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

This reverts commit da1c49171d0df185545cfbbd600e287f7c6160fa.

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>

diff --git a/src/compiler/spirv/vtn_glsl450.c b/src/compiler/spirv/vtn_glsl450.c
index a074b6f3fb48cbb40376fb5a8f4d627bd21e6325..571f06cbed2a59ffdbd0f85ab2e6037f6e341df3 100644 (file)
--- a/src/compiler/spirv/vtn_glsl450.c
+++ b/src/compiler/spirv/vtn_glsl450.c
@@ -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;
    }