9fans - fans of the OS Plan 9 from Bell Labs
 help / color / mirror / Atom feed
From: ori@eigenstate.org
To: 9fans@9fans.net
Subject: Re: [9fans] Conversion of constants in C compiler
Date: Wed, 27 Apr 2022 22:43:59 -0400	[thread overview]
Message-ID: <3C132FBD69F1EB1594B8F56989784069@eigenstate.org> (raw)
In-Reply-To: <8530f0d7-7c38-92af-d649-23f99645b748@SDF.ORG>

Quoth adr <adr@SDF.ORG>:
> On Wed, 20 Apr 2022, ori@eigenstate.org wrote:
> > When you have a patch, let me know -- I'll happily test
> > and apply to 9front.
> 
> Hi ori, this patch applyes to the sources served at 9front.org.
> By the way, do you plan to keep in sync
> http://only9fans.com/ori/git9/HEAD/info.html or should I forget
> about that repo?
> 
> Regards,
> adr.
> 
> --- /n/9front/sys/src/cmd/cc/lex.c      Wed Apr  6 14:45:26 2022
> +++ /tmp/lex.c  Thu Apr 21 08:39:14 2022
> @@ -848,16 +848,9 @@
>                 yyerror("overflow in constant");
> 
>         vv = yylval.vval;
> -       /*
> -        * c99 is silly: decimal constants stay signed,
> -        * hex and octal go unsigned before widening.
> -        */
> -       w = 32;
> -       if((c1 & (Numdec|Numuns)) == Numdec)
> -               w = 31;
> -       if(c1 & Numvlong || (c1 & Numlong) == 0 && (uvlong)vv >= 1ULL<<w){
> -               if((c1&(Numdec|Numvlong)) == Numdec && vv < 1ULL<<32)
> -                       warn(Z, "int constant widened to vlong: %s", symb);
> +       if(c1 & Numvlong ||
> +         convvtox(vv, TUVLONG) > convvtox(vv, TULONG) ||
> +         (c1 & (Numdec|Numuns)) == Numdec && convvtox(vv, TLONG) < 0) {
>                 if((c1 & Numuns) || convvtox(vv, TVLONG) < 0) {
>                         c = LUVLCONST;
>                         t = TUVLONG;

Thanks for the patch!

I think this gets the condition slightly wrong. For a decimal
number, we should promote:

        int -> long -> vlong.

For a hex number, though, we want

        int -> uint -> long -> ulong -> vlong -> uvlong

however with the condition above,

        18446744073709551615

will get converted to a uvlong:

        c1 & Numvlong == 0:
                false -> no explicit conversion
        convvtox(vv, TUVLONG) > convvtox(vv, TULONG)
                true -> vv fits into a uvlong, but not a ulong,
                so we enter this.

so then, we end up in this branch:

        if((c1 & Numuns) || convvtox(vv, TVLONG) < 0) {

                true, because convvtox(vv, TVLONG) < 0.

I think we want:

        if(c1 & Numvlong ||
          convvtox(vv, TUVLONG) != convvtox(vv, TULONG) ||
          convvtox(vv, TLONG) < 0) {
                if((c1 & (Numdec|Numuns)) == 0 &&
                  ((c1 & Numuns) || convvtox(vv, TVLONG) < 0)) {
                        c = LUVLCONST;
                        t = TUVLONG;
                        goto nret;
                }
                c = LVLCONST;
                t = TVLONG;
                goto nret;
        }
        if(c1 & Numlong ||
          convvtox(vv, TULONG) > convvtox(vv, TUINT) ||
          convvtox(vv, TINT) < 0) {
                if((c1 & (Numdec|Numuns)) == 0 && 
                  ((c1 & Numuns) || convvtox(vv, TLONG) < 0)) {
                        c = LULCONST;
                        t = TULONG;
                        goto nret;
                }
                c = LLCONST;
                t = TLONG;
                goto nret;
        }


------------------------------------------
9fans: 9fans
Permalink: https://9fans.topicbox.com/groups/9fans/T22754f10b241991c-M4a92d1c5d54d32d3f8221d1e
Delivery options: https://9fans.topicbox.com/groups/9fans/subscription

  parent reply	other threads:[~2022-04-28  2:44 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-04-20 10:19 adr
2022-04-20 11:38 ` Charles Forsyth
2022-04-20 14:37   ` adr
2022-04-20 14:37 ` ori
2022-04-21  7:54   ` adr
2022-04-21 15:29     ` ori
2022-04-28  1:10     ` ori
2022-05-01 12:03       ` adr
2022-04-28  2:43     ` ori [this message]
2022-04-28  3:12       ` ori
2022-05-01 12:35       ` adr
2022-05-09 19:31         ` adr
2022-05-10 13:06           ` adr
2022-05-10 14:48             ` adr
2022-06-06  0:35 adr

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=3C132FBD69F1EB1594B8F56989784069@eigenstate.org \
    --to=ori@eigenstate.org \
    --cc=9fans@9fans.net \
    --subject='Re: [9fans] Conversion of constants in C compiler' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).