[Bug tree-optimization/72785] [7 Regression] kernel build error since r236831

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785

--- Comment #10 from Jeffrey A. Law <law at redhat dot com> ---
Yes, it's a fairly strict interpretation, but I think it's the right one to be
using in the absence of additional language around the semantics of b_c_p.

In particular refer to c#6 in BZ38789.

Essentially b_c_p or b_o_s is more like a PHI than a function call.  All paths
to the b_c_p/b_o_s site participate in the result.

Thus isolating a path where there's a b_c_p/b_o_s is an invalid transformation
unless we prove the original and isolated paths produce the same results for
the b_c_p/b_o_s call.  But there's no real infrastructure in place to do that.

Even something like block duplication for loop header copying or unrolling has
the potential to make a b_c_p/b_o_s call give different results between the two
paths.  So ISTM the only correct thing to do is disable duplication of blocks
with those builtins.