Re: [Tinycc-devel] MAJOR bug: tcc doesn't detect duplicate cases in switch statements

[Michael Matz]

Hi,

On Sat, 12 Mar 2016, address@hidden wrote:

> > First, the bug can't be as MAJOR as the subject wants to make us 
> > believe.  After all it wasn't noticed since the last $MANY years.
>
> The bug caused me to push bad code to gawk's repo.

I assumed something like that.  It still doesn't make the bug any more 
major than many of the other bugs that tcc has.

> It's only luck that it wasn't noticed, but a bug is a bug. Geting the 
> right answer a little more slowly is always better than getting the 
> wrong answer super fast.

No, I don't agree with you that a missed diagnostics is in the same class 
like wrong-code bugs.  Especially if it's costly to implement a diagnostic 
it's quite reasonable to not implement it in a compiler that's supposed to 
be quick (at least as long as there's other missing diagnostics).  Unlike 
wrong-code bugs for instance (well, even there the world is not always 
black and white, only mostly so).

> The issue isn't just the lack of diagnostic - what kind of erroneous 
> code is being generated in this case?

Any we want.  As the behaviour of the input was undefined the generated 
code can't be erroneous.  In practice I can tell you: tcc translates a 
switch literally into a sequence of if-else tests, so the code with 
overlapping cases is simply:
  if (a == 1) ...
  else if (a == 1) ...

So, first one matching wins.

> TCC is valuable for being both fast and correct.  Lose the correctness 
> and you lose ALL the value.

Given those absolute terms, I don't know of any compiler that's correct.
TCC certainly isn't.  TCC isn't bad either in correctness, it's actually 
quite good, but not 100%.  I hope you still continue to find TCC valuable, 
your use of it for serious development is much welcome :)

And, of course, I'm not dead-set against giving a diagnostic.  An 
implementation that's not making eyes bleed and is fast would be nice. 
E.g. remember cases in an array of ranges while parsing, qsort them after 
switch is finished, generated diagnostics on overlap.


Ciao,
Michael.