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

["jk500500 at yahoo dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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;
    }

#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