signed overflow in atan2

[Eitan Adler <lists%eitanadler.com@localhost>]

Hi all,

I'd like to commit the patch below. Does anyone have concerns with it?

Previous discussion:
https://lists.freebsd.org/pipermail/freebsd-numerics/2018-February/thread.html
https://marc.info/?l=openbsd-tech&m=151859583112854&w=2

Original submission: https://github.com/freebsd/freebsd/pull/130


===

As a component of atan2(y, x), the case of x == 1.0 is farmed out to
atan(y). The current implementation of this comparison is vulnerable
to signed integer underflow (that is, undefined behavior), and it's
performed in a somewhat more complicated way than it need be. Change
it to not be quite so cute, rather directly comparing the high/low
bits of x to the specific IEEE-754 bit pattern that encodes 1.0.

Note that while there are three different e_atan* files in the
relevant directory, only this one needs fixing. e_atan2f.c already
compares against the full bit pattern encoding 1.0f, while
e_atan2l.cuses bitwise-ands/ors/nots and so doesn't require a change.
===



Index: e_atan2.c
===================================================================
RCS file: /cvs/src/lib/libm/src/e_atan2.c,v
retrieving revision 1.13
diff -u -r1.13 e_atan2.c
--- e_atan2.c 12 Sep 2016 19:47:02 -0000 1.13
+++ e_atan2.c 14 Feb 2018 08:06:05 -0000
@@ -64,7 +64,7 @@
  if(((ix|((lx|-lx)>>31))>0x7ff00000)||
     ((iy|((ly|-ly)>>31))>0x7ff00000)) /* x or y is NaN */
     return x+y;
- if(((hx-0x3ff00000)|lx)==0) return atan(y);   /* x=1.0 */
+ if(hx==0x3ff00000&&lx==0) return atan(y);   /* x=1.0 */
  m = ((hy>>31)&1)|((hx>>30)&2); /* 2*sign(x)+sign(y) */

     /* when y = 0 */

-- 
Eitan Adler