Re: -Wshift-negative-value

[Bruno H. <br...@cl...>]

Hi Daniel,

> > > > As I said on 2017-02-08, diagnostics from -Wshift-negative-value are
> > > > most probably just noise.
> > >
> > > Why not disable them then?
> >
> > Feel free to do so. Good idea.
> 
> Actually, this is is a very (very) bad idea. Applying a (left) shift to a
> negative value (or shifting by a negative value) is undefined behavior
> since (AFAIK) C99 (which is what we - at least in September - wanted to
> target).
> 
> Quote from (C11) standard (draft) [1]:
> 
> "The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits
> are filled with zeros. [..] If E1 has a signed type and nonnegative value
> [..] otherwise, the behavior is undefined."
> N1570 §6.5.7/4

It is undefined because ISO C supports not only two's complement representation
of signed integers, but also one's complement and sign-magnitude. Quote from
[1] § J.3.5:
  "The following are implementation-defined:
   ...
   — Whether signed integer types are represented using sign and magnitude, two’s
     complement, or ones’ complement, and whether the extraordinary value is a trap
     representation or an ordinary value (6.2.6.2)."

But clisp will never support anything else than two's complement arithmetic.
I haven't heard of any machine with one's complement or sign-magnitude in use
since 1980, and even gnulib - which caters to old and traditional systems -
now no longer attempts to support these odd systems [2].

> Thus every single one of those warnings is a possible Heisenbug.
> Any random optimizer pass of any compiler could horribly break our code.

Indeed. In order for this not to happen, I'm adding the gcc option "-fwrapv"
to the CFLAGS.

If, at some point, we want to get rid of this option and let the C compiler
optimize all it can, we'll need to revisit clisp's uses of signed integer
types with the help of -fsanitize=undefined (which is the successor of
the -ftrapv option [3]. -ftrapv does not catch all situations, see attached
file).

Bruno

[1] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
[2] http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=67d133ee7b512af59a392f533b17302d71e937db
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68046#c1