Useless assembly

[Sam Lauber]

When I run GCC 3.4.3 on this code:

#include <stdio.h>                                                                                                                             
int main(void)
{
        printf("Hello World!\n");
        return 0;
}
                                                                                                                                                                                                                                                          
it generates the assembly code (this is i686 assembly)

        .file   "test.c"
        .section        .rodata
.LC0:
        .string "Hello World!\n"
        .text
.globl main
        .type   main, @function
main:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        andl    $-16, %esp
        movl    $0, %eax
        addl    $15, %eax
        addl    $15, %eax
        shrl    $4, %eax
        sall    $4, %eax
        subl    %eax, %esp
        movl    $.LC0, (%esp)
        call    printf
        movl    $0, %eax
        leave
        ret
        .size   main, .-main
        .section        .note.GNU-stack,"",@progbits
        .ident  "GCC: (GNU) 3.4.3"
                                                                                                                             
By hand-optimizing the assembly, I made this:

.LC0:
        .string "Hello World!\n"
.globl main
        .type   main, @function
main:
        pushl   %ebp
        movl    %esp, %ebp
        movl    $.LC0, (%esp)
        call    printf
        leave

It worked the exact same. Clearly a lot was unnessecary. Removing "call printf" or "movl $.LC0, (%esp)" caused it not to print "Hello World!". Removing ".globl main" caused the linker to fail with an undefined reference to main. Removing anything else caused it to print Hello World and then generate a segmentation fault. Optimizing it at -O3 (which I'm sure includes SSA, DCE, and CCP) just made the compilation take longer and generate more assembly then the first time. If someone could just automate removing that code in cc1, it would probably mean a lot of optimization. Also, when I just compiled a "return 0;", I would expect that to just generate main and the instruction "ret" (or is it "retn?"). Instead, it generated a lot of extra instructions, then finally "ret". Clearly this is unsatisfactory. It makes cc1, as, and ld slower because they have to compile, assemble, and link more instructions. Each extra instruction makes a compilation time penalty of -3 and a runtime penalty of -1. At least that could be put onto DCE. If extra code gets generated by cc1, as and ld can't optimize it.

Samuel Lauber
-- 
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