On Wed, Jan 28, 2015 at 11:20 AM, Ian Romanick <i...@freedesktop.org> wrote: > On 01/28/2015 10:31 AM, Matt Turner wrote: >> Note: this will round differently for x.5 where x is even. >> >> total instructions in shared programs: 5953897 -> 5948654 (-0.09%) >> instructions in affected programs: 88619 -> 83376 (-5.92%) >> helped: 696 >> --- >> If we implemented round() differently from roundEven(), we should >> use it instead. >> >> (mul (floor (add (abs x) 0.5) (sign x))) is 6 i965 instructions. >> (roundEven x) is 1 instruction. >> >> Most shaders with this pattern wrap it in int(...), which increases >> the counts by one, to 7 and 2 respectively. >> >> Alternatively, we could optimize this as >> >> (trunc (add f (mul 0.5 (sign f)))), which would be 6 instructions, >> and the int() conversion would be free. We could also apply f's sign >> to 0.5 in two instructions, cutting the total to 4. >> >> What do you think? Should we do precisely as they say? All but two >> of the affected shaders seem to be translated from DX. >> >> src/glsl/opt_algebraic.cpp | 32 ++++++++++++++++++++++++++++++++ >> 1 file changed, 32 insertions(+) >> >> diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp >> index c6f4a9c..eaa5f47 100644 >> --- a/src/glsl/opt_algebraic.cpp >> +++ b/src/glsl/opt_algebraic.cpp >> @@ -514,6 +514,38 @@ ir_algebraic_visitor::handle_expression(ir_expression >> *ir) >> if (op_const[1] && !op_const[0]) >> reassociate_constant(ir, 1, op_const[1], op_expr[0]); >> >> + /* Optimizes >> + * >> + * (mul (floor (add (abs x) 0.5) (sign x))) > > If I'm not mistaken, this isn't round-to-even. Doesn't this round 4.5 > to 5? roundEven(4.5) should be 4. This looks like "half-up" rounding. > Which is very different. See > http://userguide.icu-project.org/formatparse/numbers/rounding-modes
Isn't this what I said? I've suggested a way to cut a six instruction sequence to one, with the caveat that it doesn't do the right thing for x.5 where x is even. What I'm asking is whether we suspect that they specifically want half-up behavior (speculation, so not likely insightful), or if there's a way we can emulate round-half-up behavior using round-even in fewer than four instructions. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
