Extended Assembler gets goto support

[Iain Buclaw via D.gnu]

I've been promising myself to get round to adding support for 
this, well now it is very close to hitting the master repos.

Say hello to GCC's "asm goto" support in GDC.

---
int main()
{
   asm { "jmp %l[MyLabel]" : : : : MyLabel; }
   return 0;

MyLabel:
   return 1;
}
---

The added fourth section tells the GCC optimizer that this asm 
statement may jump to the label 'MyLabel', so the block following 
doesn't get marked as unreachable code.


Also hooked into this, are the same D language front-end 
heuristics for goto statements.  Meaning the following is an 
error:

---
int main()
{
   asm {  // goto skips initialization of 'skipme'
     "jmp %l[Lerror]" : : : : Lerror;
   }
   return 0;

   int skipme = 1;
Lerror:
     return skipme;
}
---

This doesn't say that the compiler actually checks that a jump 
actually happens, it assumes the worst based on the information 
you've provided it.


See the GCC's documentation on Extended Assembler for more 
details[1], but these are the sort of things you could do with it.

---
int btl(int a, int b)
{
   asm {
       "btl %1, %0;"
       "jc %l2"
         : /* No outputs. */
         : "r" (a), "r" (b)
         : "cc"
         : carry;
   }
   return 0;

carry:
   return 1;
}
---

[1]: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html


Enjoy...

Iain.