Control dependence vs. builtin_unreachable

[Jeff Law]

On 01/03/2013 12:01 PM, Steven Bosscher wrote:
> Hello,
>
> Consider the following test case:
>
> void bar (void);
> int foo (int b, int c, int d)
> {
>    int r = 0;
>    if (b)
>      res = b * 2 + 4;
>    if (c)
>      {
>        if (d)
>          r = res;
>        else
>          __builtin_unreachable ();
>      }
>    return r;
> }
>
> This is typical for code in GCC itself in places where
> gcc_unreachable() is used.

>
> The corresponding CFG looks like this:
>
>              +-----+
>              | bb0 |
>              +-----+
>                |
>                |
>                v
>              +-----+
>              | bb2 | -+
>              +-----+  |
>                |      |
>                |      |
>                v      |
>              +-----+  |
>              | bb3 |  |
>              +-----+  |
>                |      |
>                |      |
>                v      |
> +-----+     +-----+  |
> | bb8 | <-- | bb4 | <+
> +-----+     +-----+
>    |           |
>    |           |
>    |           v
>    |         +-----+     +-----+
>    |         | bb5 | --> | bb7 |
>    |         +-----+     +-----+
>    |           |
>    |           |
>    |           v
>    |         +-----+
>    |         | bb6 |
>    |         +-----+
>    |           |
>    |           |
>    |           v
>    |         +-----+
>    +-------> | bb9 |
>              +-----+
>                |
>                |
>                v
>              +-----+
>              | bb1 |
>              +-----+
Presumably BB7 was created in response to the builtin_unreachable?  One 
could argue that an empty dead-end basic block should just be removed 
and the CFG appropriately simplified.

You might want to look  at a discussion from Oct/Nov 2011 "New pass to 
delete unexecutable paths in the CFG" which touches on some of this 
stuff.  It's not 100% the same, but the concept of eliminating edges 
from the CFG which we can never traverse in a conforming program applies 
to both your example and the stuff I was playing with.

jeff