[Mesa-dev] [PATCH v3 12/44] nir: add new fadd, fsub, fmul opcodes taking into account rounding mode

[Samuel Iglesias Gonsálvez]

On 2/6/19 12:47 PM, Connor Abbott wrote:
> 
> 
> On Wed, Feb 6, 2019 at 11:46 AM Samuel Iglesias Gonsálvez
> <siglesias at igalia.com <mailto:siglesias at igalia.com>> wrote:
> 
>     According to Vulkan spec, the new execution modes affect only
>     correctly rounded SPIR-V instructions, which includes fadd,
>     fsub and fmul.
> 
>     Signed-off-by: Samuel Iglesias Gonsálvez <siglesias at igalia.com
>     <mailto:siglesias at igalia.com>>
>     ---
>      src/compiler/nir/nir_opcodes.py | 17 +++++++++++++++++
>      1 file changed, 17 insertions(+)
> 
>     diff --git a/src/compiler/nir/nir_opcodes.py
>     b/src/compiler/nir/nir_opcodes.py
>     index f8997444c28..7b45d38f460 100644
>     --- a/src/compiler/nir/nir_opcodes.py
>     +++ b/src/compiler/nir/nir_opcodes.py
>     @@ -453,9 +453,15 @@ def binop_convert(name, out_type, in_type,
>     alg_props, const_expr):
>         opcode(name, 0, out_type, [0, 0], [in_type, in_type],
>                False, alg_props, const_expr, "")
> 
>     +def binop_convert_rounding_mode(name, out_type, in_type, alg_props,
>     const_expr, rounding_mode):
>     +   opcode(name, 0, out_type, [0, 0], [in_type, in_type], False,
>     alg_props, const_expr, rounding_mode)
>     +
>      def binop(name, ty, alg_props, const_expr):
>         binop_convert(name, ty, ty, alg_props, const_expr)
> 
>     +def binop_rounding_mode(name, ty, alg_props, const_expr,
>     rounding_mode):
>     +   binop_convert_rounding_mode(name, ty, ty, alg_props, const_expr,
>     rounding_mode)
>     +
>      def binop_compare(name, ty, alg_props, const_expr):
>         binop_convert(name, tbool1, ty, alg_props, const_expr)
> 
>     @@ -490,13 +496,24 @@ def binop_reduce(name, output_size,
>     output_type, src_type, prereduce_expr,
>                final(reduce_(reduce_(src0, src1), reduce_(src2, src3))), "")
> 
>      binop("fadd", tfloat, commutative + associative, "src0 + src1")
>     +binop_rounding_mode("fadd_rtne", tfloat, commutative + associative,
>     +                    "_mesa_roundeven(src0 + src1)", "_rtne")
> 
> 
> There are two things wrong here:
> - The default floating-point environment, and the one Mesa expects, does
> round-to-nearest-even so it's rtz that needs special handling.
> - _mesa_roundeven here is a no-op (it'll implicitly expand to a double
> and then convert back to a float). The rounding actually happens as part
> of the addition itself. I'm not sure if adding as double (with
> round-to-nearest-even) and then rounding back to a float will work, but
> that definitely won't work for doubles. I think you'll have to implement
> round-to-zero addition yourself, or fiddle with the CPU's floating-point
> environment.
> 
> The same goes multiply and fma.
>  

OK, thanks for the feedback. I will implement the needed RTZ ops and
send a new version.

Sam

> 
>     +binop_rounding_mode("fadd_rtz", tfloat, commutative + associative,
>     "src0 + src1", "_rtz")
>     +
>      binop("iadd", tint, commutative + associative, "src0 + src1")
>      binop("uadd_sat", tuint, commutative,
>            "(src0 + src1) < src0 ? UINT64_MAX : (src0 + src1)")
>      binop("fsub", tfloat, "", "src0 - src1")
>     +binop_rounding_mode("fsub_rtne", tfloat, "",
>     +                    "_mesa_roundeven(src0 - src1)", "_rtne")
>     +binop_rounding_mode("fsub_rtz", tfloat, "", "src0 - src1", "_rtz")
>      binop("isub", tint, "", "src0 - src1")
> 
>      binop("fmul", tfloat, commutative + associative, "src0 * src1")
>     +binop_rounding_mode("fmul_rtne", tfloat, commutative + associative,
>     +                    "_mesa_roundeven(src0 * src1)", "_rtne")
>     +binop_rounding_mode("fmul_rtz", tfloat, commutative + associative,
>     "src0 * src1", "_rtz")
>     +
>      # low 32-bits of signed/unsigned integer multiply
>      binop("imul", tint, commutative + associative, "src0 * src1")
> 
>     -- 
>     2.19.1
> 
>     _______________________________________________
>     mesa-dev mailing list
>     mesa-dev at lists.freedesktop.org <mailto:mesa-dev at lists.freedesktop.org>
>     https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> 

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: OpenPGP digital signature
URL: <https://lists.freedesktop.org/archives/mesa-dev/attachments/20190207/b0c5b373/attachment.sig>