Re: [Tinycc-devel] Regarding Long double Constant

[Michael Matz]

Hello,

On Thu, 11 Feb 2021, Ayush Varshney wrote:

> [1] https://www.researchgate.net/publication/245578769_Fully_Countering_Tru
> sting_Trust_through_Diverse_Double-Compiling
>
> On Thu, Feb 11, 2021 at 9:27 AM Ayush Varshney <varshneyayush90@gmail.com>
> wrote:
>       Hi everyone,
> Hope you all are doing well!!
> I am new to the tinycc community.
> I was working over Diverse Double-compiling technique [1]. And found
> there is a bad optimization performed by tcc. The problem is called
> Long double constant problem.
>
> Long double constant problem is for storing the value of 0.0, tcc stores 
> 0.0 in memory as long double value but long double value in tcc takes 
> only 10 bytes but the source code stores 0.0 value in 12 bytes.

Can you show this with a small example?  Because TCC, when allocating 
storage in data sections (which is where float constants are stored) zeros 
the memory.  (And the trivial example ala:

  long double ld;
  int foo(void) { return ld == 0.0; }

shows that indeed, .data contains only zeros. )

If you are talking about a different compiler than TCC having this problem 
(i.e. initializing only 10 of the 12 bytes), then _that_ needs to be fixed 
instead of putting such code in TCC.

Also, if TCC really has a problem in initializing some memory for 
reproducability it probably doesn't affect only long double, in which case 
fixing the root cause also will be better.

> The extra two bytes creates variations in the output. The problem is 
> solved using (f1-f1) instead of 0.0, it is semantically same and 
> produces the same output. For example, Instead of ‘if(f2==0.0)’ use 
> ‘if(f2==f1-f1)’

That's only a work around, for a compiler not initializing memory fully 
(and if it's TCC doing that, please show us where).  Your change in 
init_putv also doesn't compile.  Please don't push anything to mob that 
you haven't at least tested for that.  But even better would be to give us 
some more details.

Meanwhile I've reverted the change as it broke mob.


Ciao,
Michael.