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Re: [PATCH][RFC] Bit CCP and pointer alignment propagation

On Fri, 30 Jul 2010, Paolo Bonzini wrote:

> On 07/30/2010 03:15 PM, Richard Guenther wrote:
> > I think we can have negative shift counts (at least the constant folding
> > code suggests so), this is why I have the code as-is.
> No, that seems very weird.  Sure expand does not handle it, and the
> implementation-defined section of the manual does not mention it.  I'm more
> inclined to consider it a historical wart, given this comment:
>   /* Previously detected shift-counts computed by NEGATE_EXPR
>      and shifted in the other direction; but that does not work
>      on all machines.  */
> dating back to the beginning of the GCC repo.  I wonder what the attached
> patch would do.

Maybe - at least with 2 << -1 we only warn:

t.c:3: warning: left shift count is negative

and happily continue, doing a constant right shift.  With -1 put
into a temporary we get sall with a negative shift at -O0 (and
zero as a result) and one as result when optimizing.

> BTW the SHIFT_COUNT_TRUNCATED handling is not needed because you get it from
> lshift_double.  However, this opens another small can of worms. lshift_double
> does
>     count %= prec;
> which makes bit-level analysis totally wrong for non-power-of-two precisions,
> including IIRC bitfields bigger than sizeof(int).  I think the shifting
> functions are wrong however, and should round prec up to the next power of two
> before applying the truncation.

Ick.  Indeed ...


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