From 68d71289425bb133c6cbf0f5065da6b1d99f81fc Mon Sep 17 00:00:00 2001
From: Vincent Lefevre <vincent@vinc17.net>
Date: Fri, 22 Nov 2024 13:54:53 -0300
Subject: [PATCH] math: Fix non-portability in the computation of signgam in
lgammaf
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The k>>31 in signgam = 1 - (((k&(k>>31))&1)<<1); is not portable:
* The ISO C standard says "If E1 has a signed type and a negative
value, the resulting value is implementation-defined." (this is
still in C23).
* If the int type is larger than 32 bits (e.g. a 64-bit type),
then k = INT_MAX; line 144 will make k>>31 put 1 in bit 0
(thus signgam will be -1) while 0 is expected.
Moreover, instead of the fx >= 0x1p31f condition, testing fx >= 0
is probably better for 2 reasons:
The signgam expression has more or less a condition on the sign
of fx (the goal of k>>31, which can be dropped with this new
condition). Since fx ≥ 0 should be the most common case, one can
get signgam directly in this case (value 1). And this simplifies
the expression for the other case (fx < 0).
This new condition may be easier/faster to test on the processor
(e.g. by avoiding a load of a constant from the memory).
This is commit d41459c731865516318f813cf4c966dafa0eecbf from CORE-MATH.
Checked on x86_64-linux-gnu.
---
sysdeps/ieee754/flt-32/e_lgammaf_r.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)
diff --git a/sysdeps/ieee754/flt-32/e_lgammaf_r.c b/sysdeps/ieee754/flt-32/e_lgammaf_r.c
index cb65513056..447376dc55 100644
--- a/sysdeps/ieee754/flt-32/e_lgammaf_r.c
+++ b/sysdeps/ieee754/flt-32/e_lgammaf_r.c
@@ -181,12 +181,11 @@ __ieee754_lgammaf_r (float x, int *signgamp)
Note that for a binary32 |x| >= 2^23, x is necessarily an integer,
and we already dealed with negative integers, thus now:
-2^23 < x < +Inf and x is not a negative integer nor 0, 1, 2. */
- int k;
- if (__builtin_expect (fx >= 0x1p31f, 0))
- k = INT_MAX;
+ if (__glibc_unlikely (fx >= 0))
+ *signgamp = 1;
else
- k = fx;
- *signgamp = 1 - (((k & (k >> 31)) & 1) << 1);
+ /* gamma(x) is negative in (-2n-1,-2n), thus when fx is odd. */
+ *signgamp = 1 - ((((int) fx) & 1) << 1);
double z = ax, f;
if (__glibc_unlikely (ax < 0x1.52p-1f))