[Bug rtl-optimization/35643] New: gcc-4.2 -O1 -fstrict-aliasing misoptimization

[Bug rtl-optimization/35643] New: gcc-4.2 -O1 -fstrict-aliasing misoptimization

[edwintorok at gmail dot com]

When I compile the testcase with -O2, or with '-O1 -fstrict-aliasing' incorrect
code is generated. The testcase calls abort() in this case.

This is a regression in gcc-4.2 only: gcc-4.1 and gcc-4.3 do not show the bug.

$ gcc-4.2 -O1 test.i -o ok && ./ok
$ gcc-4.2 -O2 test.i -o fails && ./fails
Aborted
$ gcc-4.2 -O1 -fstrict-aliasing test.i -o fails2 && ./fails2
Aborted
$ gcc-4.2 -O3 test.i -o fails3 && ./fails3
Aborted

The testcase gives no warnings:
$ gcc-4.2 -Wall -W -O2 -Wstrict-aliasing test.i

The original code that  is miscompiled is ClamAV, but only if -O3 is used.
I created an initial reduced testcase using delta. 
Manually inlining some functions show this bug even at -O2 on the reduced
testcase.

I reproduced this on 2 different systems:

edwin@lightspeed2:~$ uname -a
Linux lightspeed2 2.6.25-rc4-00134-g84c6f60 #4 Sun Mar 9 19:40:34 EET 2008
x86_64 GNU/Linux

edwin@lightspeed2:~$ gcc-4.2 -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr
--disable-libmudflap --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.2.3 (Debian 4.2.3-2)

I also reproduced the bug with gcc 4.2.3 built with --enable-checking=yes:
edwin@lightspeed2:~$ gcc-4.2.3/inst/bin/gcc -O1 -fstrict-aliasing test.i -o
fails && ./fails
Aborted

edwin@lightspeed2:~$ gcc-4.2.3/inst/bin/gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../configure --enable-languages=c
--prefix=/home/edwin/gcc-4.2.3/obj/../inst --enable-checking=yes
--disable-multilib --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu --disable-bootstrap
Thread model: posix
gcc version 4.2.3

The second system:
edwin@thunder:~$ uname -a
Linux thunder 2.6.25-rc5-00323-g9a9e0d6 #22 SMP PREEMPT Sun Mar 16 10:23:46 EET
2008 i686 GNU/Linux

edwin@thunder:~$ gcc-4.2 -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr
--disable-libmudflap --enable-targets=all --enable-checking=release
--build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.2.3 (Debian 4.2.3-2)


-- 
           Summary: gcc-4.2 -O1 -fstrict-aliasing misoptimization
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: edwintorok at gmail dot com
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35643

[Bug rtl-optimization/35643] gcc-4.2 -O1 -fstrict-aliasing misoptimization

[edwintorok at gmail dot com]

------- Comment #1 from edwintorok at gmail dot com  2008-03-19 18:54 -------
Created an attachment (id=15345)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15345&action=view)
testcase

Some observations:
If you remove some dead code the optimization bug goes away:

Remove either of these and you'll get correct code, for example:
if (metadata->chunk_current >= metadata->chunk_end) {
}

If I remove that if and compare assembly output, I see this:

$ diff -wu bad.s ok.s
--- bad.s       2008-03-19 20:51:59.000000000 +0200
+++ ok.s        2008-03-19 20:51:47.000000000 +0200
@@ -60,7 +60,7 @@
        testb   %al, %al
        js      .L9
        movsbq  %sil,%rax
-       addq    %rax, %rcx
+       leaq    1(%rcx,%rax), %rcx
 .L11:
        movzbl  (%rdx), %eax
        addq    $1, %rdx

Of course the original code that is miscompiled doesn't have dead code, it is
just because of the reduction.

If you need more info please ask.

This is related to this bug (but that occurs only at -O3): 
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=470557


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35643

[Bug rtl-optimization/35643] gcc-4.2 -O1 -fstrict-aliasing misoptimization

[edwintorok at gmail dot com]

------- Comment #2 from edwintorok at gmail dot com  2008-03-19 19:35 -------
Created an attachment (id=15346)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15346&action=view)
another reduced testcase

I reduced the testcase further using delta, however I had to be careful to
avoid deleting initializations. This testcase is valgrind clean.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35643

[Bug rtl-optimization/35643] [4.2 Regression] gcc-4.2 -O1 -fstrict-aliasing misoptimization

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2008-03-19 19:56 -------
-O vs. -O -fstrict-aliasing.  This is another flow-insensitive alias problem.

diff -u t.c.099t.optimized t.c.099t.optimizedx 
--- t.c.099t.optimized  2008-03-19 20:45:05.000000000 +0100
+++ t.c.099t.optimizedx 2008-03-19 20:45:00.000000000 +0100
@@ -16,9 +16,9 @@
 Analyzing Edge Insertions.
 cli_chm_prepare_file (metadata)
 {
-  char * temp.77;
   uint64_t section;
   char * D.1574;
+  char * D.1572;
   char * D.1565;

 <bb 2>:
@@ -30,12 +30,10 @@
   metadata->chunk_data = D.1565 + (char *) (unsigned int)
metadata->chunk_offset;

 <L3>:;
-  D.1574 = metadata->chunk_data + 20B;
-  metadata->chunk_current = D.1574;
-  temp.77 = D.1574 + 1B + (char *) (uint64_t) *D.1574;
-  metadata->chunk_current = temp.77;
-  section = (uint64_t) *temp.77;
-  metadata->chunk_current = temp.77 + 1B;
+  D.1572 = metadata->chunk_data + 20B;
+  D.1574 = D.1572 + (char *) (uint64_t) *D.1572;
+  section = (uint64_t) *D.1574;
+  metadata->chunk_current = D.1574 + 1B;
   if (section > 1) goto <L9>; else goto <L10>;

 <L9>:;


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |alias, wrong-code
      Known to fail|                            |4.2.3
      Known to work|                            |4.1.3 4.3.0
   Last reconfirmed|0000-00-00 00:00:00         |2008-03-19 19:56:20
               date|                            |
            Summary|gcc-4.2 -O1 -fstrict-       |[4.2 Regression] gcc-4.2 -O1
                   |aliasing misoptimization    |-fstrict-aliasing
                   |                            |misoptimization
   Target Milestone|---                         |4.2.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35643

[Bug rtl-optimization/35643] [4.2 Regression] gcc-4.2 -O1 -fstrict-aliasing misoptimization

[jsm28 at gcc dot gnu dot org]

------- Comment #4 from jsm28 at gcc dot gnu dot org  2008-05-19 20:25 -------
4.2.4 is being released, changing milestones to 4.2.5.


-- 

jsm28 at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.2.4                       |4.2.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35643

[Bug rtl-optimization/35643] [4.2 Regression] gcc-4.2 -O1 -fstrict-aliasing misoptimization

[jsm28 at gcc dot gnu dot org]

------- Comment #5 from jsm28 at gcc dot gnu dot org  2009-03-31 15:35 -------
Closing 4.2 branch, fixed in 4.3.


-- 

jsm28 at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
      Known to fail|4.2.3                       |4.2.3 4.2.5
         Resolution|                            |FIXED
   Target Milestone|4.2.5                       |4.3.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35643