[Mesa-dev] [PATCH 05/13] nir/lower_double_ops: lower trunc()
Iago Toral
itoral at igalia.com
Wed Apr 20 06:37:31 UTC 2016
On Tue, 2016-04-19 at 15:32 -0700, Jason Ekstrand wrote:
>
>
> On Tue, Apr 12, 2016 at 1:05 AM, Samuel Iglesias Gonsálvez
> <siglesias at igalia.com> wrote:
> From: Iago Toral Quiroga <itoral at igalia.com>
>
> At least i965 hardware does not have native support for
> truncating doubles.
> ---
> src/compiler/nir/nir.h | 1 +
> src/compiler/nir/nir_lower_double_ops.c | 83
> +++++++++++++++++++++++++++++++++
> 2 files changed, 84 insertions(+)
>
> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> index 434d92b..f83b2e0 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -2286,6 +2286,7 @@ typedef enum {
> nir_lower_drcp = (1 << 0),
> nir_lower_dsqrt = (1 << 1),
> nir_lower_drsq = (1 << 2),
> + nir_lower_dtrunc = (1 << 3),
> } nir_lower_doubles_options;
>
> void nir_lower_doubles(nir_shader *shader,
> nir_lower_doubles_options options);
> diff --git a/src/compiler/nir/nir_lower_double_ops.c
> b/src/compiler/nir/nir_lower_double_ops.c
> index 4cd153c..9eec858 100644
> --- a/src/compiler/nir/nir_lower_double_ops.c
> +++ b/src/compiler/nir/nir_lower_double_ops.c
> @@ -302,6 +302,81 @@ lower_sqrt_rsq(nir_builder *b,
> nir_ssa_def *src, bool sqrt)
> return res;
> }
>
> +static nir_ssa_def *
> +lower_trunc(nir_builder *b, nir_ssa_def *src)
> +{
> + nir_ssa_def *unbiased_exp = nir_isub(b, get_exponent(b,
> src),
> + nir_imm_int(b,
> 1023));
> +
> + nir_ssa_def *frac_bits = nir_isub(b, nir_imm_int(b, 52),
> unbiased_exp);
> +
> + /*
> + * Depending on the exponent, we compute a mask with the
> bits we need to
> + * remove in order to trunc the double. The mask is
> computed like this:
> + *
> + * if (unbiased_exp < 0)
> + * mask = 0x0
> + * else if (unbiased_exp > 52)
> + * mask = 0x7fffffffffffffff
> + * else
> + * mask = (1LL < frac_bits) - 1
>
>
> I'm having a bit of trouble convincing myself that this is correct.
> Let me walk through it one case at a time:
>
>
> unbiased_exp < 0:
>
> In this case, 2^exp <= 2 so src < 1 and the result should be zero. In
> that case we want to stomp all the bits to zero, not keep them all.
>
>
> unbiased_exp > 52:
>
> In this case 2^exp is large enough that all of the bits matter. We
> want to keep them all not zero them out.
>
>
> else:
>
> In this case, 2^exp >= 1 but not big enough to make all the mantissa
> bits matter. We need to mask off the bottom 52-exp many bits.
>
>
> If I'm getting this backwards, please let me know. If it's doing what
> I think it's doing, there are several cases this should be getting
> wrong. Are we testing all of those cases?
Yes, I think you are getting it backwards. The mask is used to select
the bits from the src that we want to keep. So in the case that
unbiased_exp < 0, a mask of 0 means that we effectively discard all the
bits, as you expect. If unbiased_exp > 52 then mask is all 1's, meaning
that we take all the bits.
> One other aside: I think it's more efficient to generate the masks
> with either (~0u >> (32 - bits)) or (0x80000000 >> (bits - 1)) if you
> want the top bits. NIR should be able to easily get rid of the
> integer adds and subtracts. Getting rid of the -1 on (1 << frac_bits)
> - 1 is much harder.
Sure, we can do that.
> + *
> + * Notice that the else branch is a 64-bit integer
> operation that we need
> + * to implement in terms of 32-bit integer arithmetics (at
> least until we
> + * support 64-bit integer arithmetics).
> + */
> +
> + /* Compute "mask = (1LL << frac_bits) - 1" in terms of
> hi/lo 32-bit chunks
> + * for the else branch
> + */
> + nir_ssa_def *mask_lo =
> + nir_bcsel(b,
> + nir_ige(b, frac_bits, nir_imm_int(b, 32)),
> + nir_imm_int(b, 0xffffffff),
> + nir_isub(b,
> + nir_ishl(b,
> + nir_imm_int(b, 1),
> + frac_bits),
> + nir_imm_int(b, 1)));
> +
> + nir_ssa_def *mask_hi =
> + nir_bcsel(b,
> + nir_ilt(b, frac_bits, nir_imm_int(b, 33)),
> + nir_imm_int(b, 0),
> + nir_isub(b,
> + nir_ishl(b,
> + nir_imm_int(b, 1),
> + nir_isub(b,
> + frac_bits,
> + nir_imm_int(b,
> 32))),
> + nir_imm_int(b, 1)));
> +
> + /* Compute the correct mask to use based on unbiased_exp
> */
> + nir_ssa_def *mask =
> + nir_bcsel(b,
> + nir_ilt(b, unbiased_exp, nir_imm_int(b, 0)),
> + nir_pack_double_2x32_split(b,
> + nir_imm_int(b,
> 0xffffffff),
> + nir_imm_int(b,
> 0x7fffffff)),
> + nir_bcsel(b, nir_ige(b, unbiased_exp,
> nir_imm_int(b, 53)),
> + nir_imm_double(b, 0.0),
> + nir_pack_double_2x32_split(b,
> mask_lo, mask_hi)));
> +
> + /* Mask off relevant mantissa bits (0..31 in the low
> 32-bits
> + * and 0..19 in the high 32 bits)
> + */
> + mask_lo = nir_unpack_double_2x32_split_x(b, mask);
> + mask_hi = nir_unpack_double_2x32_split_y(b, mask);
> +
> + nir_ssa_def *src_lo = nir_unpack_double_2x32_split_x(b,
> src);
> + nir_ssa_def *src_hi = nir_unpack_double_2x32_split_y(b,
> src);
> +
> + nir_ssa_def *zero = nir_imm_int(b, 0);
> + nir_ssa_def *new_src_lo = nir_bfi(b, mask_lo, zero,
> src_lo);
> + nir_ssa_def *new_src_hi = nir_bfi(b, mask_hi, zero,
> src_hi);
> + return nir_pack_double_2x32_split(b, new_src_lo,
> new_src_hi);
Oh, this is silly, we should be doing a nir_iand instead of a nir_bfi
with zero, I'll fix this too.
> +}
> +
> static void
> lower_doubles_instr(nir_alu_instr *instr,
> nir_lower_doubles_options options)
> {
> @@ -325,6 +400,11 @@ lower_doubles_instr(nir_alu_instr *instr,
> nir_lower_doubles_options options)
> return;
> break;
>
> + case nir_op_ftrunc:
> + if (!(options & nir_lower_dtrunc))
> + return;
> + break;
> +
> default:
> return;
> }
> @@ -348,6 +428,9 @@ lower_doubles_instr(nir_alu_instr *instr,
> nir_lower_doubles_options options)
> case nir_op_frsq:
> result = lower_sqrt_rsq(&bld, src, false);
> break;
> + case nir_op_ftrunc:
> + result = lower_trunc(&bld, src);
> + break;
> default:
> unreachable("unhandled opcode");
> }
> --
> 2.5.0
>
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