On Sun, 2010-01-03 at 11:24 +0530, prashant rawat wrote:
> I have a simple C code :
>
> #include<stdio.h>
> int *c;
> int main ()
> {
> int a=0, b;
> while (a < 100)
> {
> b = 10;
> a = a + 1;
> }
> c = &b;
> printf ("%d\n", *c);
> return 0;
> }
>
> I would basically expect the b = 10 statement to be moved out of the
> while loop, as it is an invariant.
> However, it does not seem to happen in the *.lim dump file.
> I am using gcc 4.4.2 to compile, and the cmd line options are
> install/bin/gcc -fdump-tree-all -O3 filename.c
>
> Can anyone explain what went wrong?
>
My version of gcc (4.4.1) completely eliminates the loop since it it
irrelevant. Notice that a is not used beyond the loop. gcc generates the
following assembly language:
.file "loop_invar.c"
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "%d\n"
.text
.p2align 4,,15
.globl main
.type main, @function
main:
.LFB22:
.cfi_startproc
subq $24, %rsp
.cfi_def_cfa_offset 32
movl $10, %edx # 3rd argument to printf
movl $.LC0, %esi # address of format string
leaq 12(%rsp), %rax # address of b
movl $1, %edi # check stack overflow
movl $10, 12(%rsp) # b = 10;
movq %rax, c(%rip) # c = &b;
xorl %eax, %eax # no floats
call __printf_chk
xorl %eax, %eax # return 0;
addq $24, %rsp
ret
.cfi_endproc
.LFE22:
.size main, .-main
.comm c,8,8
.ident "GCC: (Ubuntu 4.4.1-4ubuntu8) 4.4.1"
.section .note.GNU-stack,"",@progbits
which is the equivalent of:
#include<stdio.h>
int *c;
int main ()
{
int b;
b = 10;
c = &b;
printf ("%d\n", 10);
return 0;
}
--Bob
