse()
> > > > -- 
> > > > 2.17.1
> > > > 
> > > > _______________________________________________
> > > > mesa-dev mailing list
> > > > mesa-dev at lists.freedesktop.org
> > > > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> > > 
> > > 
> 
>