GNU bug report logs - #16535
Compiler bug ?

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#16535: closed (Compiler bug ?)
Date: Fri, 24 Jan 2014 23:36:02 +0000

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Your message dated Fri, 24 Jan 2014 15:35:21 -0800
with message-id <52E2F8B9.7080101 <at> cs.ucla.edu>
and subject line Re: Compiler bug ?
has caused the debbugs.gnu.org bug report #16535,
regarding Compiler bug ?
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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16535: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16535
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From: Fabrice Popineau <fabrice.popineau <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: Compiler bug ?
Date: Fri, 24 Jan 2014 16:03:17 +0100

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I hit a bug while compiling Gnu Emacs trunk development.

The following test :
 passed    17/1152  bool-vector-count-population-1-nil
fails unless I apply the following patch :

 === modified file 'src/data.c'
--- src/data.c  2014-01-03 06:47:27 +0000
+++ src/data.c  2014-01-24 14:59:16 +0000
@@ -3012,8 +3012,10 @@
     {
       int i = 0, count = 0;
       while (count += count_one_bits_ll (w),
-            BITS_PER_BITS_WORD <= (i += BITS_PER_ULL))
+            BITS_PER_BITS_WORD <= i) {
+       i += BITS_PER_ULL;
        w = shift_right_ull (w);
+      }
       return count;
     }
 }

This is with :
$ gcc --version
gcc.exe (Rev6, Built by MSYS2 project) 4.8.2
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Given the nature of the patch, I don't think it is specific of th MinGW64
gcc version.

Regards,

Fabrice Popineau

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Fabrice Popineau <fabrice.popineau <at> gmail.com>
Cc: 16535-done <at> debbugs.gnu.org
Subject: Re: Compiler bug ?
Date: Fri, 24 Jan 2014 15:35:21 -0800

On 01/24/2014 03:06 PM, Fabrice Popineau wrote:
> Actually, afaik BITS_PER_ULL == BITS_PER_BITS_WORD

Ah, OK, thanks for following up. It looks like the negated-comparison 
bug caused undefined behavior due to integer overflow in 
count_one_bits_word; once that happens it's anybody's guess what 
compilers will do; maybe you hit the jackpot. Anyway, the Emacs bug is 
fixed (and there doesn't seem to have been a compiler bug) so I'll close 
this report.

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