Re: Patch 4/6 randomize the stack pointer

[H. Peter Anvin]

Roland Dreier wrote:
>     Julien> Not very important but ((get_random_int() % 4096) << 4)
>     Julien> could be optimized into get_random_int() & 0xFFF0.
>
>     HPA> .. and gcc knows that.
>
>     HPA>    8:   25 ff 0f 00 00          and    $0xfff,%eax
>     HPA>    d:   83 c4 0c                add    $0xc,%esp
>     HPA>   10:   c1 e0 04                shl    $0x4,%eax
>
> Actually gcc isn't quite that smart (since it obviously can't
> understand the semantics of get_random int()).  The original point was
> that the "shl $0x4" can be avoided by directly &'ing with 0xfff0, not
> that "% 4096" can be strength reduced to "& 0xfff".

Oh, right.  D'oh!  :)

	-hpa

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