[PATCH v2 2/4] math: Optimize fma call on log2pf1

[Adhemerval Zanella]

The fma is required only for x == -0x1.da285cp-5 in FE_TONEAREST
to provide correctly rounded results.

Checked on x86_64-linux-gnu and i686-linux-gnu.
---
 math/auto-libm-test-in             |  1 +
 math/auto-libm-test-out-log2p1     | 25 +++++++++++++++++++++++++
 sysdeps/ieee754/flt-32/s_log2p1f.c |  7 ++++++-
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/math/auto-libm-test-in b/math/auto-libm-test-in
index 7e8cb4cef83..03f6fee1f00 100644
--- a/math/auto-libm-test-in
+++ b/math/auto-libm-test-in
@@ -7714,6 +7714,7 @@ log2p1 -0x4.f37d3c9ce0b14bdd86eb157df5d4p-4
 log2p1 0x7.2eca50c4d93196362b4f37f6e8dcp-4
 log2p1 -0x6.3fef3067427e43dfcde9e48f74bcp-4
 log2p1 0x6.af53d00fd2845d4772260ef5adc4p-4
+log2p1 -0x1.da285cp-5
 
 mul 0 0
 mul 0 -0
diff --git a/math/auto-libm-test-out-log2p1 b/math/auto-libm-test-out-log2p1
index 5e395fffa47..3902600a340 100644
--- a/math/auto-libm-test-out-log2p1
+++ b/math/auto-libm-test-out-log2p1
@@ -4467,3 +4467,28 @@ log2p1 0x6.af53d00fd2845d4772260ef5adc4p-4
 = log2p1 tonearest ibm128 0x6.af53d00fd2845d4772260ef5acp-4 : 0x8.0efc6087a73bba3b9eb65a673p-4 : inexact-ok
 = log2p1 towardzero ibm128 0x6.af53d00fd2845d4772260ef5acp-4 : 0x8.0efc6087a73bba3b9eb65a673p-4 : inexact-ok
 = log2p1 upward ibm128 0x6.af53d00fd2845d4772260ef5acp-4 : 0x8.0efc6087a73bba3b9eb65a6734p-4 : inexact-ok
+log2p1 -0x1.da285cp-5
+= log2p1 downward binary32 -0xe.d142ep-8 : -0x1.60549p-4 : inexact-ok
+= log2p1 tonearest binary32 -0xe.d142ep-8 : -0x1.60549p-4 : inexact-ok
+= log2p1 towardzero binary32 -0xe.d142ep-8 : -0x1.60548ep-4 : inexact-ok
+= log2p1 upward binary32 -0xe.d142ep-8 : -0x1.60548ep-4 : inexact-ok
+= log2p1 downward binary64 -0xe.d142ep-8 : -0x1.60548f0000002p-4 : inexact-ok
+= log2p1 tonearest binary64 -0xe.d142ep-8 : -0x1.60548f0000001p-4 : inexact-ok
+= log2p1 towardzero binary64 -0xe.d142ep-8 : -0x1.60548f0000001p-4 : inexact-ok
+= log2p1 upward binary64 -0xe.d142ep-8 : -0x1.60548f0000001p-4 : inexact-ok
+= log2p1 downward intel96 -0xe.d142ep-8 : -0x1.60548f00000016dp-4 : inexact-ok
+= log2p1 tonearest intel96 -0xe.d142ep-8 : -0x1.60548f00000016dp-4 : inexact-ok
+= log2p1 towardzero intel96 -0xe.d142ep-8 : -0x1.60548f00000016cep-4 : inexact-ok
+= log2p1 upward intel96 -0xe.d142ep-8 : -0x1.60548f00000016cep-4 : inexact-ok
+= log2p1 downward m68k96 -0xe.d142ep-8 : -0x1.60548f00000016dp-4 : inexact-ok
+= log2p1 tonearest m68k96 -0xe.d142ep-8 : -0x1.60548f00000016dp-4 : inexact-ok
+= log2p1 towardzero m68k96 -0xe.d142ep-8 : -0x1.60548f00000016cep-4 : inexact-ok
+= log2p1 upward m68k96 -0xe.d142ep-8 : -0x1.60548f00000016cep-4 : inexact-ok
+= log2p1 downward binary128 -0xe.d142ep-8 : -0x1.60548f00000016cf47431656929dp-4 : inexact-ok
+= log2p1 tonearest binary128 -0xe.d142ep-8 : -0x1.60548f00000016cf47431656929dp-4 : inexact-ok
+= log2p1 towardzero binary128 -0xe.d142ep-8 : -0x1.60548f00000016cf47431656929cp-4 : inexact-ok
+= log2p1 upward binary128 -0xe.d142ep-8 : -0x1.60548f00000016cf47431656929cp-4 : inexact-ok
+= log2p1 downward ibm128 -0xe.d142ep-8 : -0x1.60548f00000016cf4743165693p-4 : inexact-ok
+= log2p1 tonearest ibm128 -0xe.d142ep-8 : -0x1.60548f00000016cf47431656928p-4 : inexact-ok
+= log2p1 towardzero ibm128 -0xe.d142ep-8 : -0x1.60548f00000016cf47431656928p-4 : inexact-ok
+= log2p1 upward ibm128 -0xe.d142ep-8 : -0x1.60548f00000016cf47431656928p-4 : inexact-ok
diff --git a/sysdeps/ieee754/flt-32/s_log2p1f.c b/sysdeps/ieee754/flt-32/s_log2p1f.c
index 09e77dc08ad..6fa7e5dc7a4 100644
--- a/sysdeps/ieee754/flt-32/s_log2p1f.c
+++ b/sysdeps/ieee754/flt-32/s_log2p1f.c
@@ -231,7 +231,12 @@ __log2p1f (float x)
       int j = (m + ((int64_t) 1 << (52 - 8))) >> (52 - 7), k = j > 53;
       e += k;
       double xd = asdouble (m | (uint64_t) 0x3ff << 52);
-      z = fma (xd, ix[j], -1.0);
+#ifndef __FP_FAST_FMA
+      if (__glibc_unlikely (x == -0x1.da285cp-5f))
+	z = fma (xd, ix[j], -1.0);
+      else
+#endif
+        z = xd * ix[j] - 1.0;
       static const double c[] =
 	{
 	   0x1.71547652b82fep+0, -0x1.71547652b82ffp-1,  0x1.ec709dc32988bp-2,
-- 
2.43.0