[Bug c/33262] New: Disappearing loop conditions

mj at ucw dot cz gcc-bugzilla@gcc.gnu.org
Fri Aug 31 11:37:00 GMT 2007

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:

        addl    $1, %esi
        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)
        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


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
#include "..." search starts here:
#include <...> search starts here:
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

           Summary: Disappearing loop conditions
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mj at ucw dot cz
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


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