[Bug c/39504] Incorrect code at -O2 and -O3

[pinskia at gmail dot com]

------- Comment #4 from pinskia at gmail dot com  2009-03-19 16:01 -------
Subject: Re:   New: Incorrect code at -O2 and -O3



Sent from my iPhone

On Mar 19, 2009, at 8:38 AM, "jk500500 at yahoo dot com"
<gcc-bugzilla@gcc.gnu.org 
 > wrote:

> The attached test program -- which I extracted and simplified from the
> '176.gcc'
> SPEC2000 benchmark -- is compiled incorrectly at -O2 and -O3.   The  
> code is
> correct at -O1 and -O0.
>
> The bad code I am reporting here is produced by a MIPS gcc-4.3.3
> cross-compiler.
> However, I see the same problem with an in-house port of gcc-4.3.3  
> to an
> in-house-developed CPU, so I believe the problem is generic to GCC and
> not specific to the MIPS version.
>
> I am attaching the complete testcase (a self-contained C file), but  
> the
> problem is specifically with this C code:
>
> rtx
> rtx_alloc (enum rtx_code jsk_code_arg) {
>    rtx rt;
>    struct obstack *ob = &rtl_obstack;
>    int length;
>    _obstack_newchunk(ob);
>    rt = (rtx)ob->object_base;
>    length = (sizeof (struct rtx_def) - sizeof (rtunion) - 1) /  
> sizeof (int);
>    for (; length >= 0; length--) {
>        ((int *) rt)[length] = 0;
>    }

This is undefined code. The code in spec is known to violate C90/C99/C+ 
+98 aliasing rules.

>
>
> #ifdef WORKAROUND
>    /* If enabled, will fix the issue */
>    __asm__ __volatile__ ( "sll r0,r0,0" );
> #endif
>
>    rt->jsk_code_val = jsk_code_arg;
>    return rt;
> }
>
>
> The "rt->jsk_code_val = jsk_code_arg" assignment is incorrectly  
> dropped
> from the generated code.  As the comment in the C code indicates, if I
> insert a volatile asm statement between the zero'ing of the *rt struct
> and the jsk_code_val assignment, correct code results at all  
> optimization
> levels.
>
> At -O2, the MIPS assembly code is below.  There is a 'sw' (store 32- 
> bit word)
> instruction near the end that results in the jsk_code_val field being
> set to zero rather than to the value of jsk_code_arg.
>
> rtx_alloc:
>        .frame  $sp,16,$31              # vars= 0, regs= 3/0, args=  
> 0, gp= 0
>        .mask   0x80030000,-4
>        .fmask  0x00000000,0
>        .set    noreorder
>        .set    nomacro
>
>        addiu   $sp,$sp,-16
>        sw      $16,4($sp)
>        lui     $16,%hi(rtl_obstack)
>        addiu   $4,$16,%lo(rtl_obstack)
>        addiu   $16,$16,%lo(rtl_obstack)
>        sw      $31,12($sp)
>        jal     _obstack_newchunk
>        sw      $17,8($sp)
>
>        lw      $2,8($16)
>        lw      $31,12($sp)
>        lw      $17,8($sp)
>        lw      $16,4($sp)
>        sw      $0,4($2)
>        sw      $0,0($2)   <--- Writes 'jsk_code_val' to zero
>        j       $31
>        addiu   $sp,$sp,16
>
>
>
> With the WORKAROUND define enabled, the code becomes as shown  
> below.  Now
> there is the correct 'sh' (store 16-bit halfword) instruction to set
> jsk_code_arg to the value of jsk_code_val.
>
> rtx_alloc:
>        .frame  $sp,16,$31              # vars= 0, regs= 3/0, args=  
> 0, gp= 0
>        .mask   0x80030000,-4
>        .fmask  0x00000000,0
>        addiu   $sp,$sp,-16
>        sw      $16,4($sp)
>        lui     $16,%hi(rtl_obstack)
>        sw      $17,8($sp)
>        move    $17,$4
>        addiu   $4,$16,%lo(rtl_obstack)
>        sw      $31,12($sp)
>        .set    noreorder
>        .set    nomacro
>        jal     _obstack_newchunk
>        addiu   $16,$16,%lo(rtl_obstack)
>        .set    macro
>        .set    reorder
>
>        lw      $2,8($16)
>        sw      $0,4($2)
>        sw      $0,0($2)
> #APP
> # 78 "./gcc0.c" 1
>        sll r0,r0,0
> # 0 "" 2
> #NO_APP
>        lw      $31,12($sp)
>        sh      $17,0($2)    <--------- sets jsk_code_val to  
> jsk_code_arg
>        lw      $16,4($sp)
>        lw      $17,8($sp)
>        .set    noreorder
>        .set    nomacro
>        j       $31
>        addiu   $sp,$sp,16
>
>
> -- 
>           Summary: Incorrect code at -O2 and -O3
>           Product: gcc
>           Version: 4.3.3
>            Status: UNCONFIRMED
>          Severity: normal
>          Priority: P3
>         Component: c
>        AssignedTo: unassigned at gcc dot gnu dot org
>        ReportedBy: jk500500 at yahoo dot com
> GCC build triplet: x86_64-unknown-linux-gnu
>  GCC host triplet: x86_64-unknown-linux-gnu
> GCC target triplet: mipsisa32-unknown-elf
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39504
>


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39504