[Bug c/14896] New: Fails to generate code for structure field assignment when assigning first field and assignment is within an infinite loop

["aweiner at lsil dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Version: powerpcle-440-eabi-gcc (GCC) 3.4.0 20040225 (prerelease)

When using the "-Os" compiler option, the optimizer will improperly optimize-
out the assignment of a structure field if that field is the FIRST field within 
the structure AND the assignemnt is within an infinite-loop block. Example:

typedef struct {
    int a;
    int b;
} TEST;

TEST    t; /* also fails if 't' is local to foo() */

void foo(void) {
    for (;;) { /* while (1) also fails */
        t.a = 5;
    }
}

Code generated:

00000000 <foo>:
   0:	00 00 00 48 	b	0 <foo>

Any of the following changes to the above source cause the compiler to generate 
the proper code:
     * Assign second field ('b') of structure rather than first ('a')
     * Move assignemnt outside of "for(;;)" loop
     * Access field through pointer rather than instance
     * Call to a non-inlined function is made within loop block
     * Compile without "-Os"

Below is the compiler output when the second field ('b') is assigned instead of 
the first ("t.b = 5" vs. "t.a = 5"):

00000000 <foo>:
   0:	00 00 20 3d 	lis	r9,0
   4:	05 00 00 38 	li	r0,5
   8:	00 00 29 39 	addi	r9,r9,0
   c:	04 00 09 90 	stw	r0,4(r9)
  10:	fc ff ff 4b 	b	c <foo+0xc>

-- 
           Summary: Fails to generate code for structure field assignment
                    when assigning first field and assignment is within an
                    infinite loop
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P1
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: aweiner at lsil dot com
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14896