Re: [Tinycc-devel] [PATCH] tccgen.c: Optimise 0<<x, 0>>x, -1>>x, x&0, x*0, x|-1, x%1.

[Sergey Korshunoff]

> +            if (l2 == 1)
> +                memset(&vtop->c, 0, sizeof(vtop->c));

What this means?

2015-03-06 1:09 GMT+03:00, Edmund Grimley Evans
<address@hidden>:
> Any thoughts on this? There are lots of opportunities to introduce a
> bug here so it would be good if someone else could take a look.
>
> I'll push it at the weekend if there are no objections.
>
>
> From: Edmund Grimley Evans <address@hidden>
> Date: Wed, 4 Mar 2015 23:39:06 +0000
> Subject: [PATCH] tccgen.c: Optimise  0<<x, 0>>x, -1>>x, x&0, x*0, x|-1,
> x%1.
>
> More precisely, treat (0 << x) and so on as constant expressions, but
> not if const_wanted as we do not want to allow "case (x*0):", ...
>
> Do not optimise (0 / x) and (0 % x) here as x might be zero, though
> for an architecture that does not generate an exception for division
> by zero the back end might choose to optimise those.
> ---
>  tccgen.c |   34 ++++++++++++++++++++++++----------
>  1 file changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/tccgen.c b/tccgen.c
> index e86ff4a..a7251fb 100644
> --- a/tccgen.c
> +++ b/tccgen.c
> @@ -1476,16 +1476,30 @@ static void gen_opic(int op)
>              c2 = c1; //c = c1, c1 = c2, c2 = c;
>              l2 = l1; //l = l1, l1 = l2, l2 = l;
>          }
> -        /* Filter out NOP operations like x*1, x-0, x&-1... */
> -        if (c2 && (((op == '*' || op == '/' || op == TOK_UDIV ||
> -                     op == TOK_PDIV) &&
> -                    l2 == 1) ||
> -                   ((op == '+' || op == '-' || op == '|' || op == '^' ||
> -                     op == TOK_SHL || op == TOK_SHR || op == TOK_SAR) &&
> -                    l2 == 0) ||
> -                   (op == '&' &&
> -                    l2 == -1))) {
> -            /* nothing to do */
> +        if (!const_wanted &&
> +            c1 && ((l1 == 0 &&
> +                    (op == TOK_SHL || op == TOK_SHR || op == TOK_SAR)) ||
> +                   (l1 == -1 && op == TOK_SAR))) {
> +            /* treat (0 << x), (0 >> x) and (-1 >> x) as constant */
> +            vtop--;
> +        } else if (!const_wanted &&
> +                   c2 && ((l2 == 0 && (op == '&' || op == '*')) ||
> +                          (l2 == -1 && op == '|') ||
> +                          (l2 == 1 && (op == '%' || op == TOK_UMOD)))) {
> +            /* treat (x & 0), (x * 0), (x | -1) and (x % 1) as constant */
> +            if (l2 == 1)
> +                memset(&vtop->c, 0, sizeof(vtop->c));
> +            vswap();
> +            vtop--;
> +        } else if (c2 && (((op == '*' || op == '/' || op == TOK_UDIV ||
> +                          op == TOK_PDIV) &&
> +                           l2 == 1) ||
> +                          ((op == '+' || op == '-' || op == '|' || op ==
> '^' ||
> +                            op == TOK_SHL || op == TOK_SHR || op ==
> TOK_SAR) &&
> +                           l2 == 0) ||
> +                          (op == '&' &&
> +                           l2 == -1))) {
> +            /* filter out NOP operations like x*1, x-0, x&-1... */
>              vtop--;
>          } else if (c2 && (op == '*' || op == TOK_PDIV || op == TOK_UDIV))
> {
>              /* try to use shifts instead of muls or divs */
> --
> 1.7.10.4
>
>
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