On Mon, 21 Mar 2016 13:48:06 -0700
Scott Perry <numist at apple.com> wrote:
> Compilers allow you to choose your standard; --std=c11 means
> something very specific (and unchanging)
They do. And that covers what the standard covers. The standard also
has limits. It includes constructs that are syntactically permitted
but whose behavior is left undefined, known by the scarred as "UB" for
"undefined behavior". An example from Clang's discussion is
int i = 10 << 31;
The standard says << is a shift operator. It places no limit on the
number of bits to be shifted. If that number is so large that the
product cannot be represented by the assigned variable, that is *not*
an error. The standard allows the compiler to do anything or nothing
with it. As you may imagine, the varieties of anything and nothing are
many.
Compiler writers are well aware that "nothing" is faster done than
"something". Over time, they have gotten more aggressive in simply
deleting UB code. As a consequence, programmers who thought they wrote
standards-conforming code get burned when they upgrade/change
compilers. Mysterious and sometimes subtle errors are introduced by
the compiler for the user's benefit.
Your googlefu will turn up lots of discussion. One I liked that wasn't
on Page 1:
http://blog.frama-c.com/index.php?post/2013/10/09/Overflow-float-integer
--jkl
