Re: [Tinycc-devel] Largish mob update from me

[Michael Matz]

Hello,

On Sun, 18 Dec 2016, grischka wrote:

> Ok, I guess the best we can do to make sure the assert of above
> will never trigger is to tell the compiler to optimize it out,
> like so:
>
>    ST_FUNC void g(int c)
>    {
>        int ind1;
>        if (nocode_wanted)
>            return;
>        if (nocode_wanted)
>             tcc_error("internal: code generated but nocode_wanted");

Yes, that's of course the nicest thing as it's the most aggressive way to 
ensure no code is emitted.  But I had problems with that that I've not 
analyzed further back then.

> Why I think this is necessary is because I got some regression wrt.
> your improved "expr_land/lor()", and I wondered whether maybe the
> code was wrong in itself or in its trust on the underlying nocode
> machinery.  So to find out I needed a machinery that works ;)

Okay, so I was now forced to indeed analyze the problems (miscompiled 
the linux kernel) ;)  Result:
  http://repo.or.cz/tinycc.git/commitdiff/42e2a67f2

> There aren't any if's with (nocode_wanted) in tccgen.c anywhere
> anymore, exept the 3 mentioned in the patch, and it I'll be happy
> if that can stay so.

Unfortunately I had to add one other place where it checks nocode_wanted 
and that's in the ugly dealing with VT_CMP/VT_JMP values in vtop.  See 
testcases in the above, and my comment included here for 
citing/discussing:

+       Don't do this when nocode_wanted.  vtop might come from
+       !nocode_wanted regions (see 88_codeopt.c) and transforming
+       it to a register without actually generating code is wrong
+       as their value might still be used for real.  All values
+       we push under nocode_wanted will eventually be popped
+       again, so that the VT_CMP/VT_JMP value will be in vtop
+       when code is unsuppressed again.  */
+    if (vtop >= vstack && !nocode_wanted) {
         v = vtop->r & VT_VALMASK;
         if (v == VT_CMP || (v & ~1) == VT_JMP)
             gv(RC_INT);

I hope I haven't broken anything that you've fixed with your three 
nocode_wanted patches but lacking testcases I can't be sure (hint hint) :)


Ciao,
Michael.