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* Re: optimization/6330: tiny C++ prog broken under -O1
@ 2002-04-22 9:36 Richard Henderson
0 siblings, 0 replies; 4+ messages in thread
From: Richard Henderson @ 2002-04-22 9:36 UTC (permalink / raw)
To: nobody; +Cc: gcc-prs
The following reply was made to PR optimization/6330; it has been noted by GNATS.
From: Richard Henderson <rth@redhat.com>
To: David Hanney <dh@digitalbrain.com>
Cc: rth@gcc.gnu.org, gcc-bugs@gcc.gnu.org, gcc-prs@gcc.gnu.org,
nobody@gcc.gnu.org, gcc-gnats@gcc.gnu.org
Subject: Re: optimization/6330: tiny C++ prog broken under -O1
Date: Mon, 22 Apr 2002 09:34:33 -0700
On Mon, Apr 22, 2002 at 10:02:00AM +0100, David Hanney wrote:
> .. would a compiler warning be useful here?
Yes. There's already an open PR about this.
r~
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: optimization/6330: tiny C++ prog broken under -O1
@ 2002-04-22 2:06 David Hanney
0 siblings, 0 replies; 4+ messages in thread
From: David Hanney @ 2002-04-22 2:06 UTC (permalink / raw)
To: nobody; +Cc: gcc-prs
The following reply was made to PR optimization/6330; it has been noted by GNATS.
From: David Hanney <dh@digitalbrain.com>
To: rth@gcc.gnu.org, gcc-bugs@gcc.gnu.org, gcc-prs@gcc.gnu.org,
nobody@gcc.gnu.org, gcc-gnats@gcc.gnu.org, gcc-bugs@gcc.gnu.org,
gcc-prs@gcc.gnu.org, nobody@gcc.gnu.org
Cc:
Subject: Re: optimization/6330: tiny C++ prog broken under -O1
Date: Mon, 22 Apr 2002 10:02:00 +0100
Many thanks for your prompt response!
I assumed (wrongly, I know) that because "the cast-output is the same four
bytes
as the cast-input" that it would be at the same address (union-style). My bad.
But, given this behavior-set ..
gcc with plain C++ works. (no compiler warning)
gcc with optimized C++ doesn't work. (no compiler warning)
gcc with C doesn't compile.
(under gcc 2.95.4 and gcc 3)
.. would a compiler warning be useful here?
Especially as the pointer may be formed implicitly via a reference
(as shown in the code quoted below)
cheers,
DH
-------------------------------------------
#!/bin/sh
# what compiler/OS version is it?
gcc-3.0 -v
uname -a
# create a program
echo '#include <stdio.h>' > x.cpp
echo '#include <stdlib.h>' >> x.cpp
echo 'typedef unsigned char byte;' >> x.cpp;
echo 'typedef byte *pbyte;' >> x.cpp;
echo 'void change(pbyte &z) { z = (pbyte)"correct"; }' >> x.cpp
echo 'int main() {' >> x.cpp
echo ' char *s = "incorrect";' >> x.cpp
echo ' change((byte*)s);' >> x.cpp
echo ' printf("%s\n", s);' >> x.cpp
echo ' exit(0);' >> x.cpp
echo '}' >> x.cpp
# compile and run it
gcc-3.0 x.cpp
./a.out
# compile and run The Same Program optimized
gcc-3.0 -O1 x.cpp
./a.out
-------------------------------------------
At 00:41 18/04/02 +0000, rth@gcc.gnu.org wrote:
>Synopsis: tiny C++ prog broken under -O1
>
>State-Changed-From-To: open->closed
>State-Changed-By: rth
>State-Changed-When: Wed Apr 17 17:41:35 2002
>State-Changed-Why:
> Not a bug. You have taken the address of a temporary.
> You wanted "change((byte**)&s)".
>
>http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6330
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: optimization/6330: tiny C++ prog broken under -O1
@ 2002-04-17 17:41 rth
0 siblings, 0 replies; 4+ messages in thread
From: rth @ 2002-04-17 17:41 UTC (permalink / raw)
To: dh, gcc-bugs, gcc-prs, nobody
Synopsis: tiny C++ prog broken under -O1
State-Changed-From-To: open->closed
State-Changed-By: rth
State-Changed-When: Wed Apr 17 17:41:35 2002
State-Changed-Why:
Not a bug. You have taken the address of a temporary.
You wanted "change((byte**)&s)".
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6330
^ permalink raw reply [flat|nested] 4+ messages in thread
* optimization/6330: tiny C++ prog broken under -O1
@ 2002-04-17 2:16 dh
0 siblings, 0 replies; 4+ messages in thread
From: dh @ 2002-04-17 2:16 UTC (permalink / raw)
To: gcc-gnats
>Number: 6330
>Category: optimization
>Synopsis: tiny C++ prog broken under -O1
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: sw-bug
>Submitter-Id: net
>Arrival-Date: Wed Apr 17 02:16:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator: dh@digitalbrain.com
>Release: 2.95.4 20011002 (Debian prerelease)
>Organization:
>Environment:
gcc version 2.95.4 20011002 (Debian prerelease)
Linux fee 2.4.18-p7-rmap12c-r #267 SMP Mon Feb 4 12:05:56 GMT 2002 i686 unknown
>Description:
Taking the address of an automatic variable says to the compiler "don't use a register for this!".
But I suspect that casting an automatic variable into another type and then taking the address of That
doesn't say anything of the kind
(even though it should)
>How-To-Repeat:
this linux console output shows a tiny shell script
being run that duplicates the problem ..
5750:fee:~: cat ./show_gcc_bug.sh
#!/bin/sh
# what compiler/OS version is it?
gcc -v
uname -a
# create a program
echo '#include <stdio.h>' > x.cpp
echo 'typedef unsigned char byte;' >> x.cpp;
echo 'void change(byte **z) { *z = (byte*)"correct"; }' >> x.cpp
echo 'int main() {' >> x.cpp
echo ' char *s = "incorrect";' >> x.cpp
echo ' change(&(byte*)s);' >> x.cpp
echo ' printf("%s\n", s);' >> x.cpp
echo ' exit(0);' >> x.cpp
echo '}' >> x.cpp
# compile and run it
gcc x.cpp
./a.out
# compile and run The Same Program optimized
gcc -O1 x.cpp
./a.out
5751:fee:~: ./show_gcc_bug.sh
Reading specs from /usr/lib/gcc-lib/i386-linux/2.95.4/specs
gcc version 2.95.4 20011002 (Debian prerelease)
Linux fee 2.4.18-p7-rmap12c-r #267 SMP Mon Feb 4 12:05:56 GMT 2002 i686 unknown
correct
incorrect
5752:fee:~:
>Fix:
no fix known ..
work arounds:
1)
change the x.cpp from &(byte*)s
to (byte**)&s
2)
rename x.cpp to x.c
because the C front end will give the error
"x.c:6: invalid lvalue in unary `&'"
rather than make incorrect code
>Release-Note:
>Audit-Trail:
>Unformatted:
^ permalink raw reply [flat|nested] 4+ messages in thread
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2002-04-22 9:36 optimization/6330: tiny C++ prog broken under -O1 Richard Henderson
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2002-04-22 2:06 David Hanney
2002-04-17 17:41 rth
2002-04-17 2:16 dh
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