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pan/bi: Lower flog2 to a table and polynomial

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Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8894>
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Alyssa Rosenzweig 2021-01-15 16:34:41 -05:00 • committed by Marge Bot
parent d4c028f770
commit 0bdd4cbb57
1 changed files with 34 additions and 0 deletions
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34
src/panfrost/bifrost/bifrost_compile.c
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@ -1036,6 +1036,40 @@ bi_lower_fexp2_32(bi_builder *b, bi_index dst, bi_index s0)
max->sem = BI_SEM_NAN_PROPAGATE;
}
static void
bi_lower_flog2_32(bi_builder *b, bi_index dst, bi_index s0)
{
/* s0 = a1 * 2^e, with a1 in [0.75, 1.5) */
bi_index a1 = bi_frexpm_f32(b, s0, true, false);
bi_index ei = bi_frexpe_f32(b, s0, true, false);
bi_index ef = bi_s32_to_f32(b, ei, BI_ROUND_RTZ);
/* xt estimates -log(r1), a coarse approximation of log(a1) */
bi_index r1 = bi_flog_table_f32(b, s0, BI_MODE_RED, BI_PRECISION_NONE);
bi_index xt = bi_flog_table_f32(b, s0, BI_MODE_BASE2, BI_PRECISION_NONE);
/* log(s0) = log(a1 * 2^e) = e + log(a1) = e + log(a1 * r1) -
* log(r1), so let x1 = e - log(r1) ~= e + xt and x2 = log(a1 * r1),
* and then log(s0) = x1 + x2 */
bi_index x1 = bi_fadd_f32(b, ef, xt, BI_ROUND_NONE);
/* Since a1 * r1 is close to 1, x2 = log(a1 * r1) may be computed by
* polynomial approximation around 1. The series is expressed around
* 1, so set y = (a1 * r1) - 1.0 */
bi_index y = bi_fma_f32(b, a1, r1, bi_imm_f32(-1.0), BI_ROUND_NONE);
/* x2 = log_2(1 + y) = log_e(1 + y) * (1/log_e(2)), so approximate
* log_e(1 + y) by the Taylor series (lower precision than the blob):
* y - y^2/2 + O(y^3) = y(1 - y/2) + O(y^3) */
bi_index loge = bi_fmul_f32(b, y,
bi_fma_f32(b, y, bi_imm_f32(-0.5), bi_imm_f32(1.0), BI_ROUND_NONE));
bi_index x2 = bi_fmul_f32(b, loge, bi_imm_f32(1.0 / logf(2.0)));
/* log(s0) = x1 + x2 */
bi_fadd_f32_to(b, dst, x1, x2, BI_ROUND_NONE);
}
static void
bi_emit_alu(bi_builder *b, nir_alu_instr *instr)
{