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[v5,03/12] math: Improve hypot performance with FMA

Message ID 20211207190353.3282666-4-adhemerval.zanella@linaro.org
State Committed
Commit ccfa865a82c648fde56864ea094f70ee1a8a944b
Headers show
Series Improve hypot | expand

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Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent

Commit Message

Adhemerval Zanella Dec. 7, 2021, 7:03 p.m. UTC
From: Wilco Dijkstra <Wilco.Dijkstra@arm.com>

Improve hypot performance significantly by using fma when available. The
fma version has twice the throughput of the previous version and 70% of
the latency.  The non-fma version has 30% higher throughput and 10%
higher latency.

Max ULP error is 0.949 with fma and 0.792 without fma.

Passes GLIBC testsuite.
---
 sysdeps/ieee754/dbl-64/e_hypot.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)
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Patch

diff --git a/sysdeps/ieee754/dbl-64/e_hypot.c b/sysdeps/ieee754/dbl-64/e_hypot.c
index 75bce2df4e..6fedf0d61f 100644
--- a/sysdeps/ieee754/dbl-64/e_hypot.c
+++ b/sysdeps/ieee754/dbl-64/e_hypot.c
@@ -26,7 +26,11 @@ 
      rounding mode.
    - Handle required underflow exception for subnormal results.
 
-   The expected ULP is ~0.792.
+   The expected ULP is ~0.792 or ~0.948 if FMA is used.  For FMA, the
+   correction is not used and the error of sqrt (x^2 + y^2) is below 1 ULP
+   if x^2 + y^2 is computed with less than 0.707 ULP error.  If |x| >= |2y|,
+   fma (x, x, y^2) has ~0.625 ULP.  If |x| < |2y|, fma (|2x|, |y|, (x - y)^2)
+   has ~0.625 ULP.
 
    [1] https://arxiv.org/pdf/1904.09481.pdf  */
 
@@ -48,6 +52,16 @@  static inline double
 kernel (double ax, double ay)
 {
   double t1, t2;
+#ifdef __FP_FAST_FMA
+  t1 = ay + ay;
+  t2 = ax - ay;
+
+  if (t1 >= ax)
+    return sqrt (fma (t1, ax, t2 * t2));
+  else
+    return sqrt (fma (ax, ax, ay * ay));
+
+#else
   double h = sqrt (ax * ax + ay * ay);
   if (h <= 2.0 * ay)
     {
@@ -64,6 +78,7 @@  kernel (double ax, double ay)
 
   h -= (t1 + t2) / (2.0 * h);
   return h;
+#endif
 }
 
 double