From RISKS: secret scrubbing code removed by optimizers

[Ben Lindstrom]

On Fri, 8 Nov 2002, Darren Tucker wrote:

> 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.
>

does setting -fno-builtin-memset at compile time stop gcc from
miscompiling?

- Ben


> 		-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.
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