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math: Optimize fma call on log2pf1

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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.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>
This commit is contained in:
Adhemerval Zanella
2025-10-10 09:52:35 -03:00
parent 82a4f50b4e
commit ae49afe74d
3 changed files with 34 additions and 1 deletions
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1
math/auto-libm-test-in
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@@ -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

25
math/auto-libm-test-out-log2p1
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@@ -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

9
sysdeps/ieee754/flt-32/s_log2p1f.c
View File

@@ -231,7 +231,14 @@ __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
/* The fma is required only for x == -0x1.da285cp-5f in FE_TONEAREST
to provide correctly rounded results. */
if (__glibc_likely (x != -0x1.da285cp-5f))
z = xd * ix[j] - 1.0;
else
#endif
z = fma (xd, ix[j], -1.0);
static const double c[] =
{
0x1.71547652b82fep+0, -0x1.71547652b82ffp-1, 0x1.ec709dc32988bp-2,