From: James Chesterman <James.Chesterman@arm.com>
Date: Wed, 19 Nov 2025 21:40:40 +0000 (+0000)
Subject: aarch64: Optimise AdvSIMD log1p
X-Git-Tag: glibc-2.43~132
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=015a13e780dd26d2da1ada6baa0621f49b7c0eb8;p=thirdparty%2Fglibc.git

aarch64: Optimise AdvSIMD log1p

Optimise AdvSIMD log1p by vectorising the special case.
The special cases are for when the input is:
Less than or equal to -1
+/- INFINITY
+/- NaN
---

diff --git a/sysdeps/aarch64/fpu/log1p_advsimd.c b/sysdeps/aarch64/fpu/log1p_advsimd.c
index 9bf11ffb2a..890dd3ed76 100644
--- a/sysdeps/aarch64/fpu/log1p_advsimd.c
+++ b/sysdeps/aarch64/fpu/log1p_advsimd.c
@@ -23,34 +23,42 @@
 const static struct data
 {
   struct v_log1p_data d;
-  uint64x2_t inf, minus_one;
-} data = { .d = V_LOG1P_CONSTANTS_TABLE,
-	   .inf = V2 (0x7ff0000000000000),
-	   .minus_one = V2 (0xbff0000000000000) };
+  float64x2_t nan, pinf, minf;
+} data = {
+  .d = V_LOG1P_CONSTANTS_TABLE,
+  .nan = V2 (NAN),
+  .pinf = V2 (INFINITY),
+  .minf = V2 (-INFINITY),
+};
 
-#define BottomMask v_u64 (0xffffffff)
-
-static float64x2_t NOINLINE VPCS_ATTR
+static inline float64x2_t
 special_case (float64x2_t x, uint64x2_t cmp, const struct data *d)
 {
-  /* Side-step special lanes so fenv exceptions are not triggered
-     inadvertently.  */
-  return v_call_f64 (log1p, x, log1p_inline (x, &d->d), cmp);
+  float64x2_t y = log1p_inline (x, ptr_barrier (&d->d));
+  y = vbslq_f64 (cmp, d->nan, y);
+  uint64x2_t ret_pinf = vceqq_f64 (x, d->pinf);
+  uint64x2_t ret_minf = vceqq_f64 (x, v_f64 (-1.0));
+
+  y = vbslq_f64 (ret_pinf, d->pinf, y);
+  return vbslq_f64 (ret_minf, d->minf, y);
 }
 
 /* Vector log1p approximation using polynomial on reduced interval. Routine is
    a modification of the algorithm used in scalar log1p, with no shortcut for
-   k=0 and no narrowing for f and k. Maximum observed error is 2.45 ULP:
+   k=0 and no narrowing for f and k.
+   Maximum observed error is 1.95 + 0.5 ULP
    _ZGVnN2v_log1p(0x1.658f7035c4014p+11) got 0x1.fd61d0727429dp+2
 					want 0x1.fd61d0727429fp+2 .  */
-VPCS_ATTR float64x2_t V_NAME_D1 (log1p) (float64x2_t x)
+float64x2_t VPCS_ATTR NOINLINE V_NAME_D1 (log1p) (float64x2_t x)
 {
   const struct data *d = ptr_barrier (&data);
-  uint64x2_t ix = vreinterpretq_u64_f64 (x);
-  uint64x2_t ia = vreinterpretq_u64_f64 (vabsq_f64 (x));
 
-  uint64x2_t special_cases
-      = vorrq_u64 (vcgeq_u64 (ia, d->inf), vcgeq_u64 (ix, d->minus_one));
+  /* Use signed integers here to ensure that negative numbers between 0 and -1
+    don't make this expression true.  */
+  uint64x2_t is_infnan
+      = vcgeq_s64 (vreinterpretq_s64_f64 (x), vreinterpretq_s64_f64 (d->pinf));
+  /* The OR-NOT is needed to catch -NaN.  */
+  uint64x2_t special_cases = vornq_u64 (is_infnan, vcgtq_f64 (x, v_f64 (-1)));
 
   if (__glibc_unlikely (v_any_u64 (special_cases)))
     return special_case (x, special_cases, d);