Nix wrote:
I've grepped all the source on my system (1148 expanded upstream source
tarballs or git/cvs/svn trees including the Linux kernel, most of GNOME,
and all of KDE and X.org) and found that hits are extremely rare: not as
rare as calls to seekdir() and telldir() :) but rare. (Quite a lot of
things multiply by negative constants *inside* a call to abs(), but this
should be unaffected.)
I implemented a different approach: patched gcc with the official fix plus a
call to emit a warning (see below), and recompiled the whole LFS LiveCD (see the
list of packages at
http://wiki.linuxfromscratch.org/livecd/browser/trunk/packages). Only libtheora
emitted a warning.
Certain hits:
./nethack/3.4.3/src/cmd.c: else if(x < -2*abs(y))
./nethack/3.4.3/src/cmd.c: else if(y < -2*abs(x))
Sure, this is a hit, but nethack is not on my LiveCD.
Possible hits (I'm not sure what the folder would do with this: the
extra level of brackets in the way might affect things but I don't think
so):
./libtheora/libtheora/lib/enc/pp.c: TmpMod = 32 + QValue -
2*(abs(Src[j+Pitch]-Src[j]));
./libtheora/libtheora/lib/enc/pp.c: TmpMod = 32 + QValue -
2*(abs(Src[j+1]-Src[j]));
This did emit a warning, I have already reported it:
https://trac.xiph.org/ticket/1260
./xmms/modules/projectM-0.94.20/main.c:
wave_x_temp=-2*0.4142*(abs(abs(wave_mystery)-.5)-.5);
./xmms/modules/projectM-0.94.20/main.c:
wave_x_temp=-2*0.4142*(abs(abs(wave_mystery)-.5)-.5);
Not a hit, probably due to conversions between int and double.
--- trunk/gcc/fold-const.c 2007/11/17 13:46:53 130257
+++ trunk/gcc/fold-const.c 2007/11/17 14:22:42 130258
@@ -6095,6 +6095,9 @@
}
break;
}
+ /* If the constant is negative, we cannot simplify this. */
+ if (tree_int_cst_sgn (c) == -1)
+ { warning(0, "Unpatched gcc miscompiles this"); break; }
/* FALLTHROUGH */
case NEGATE_EXPR:
if ((t1 = extract_muldiv (op0, c, code, wide_type, strict_overflow_p))
--
Alexander E. Patrakov
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