[PATCH][RFC] middle-end/106811 - document GENERIC/GIMPLE undefined behavior

Richard Sandiford richard.sandiford@arm.com
Wed Sep 20 14:56:54 GMT 2023


Richard Biener <rguenther@suse.de> writes:
> On Wed, 20 Sep 2023, Richard Sandiford wrote:
>
>> Thanks for doing this.  Question below...
>> 
>> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>> > The following attempts to provide a set of conditions GENERIC/GIMPLE
>> > considers invoking undefined behavior, leaning on the C standards
>> > Annex J, as to provide portability guidance to language frontend
>> > developers.
>> >
>> > I've both tried to remember cases we exploit undefined behavior
>> > and went over C2x Annex J to catch more stuff.  I'd be grateful
>> > if people could point out obvious omissions or cases where the
>> > wording isn't clear.  I plan to check/amend the individual operator
>> > documentations as well, but not everything fits there.
>> >
>> > I've put this into generic.texi because it applies to GENERIC as
>> > the frontend interface.  All constraints apply to GIMPLE as well.
>> > I plan to add a section to gimple.texi as to how to deal with
>> > undefined behavior.
>> >
>> > As said, every comment is welcome.
>> >
>> > For testing I've built doc and inspected the resulting pdf.
>> >
>> > 	PR middle-end/106811
>> > 	* doc/generic.texi: Add portability section with
>> > 	subsection on undefined behavior.
>> > ---
>> >  gcc/doc/generic.texi | 87 ++++++++++++++++++++++++++++++++++++++++++++
>> >  1 file changed, 87 insertions(+)
>> >
>> > diff --git a/gcc/doc/generic.texi b/gcc/doc/generic.texi
>> > index 6534c354b7a..0969f881146 100644
>> > --- a/gcc/doc/generic.texi
>> > +++ b/gcc/doc/generic.texi
>> > @@ -43,6 +43,7 @@ seems inelegant.
>> >  * Functions::           	Function bodies, linkage, and other aspects.
>> >  * Language-dependent trees::    Topics and trees specific to language front ends.
>> >  * C and C++ Trees::     	Trees specific to C and C++.
>> > +* Portability issues::          Portability summary for languages.
>> >  @end menu
>> >  
>> >  @c ---------------------------------------------------------------------
>> > @@ -3733,3 +3734,89 @@ In either case, the expression is void.
>> >  
>> >  
>> >  @end table
>> > +
>> > +
>> > +@node Portability issues
>> > +@section Portability issues
>> > +
>> > +This section summarizes portability issues when translating source languages
>> > +to GENERIC.  Everything written here also applies to GIMPLE.  This section
>> > +heavily relies on interpretation according to the C standard.
>> > +
>> > +@menu
>> > +* Undefined behavior::          Undefined behavior.
>> > +@end menu
>> > +
>> > +@node Undefined behavior
>> > +@subsection Undefined behavior
>> > +
>> > +The following is a list of circumstances that invoke undefined behavior.
>> > +
>> > +@itemize @bullet
>> > +@item
>> > +When the result of negation, addition, subtraction or division of two signed
>> > +integers or signed integer vectors not subject to @option{-fwrapv} cannot be
>> > +represented in the type.
>> 
>> Couldn't tell: is the omission of multiplication deliberate?
>
> No.  Fixed.  Do you by chance remember/know anything about RTL 'div'
> and behavior on overflow (INT_MIN/-1), in particular with -fwrapv?

No, sorry.  I thought SDIV was allowed (but not required) to trap
on overflow, but I don't know off-hand what effect -fwrapv has
on the way that we use it.

Richard


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