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Re: [bug-gnulib] Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
- From: Bruno Haible <bruno at clisp dot org>
- To: bug-gnulib at gnu dot org
- Cc: Richard Kenner <kenner at vlsi1 dot ultra dot nyu dot edu>, eggert at cs dot ucla dot edu, gcc at gcc dot gnu dot org, autoconf-patches at gnu dot org
- Date: Mon, 8 Jan 2007 18:14:19 +0100
- Subject: Re: [bug-gnulib] Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
- References: <200612291732.kBTHWNkC024013@localhost.localdomain> <email@example.com> <10612300026.AA05792@vlsi1.ultra.nyu.edu>
Richard Kenner wrote:
> (3) How many programs are known to rely on wrap semantics? For each:
> (a) How hard was it to determine there was a problem with that
A piece of data for GNU clisp and cln:
- For clisp, it was easy to find out and fix all problems, because the
package has a good testsuite coverage. It was just a matter of building
the package with CFLAGS=-ftrapv, doing "make check", and debugging the
abort() calls. 2 or 3 hours of work.
Just 2 problems were found:
- A loop:
for (x=1; x != 0; x = x+x) ...
This is the same idiom as Paul's example.
- A bitmask computation that assumed two's complement arithmetic:
if (n > 0)
*p &= (unsigned int) ((1 << (32 - n)) - 1);
- For cln, I fixed the easily spottable problems, but I completely ignore
how many other problems remain - because the package has only a small
testsuite, and gcc last week emitted no warnings when it exploits the C99
semantics of signed integer overflow.