From RISKS: secret scrubbing code removed by optimizers

[Darren Tucker]

Dan Kaminsky wrote:
>     Has somebody actually verified this optimizing behavior in any build
> of GCC?  Does voliatile actually stop it?

Yes (gcc-3.2 on a SPARC). Yes.

		-Daz.

Test function:
void myfunc1()
{
        char p[100];

        scanf("%s\n", &p);
        memset(p, 0, 100);
}

gcc -s testfunc.c gives:
myfunc1:
        !#PROLOGUE# 0
        save    %sp, -216, %sp
        !#PROLOGUE# 1
        add     %fp, -120, %o1
        sethi   %hi(.LLC0), %o0
        or      %o0, %lo(.LLC0), %o0
        call    scanf, 0
         nop
        add     %fp, -120, %o0
        mov     0, %o1
        mov     100, %o2
        call    memset, 0
         nop
        nop
        ret
        restore

gcc -s -O3 testfunc.c
myfunc1:
        !#PROLOGUE# 0
        save    %sp, -216, %sp
        !#PROLOGUE# 1
        sethi   %hi(.LLC0), %g1
        or      %g1, %lo(.LLC0), %o0
        call    scanf, 0
        add     %fp, -120, %o1
        nop
        ret
        restore

Add "volatile" and inlines and unrolls memset:
myfunc1:
        !#PROLOGUE# 0
        save    %sp, -216, %sp
        !#PROLOGUE# 1
        sethi   %hi(.LLC0), %g1
        or      %g1, %lo(.LLC0), %o0
        call    scanf, 0
        add     %fp, -120, %o1
        mov     0, %o2
        mov     0, %o3
        std     %o2, [%fp-120]
        std     %o2, [%fp-112]
        std     %o2, [%fp-104]
        std     %o2, [%fp-96]
        std     %o2, [%fp-88]
        std     %o2, [%fp-80]
        std     %o2, [%fp-72]
        std     %o2, [%fp-64]
        std     %o2, [%fp-56]
        std     %o2, [%fp-48]
        std     %o2, [%fp-40]
        std     %o2, [%fp-32]
        st      %g0, [%fp-24]
        nop
        ret
        restore

-- 
Darren Tucker (dtucker at zip.com.au)
GPG Fingerprint D9A3 86E9 7EEE AF4B B2D4  37C9 C982 80C7 8FF4 FA69
    Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.