Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

[Ian Lance Taylor <iant at google dot com>]

Robert Dewar <dewar@adacore.com> writes:

> Ian Lance Taylor wrote:
> 
> > The new option -fstrict-overflow tells gcc that it can assume the
> > strict signed overflow semantics prescribed by the language standard.
> > This option is enabled by default at -O2 and higher.  Using
> > -fno-strict-overflow will tell gcc that it can not assume that signed
> > overflow is undefined behaviour.  The general effect of using this
> > option will be that signed overflow will become implementation
> > defined.  This will disable a number of generally harmless
> > optimizations, but will not have the same effect as -fwrapv.
> 
> Can you share the implementation definition (implementation defined
> generally means that the implementation must define what it does).
> This seems awfully vague.

You're right, I shouldn't have said "implementation defined."

What will happen with -fno-strict-overflow is whatever the processor
ISA happens to do when a signed arithmetic operation overflows.  For
ordinary machines it will just wrap.

It is intentionally vague.  If you need non-vague semantics, you
should use -fwrapv.  -fno-strict-overflow is intended to provide the
vague semantics which C compilers have historically provided, in
support of existing code.  -fstrict-overflow provides the reasonably
precise semantics of the language standard.  The options are generally
analogous to -fstrict-aliasing and -fno-strict-aliasing.

Ian