[Mesa-dev] [PATCH] nv50/ir: saturate FRC result to avoid completely bogus values

[Roland Scheidegger]

Am 18.11.2014 um 05:03 schrieb Ilia Mirkin:
> For values above integer accuracy in floats, val - floor(val) might
> actually produce a value greater than 1. For such large floats, it's
> reasonable to be imprecise, but it's unreasonable for FRC to return a
> value that is not between 0 and 1.
> 
> Signed-off-by: Ilia Mirkin <imirkin at alum.mit.edu>
> ---
>  src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp b/src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp
> index 41b91e8..e5b767f 100644
> --- a/src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp
> +++ b/src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp
> @@ -2512,7 +2512,8 @@ Converter::handleInstruction(const struct tgsi_full_instruction *insn)
>           src0 = fetchSrc(0, c);
>           val0 = getScratch();
>           mkOp1(OP_FLOOR, TYPE_F32, val0, src0);
> -         mkOp2(OP_SUB, TYPE_F32, dst0[c], src0, val0);
> +         mkOp2(OP_SUB, TYPE_F32, val0, src0, val0);
> +         mkOp1(OP_SAT, TYPE_F32, dst0[c], val0);
>        }
>        break;
>     case TGSI_OPCODE_ROUND:
> 

I don't understand the math behind this. For any such large number, as
far as I can tell floor(val) == val and hence the end result ought to be
zero. Or doesn't your floor work like that?

Roland