Re: [RFC] Experimental __attribute__((saturating)) on integer types.

[Joseph Myers <joseph@codesourcery.com>]

On Mon, 27 Sep 2021, Richard Biener via Gcc-patches wrote:

> Now - ISTR that elsewhere Joseph suggested that taking on
> saturating operations by type was eventually misguided and we should
> have instead added saturating arithmetic tree codes that we could
> expose via some builtin functions like the overflow ones.

There are several issues there:

* saturating (and other fixed-point) types at the C API level;

* saturating (and other fixed-point) types in GIMPLE;

* saturating (and other fixed-point) modes in RTL.

As I said in 
<https://gcc.gnu.org/legacy-ml/gcc-patches/2011-05/msg00846.html>, I think 
having special modes for these kinds of types is a bad idea, because 
operations should be lowered to ordinary integer arithmetic at some point 
in GIMPLE, or at the latest in expand.  (Maybe a few cases would sensibly 
use libgcc functions rather than inline arithmetic, but those would be the 
exception.  We handle inline expansion of the overflow-checking built-in 
functions in general, much of that code could be shared to expand 
saturating arithmetic in general on hardware lacking the operations.)  At 
present, there are loads of fixed-point machine modes, and very many 
libgcc functions on the targets supporting fixed-point, and very little 
optimization done on these operations, when if the operations were lowered 
to normal arithmetic earlier, generic code in the compiler could optimize 
them.  (Back ends would still need to know enough about the types in 
question to be able to implement any desired ABI differences from the 
underlying ordinary integer types.)

My inclination is that GIMPLE should also use saturating operations rather 
than saturating types.

At the C API level it's less clear.  When you have saturating types in the 
front end - as in those we currently have implemented, from the Embedded C 
TR, for example - at some point they need lowering to saturating 
operations on normal types, if you follow my suggested model above.  That 
could be at gimplification, or you could allow saturating types in GIMPLE 
but then have some early pass that replaces them by normal types using 
saturating operations.

For some kinds of algorithm, saturating types may well be a convenient 
abstraction for the user.  For others, saturating operations on normal 
types may make more sense (e.g. using saturating arithmetic on size_t to 
compute an allocation size, knowing that SIZE_MAX will result in 
allocation failure if passed to an allocation function).

As for the specific patch: it looks like you create a new type every time 
the user uses the attribute.  If you allow users to create such saturating 
types (distinct from the fixed-point ones) at all, I think that every time 
someone requests int __attribute__ ((saturating)) it should produce the 
same type (and likewise for each other underlying non-saturating integer 
type, and watch out for any interactions with types created for 
bit-fields).  Then there would be API design questions to address such as 
the results of converting out-of-range integer or floating-point values - 
or, for that matter, wider pointers - to a saturating type.

-- 
Joseph S. Myers
joseph@codesourcery.com