Re: [Tinycc-devel] Tinycc-devel Digest, Vol 134, Issue 16

[Michael Matz]

Hi,

On Sun, 22 Jun 2014, jiang wrote:

> This is my patch (see Annex)
>
> tcc result is correct
> --> 254 / 30 / 126

That's not the correct result.  If you think it is the probably because 
you're confused by the semantics of assignments as rvalue.

> I guess gcc & mvc repeated use of the register, so wrong

No, GCC and MSVC are correct.

> > struct {
> > unsigned a:9, b:5, c:7;
> > } _s, *s = &_s;
> > int n = 250;
> >
> > s->a = s->b = s->c = n + 4;

To show that GCC is correct (and TCC wrong, and your patch still wrong) 
rewrite the above into the explicit expansion according to associativity 
of '=':

  s->c = n + 4;  // s->c == 254 & 127 == 126
  s->b = s->c;   // s->b == 126 & 31  == 30
  s->a = s->b;   // s->a == 30

In particular the value loaded into s->a comes from s->b, and hence is the 
truncated value, which isn't changed anymore as 5 bits fits into 9 bits.


Ciao,
Michael.