[Bug c/36232] New: Optimizer removes condition check to terminate loop

[Bug c/36232] New: Optimizer removes condition check to terminate loop

[nick dot spence at freescale dot com]

This section of code is used in a bootloader to test DRAM.

It loops over the DRAM performing various operations of reads and writes.

When optimization (O2 or Os) is switched on the second loop in this example no
longer terminates, but continues indefinitely because the loop condition has
been removed from the optimized code


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           Summary: Optimizer removes condition check to terminate loop
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: nick dot spence at freescale dot com
 GCC build triplet: gcc version 4.1.2 20070626 (Red Hat 4.1.2-13)
  GCC host triplet: i386-redhat-linux
GCC target triplet: i386-redhat-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36232

[Bug c/36232] Optimizer removes condition check to terminate loop

[nick dot spence at freescale dot com]

------- Comment #1 from nick dot spence at freescale dot com  2008-05-13 19:51 -------
Created an attachment (id=15636)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15636&action=view)
Example code source file 

Compile with 'gcc -Os test.c' or 'gcc -O2 test.c' and examine assembly output.
Note this code cannot be run in user space becuase it will generate a
segmentation fault.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36232

[Bug c/36232] Optimizer removes condition check to terminate loop

[nick dot spence at freescale dot com]

------- Comment #2 from nick dot spence at freescale dot com  2008-05-13 19:52 -------
Created an attachment (id=15637)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15637&action=view)
Compiler intermediate file


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36232

[Bug c/36232] Optimizer removes condition check to terminate loop

[nick dot spence at freescale dot com]

------- Comment #3 from nick dot spence at freescale dot com  2008-05-13 19:56 -------
The code has two loops, which compile correctly when no optimization, or
-O1 are used, but the second loop becomes infinite if -Os or -O2 is used.

        .file   "test.c"
        .section        .rodata.str1.1,"aMS",@progbits,1
.LC0:
        .string "\n reads %d, 0x%08X"
        .text
.globl main
        .type   main, @function
main:
        leal    4(%esp), %ecx
        andl    $-16, %esp
        pushl   -4(%ecx)
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %esi
        pushl   %ebx
        xorl    %ebx, %ebx
        pushl   %ecx
        subl    $12, %esp
.L2:
        movl    $0, (%ebx)
        addl    $4, %ebx
        cmpl    $64, %ebx
        jne     .L2         /* branch condition for first loop */
        xorl    %esi, %esi
.L4:
        pushl   %eax
        pushl   %ebx
        subl    $4, %ebx
        pushl   %esi
        incl    %esi
        pushl   $.LC0
        call    printf
        movl    (%ebx), %eax
        addl    $16, %esp
        jmp     .L4            /* branch condition for second loop was lost */
        .size   main, .-main
        .ident  "GCC: (GNU) 4.1.2 20070626 (Red Hat 4.1.2-13)"
        .section        .note.GNU-stack,"",@progbits


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36232

[Bug c/36232] Optimizer removes condition check to terminate loop

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2008-05-13 20:02 -------
a pointer cannot go from a valid pointer to a NULL pointer.  See PR 36124.

*** This bug has been marked as a duplicate of 36124 ***


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pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36232

[Bug c/36232] Optimizer removes condition check to terminate loop

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2008-05-13 20:02 -------
>This section of code is used in a bootloader to test DRAM.

You cannot use C for that code because of how C defines pointers.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36232

[Bug c/36232] Optimizer removes condition check to terminate loop

[nick dot spence at freescale dot com]

------- Comment #6 from nick dot spence at freescale dot com  2008-05-13 20:41 -------
1) Since most bootloaders are written in C your view is very interesting but
probably not shared by everyone else working with embedded software.

2) If the compiler now treats this as an undefined condition it should report a
warning instead of silently creating invalid code.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36232