Bug 33262 - Disappearing loop conditions
Disappearing loop conditions
Status: UNCONFIRMED
Product: gcc
Classification: Unclassified
Component: middle-end
4.2.1
: P3 normal
: ---
Assigned To: Not yet assigned to anyone
:
Depends on:
Blocks:
  Show dependency treegraph
 
Reported: 2007-08-31 11:37 UTC by Martin Mares
Modified: 2008-07-09 16:19 UTC (History)
2 users (show)

See Also:
Host: i686-pc-linux-gnu
Target: i686-pc-linux-gnu
Build: i686-pc-linux-gnu
Known to work:
Known to fail:
Last reconfirmed:


Attachments
Source file triggering the problem (86.43 KB, text/plain)
2007-08-31 11:38 UTC, Martin Mares
Details

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Description Martin Mares 2007-08-31 11:37:40 UTC
When I compile the attached source file, the s2_multiway_merge function gets apparently miscompiled. The do loop near the end of this function loses both of its terminating conditions and it is compiled to:

.L101:  
        addl    $1, %esi
.L53:   
        movl    -76(%ebp), %edi
        movl    -92(%ebp), %eax
        movl    -96(%ebp), %edx
        movl    -32(%ebp), %ecx
        movl    -36(%ebp), %ebx
        movl    %edi, -4(%eax,%esi,4)
        movl    -52(%ebp), %eax
        movl    -100(%ebp), %edi
        movl    %ecx, -4(%edx,%esi,4)
        movl    -40(%ebp), %edx
        movl    %eax, -4(%edi,%esi,4)
        movl    $-1, (%edx)
.L88:   
        shrl    %ebx
        je      .L101
[... the s2_update_tree loop got inlined here, this test is its correct terminating condition ...]

It seems that gcc thinks that `i' never changes in the loop, so it has optimized out every expression which depends in its value.

I apologize for submitting a large source file, but the problem appears to be very chaotic and even removing parts of the source which are not referenced anywhere (see for example the block with the MAGIC comment attached) makes the problem disappear.

Compilation command: gcc-4.2.1 -v -std=gnu99 -O2 -S xxx.c -Wunused -fgnu89-inline

Output:

Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ./configure --prefix=/opt/gcc-4.2.1 --enable-bootstrap
Thread model: posix
gcc version 4.2.1
 /opt/gcc-4.2.1/libexec/gcc/i686-pc-linux-gnu/4.2.1/cc1 -quiet -v xxx.c -quiet -dumpbase xxx.c -mtune=generic -auxbase xxx -O2 -Wunused -std=gnu99 -version -fgnu89-inline -o xxx.s
ignoring nonexistent directory "/opt/gcc-4.2.1/lib/gcc/i686-pc-linux-gnu/4.2.1/../../../../i686-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /opt/gcc-4.2.1/include
 /opt/gcc-4.2.1/lib/gcc/i686-pc-linux-gnu/4.2.1/include
 /usr/include
End of search list.
GNU C version 4.2.1 (i686-pc-linux-gnu)
        compiled by GNU C version 4.2.1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 9831b30df15d36dc669e016cd6c4ba43
Comment 1 Martin Mares 2007-08-31 11:38:39 UTC
Created attachment 14146 [details]
Source file triggering the problem
Comment 2 Martin Mares 2008-07-09 16:11:39 UTC
Can I do anything to help catch the bug?
Comment 3 Richard Biener 2008-07-09 16:19:22 UTC
Try with a recent GCC 4.2 version - GCC 4.2.4 is available.  Also try
GCC 4.3.1.  Try to reduce the testcase or at least make it executable and
arrange for it to abort () whenever the problem appears.