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From: Kaz Kylheku <221-501-9011@kylheku.com>
Newsgroups: comp.compilers
Subject: Re: Alternative C compilers on x86_64 Linux?
Date: Tue, 6 Sep 2016 02:15:52 +0000 (UTC)
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On 2016-09-05, BartC <bc@freeuk.com> wrote:
> On 02/09/2016 04:01, Aharon Robbins wrote:
>
>> TinyCC is blindingly fast, and can compile gawk, but is broken in that
>> it won't diagnose duplicate case statements inside switch. The developers
>> don't consider this a problem. So I refuse to use it.
>>
>> In short, I'm looking for a faster compiler that actually works.
>
> I'm quite impressed with Tiny CC. I'd considered it a toy compiler that
> could only compile a small subset of the language until I actually tried it.
>
> And yes, on my Win64 version, it does seem to ignore duplicate 'case'
> labels.
>
> But TCC does have problem with compiling switch statements; although the
> code it generates isn't that great anyway, that for switches is slower.
>
> I suspect it just compiles switch as an if-else chain. Which might
> explain why duplicate case labels are ignored if that conversion is done
> at at early stage.
>
> (Because a duplicate condition in an if-else chain is fine. For a
> jump-table of course, a duplicate label is ambiguous so can't be allowed.)

A duplicate condition in an if-else chain is unreachable code, which is
probably a bug.

   if (foo) {
     ...
   } else if (foo) {
     /* unreachable */
   } else if (bar) {
     ...
   }

Duplicate cases are a constraint violation in ISO C, requiring a
diagnostic.

A duplicate label can also be just as "fine". Look, you can build a jump
table, and then just fill it without caring about duplicates:

  case 42:
  ...
  case 42:

No problem: just write to jumptable[42] twice, letting it the most
recently established address overrule the previous one.

Duplicate case label code can still be reachable, making the bugs
potentially more interesting:

  case 42: // suppose this is used
    ...
    break;
  case 19:
    ...
    /* fallthrough */
  case 42: // this isn't used, but fallthrough makes it reachable

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