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Revert "spirv: Use a simpler and more correct implementaiton of tanh()"

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This reverts commit da1c49171d.

The reduced formula has precision problems on fp16 around 0.  Bring
back the old formula, but make sure to keep the clamping.

Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4054>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4054>
(cherry picked from commit 9f9432d56c)
This commit is contained in:
Kristian H. Kristensen 2020-02-27 11:38:53 -08:00 • committed by Dylan Baker
parent a9ee554df3
commit 45e14cbdc7
2 changed files with 14 additions and 15 deletions
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2
.pick_status.json
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@ -31,7 +31,7 @@
"description": "Revert \"spirv: Use a simpler and more correct implementaiton of tanh()\"",
"nominated": true,
"nomination_type": 2,
"resolution": 0,
"resolution": 1,
"master_sha": null,
"because_sha": "da1c49171d0df185545cfbbd600e287f7c6160fa"
},

27
src/compiler/spirv/vtn_glsl450.c
View file

@ -458,25 +458,24 @@ handle_glsl450_alu(struct vtn_builder *b, enum GLSLstd450 entrypoint,
return;
case GLSLstd450Tanh: {
/* tanh(x) := (0.5 * (e^x - e^(-x))) / (0.5 * (e^x + e^(-x)))
/* tanh(x) := (e^x - e^(-x)) / (e^x + e^(-x))
*
* With a little algebra this reduces to (e^2x - 1) / (e^2x + 1)
* We clamp x to [-10, +10] to avoid precision problems. When x > 10,
* e^x dominates the sum, e^(-x) is lost and tanh(x) is 1.0 for 32 bit
* floating point.
*
* We clamp x to (-inf, +10] to avoid precision problems. When x > 10,
* e^2x is so much larger than 1.0 that 1.0 gets flushed to zero in the
* computation e^2x +/- 1 so it can be ignored.
*
* For 16-bit precision we clamp x to (-inf, +4.2] since the maximum
* representable number is only 65,504 and e^(2*6) exceeds that. Also,
* if x > 4.2, tanh(x) will return 1.0 in fp16.
* For 16-bit precision this we clamp x to [-4.2, +4.2].
*/
const uint32_t bit_size = src[0]->bit_size;
const double clamped_x = bit_size > 16 ? 10.0 : 4.2;
nir_ssa_def *x = nir_fmin(nb, src[0],
nir_imm_floatN_t(nb, clamped_x, bit_size));
nir_ssa_def *exp2x = build_exp(nb, nir_fmul_imm(nb, x, 2.0));
val->ssa->def = nir_fdiv(nb, nir_fadd_imm(nb, exp2x, -1.0),
nir_fadd_imm(nb, exp2x, 1.0));
nir_ssa_def *x = nir_fclamp(nb, src[0],
nir_imm_floatN_t(nb, -clamped_x, bit_size),
nir_imm_floatN_t(nb, clamped_x, bit_size));
val->ssa->def =
nir_fdiv(nb, nir_fsub(nb, build_exp(nb, x),
build_exp(nb, nir_fneg(nb, x))),
nir_fadd(nb, build_exp(nb, x),
build_exp(nb, nir_fneg(nb, x))));
return;
}