Re: [PATCH] middle-end/114931 - type_hash_canon and structual equality types

[Martin Uecker <uecker@tugraz.at>]

Am Freitag, dem 03.05.2024 um 20:48 +0200 schrieb Richard Biener:
> 
> > Am 03.05.2024 um 20:37 schrieb Martin Uecker <uecker@tugraz.at>:
> > 
> > Am Freitag, dem 03.05.2024 um 20:18 +0200 schrieb Jakub Jelinek:
> > > > On Fri, May 03, 2024 at 08:04:18PM +0200, Martin Uecker wrote:
> > > > A change that is not optimal but would avoid a lot of trouble is to
> > > > only use the tag of the struct for computing a TYPE_CANONICAL, which
> > > > could then be set already for incomplete types and never needs to
> > > > change again. We would not differentiate between different struct
> > > > types with the same tag for aliasing analysis, but in most cases
> > > > I would expect different structs to have a different tag.
> > > 
> > > Having incompatible types have the same TYPE_CANONICAL would lead to wrong
> > > code IMHO, while for aliasing purposes that might be conservative (though
> > > not sure, the alias set computation is based on what types the element have
> > > etc., so if the alias set is computed for say struct S { int s; }; and
> > > then the same alias set used for struct S { long long a; double b; union {
> > > short c; float d; } c; };, I think nothing good will come out of that),
> > 
> > The C type systems requires us to form equivalence classes though.
> > For example
> > 
> > int (*r)[1];
> > int (*q)[];
> > int (*p)[3];
> > 
> > need to be in the same equivalence class even though r and p are
> > not compatible, while at the same time r and q and q and p
> > are compatible.
> 
> TYPE_CANONICAL as used by the middle-end cannot express this but

Hm. so how does it work now for arrays?


> useless_type_conversion_p is directed and has similar behavior. 
> Note the dual-use for TBAA and compatibility was convenient but
> maybe we have to separate both since making the equivalence class
> for TBAA larger is more conservative while for compatibility it’s
> the other way around…

Maybe, although I do not understand why the middle end would
need precise knowledge for checking type compatibility?  The
FE has much stricter rules. 

Martin

> 
> Richard 
> 
> > 
> > > but middle-end also uses TYPE_CANONICAL to see if types are the same,
> > > say e.g. useless_type_conversion_p says that conversions from one
> > > RECORD_TYPE to a different RECORD_TYPE are useless if they have the
> > > same TYPE_CANONICAL.
> > >  /* For aggregates we rely on TYPE_CANONICAL exclusively and require
> > >     explicit conversions for types involving to be structurally
> > >     compared types.  */
> > >  else if (AGGREGATE_TYPE_P (inner_type)
> > >           && TREE_CODE (inner_type) == TREE_CODE (outer_type))
> > >    return TYPE_CANONICAL (inner_type)
> > >           && TYPE_CANONICAL (inner_type) == TYPE_CANONICAL (outer_type);
> > > So, if you have struct S { int s; } and struct S { short a, b; }; and
> > > VIEW_CONVERT_EXPR between them, that VIEW_CONVERT_EXPR will be removed
> > > as useless, etc.
> > 
> > Maybe we could limit for purposes of computing TYPE_CANONICAL of derived
> > types, e.g. TYPE_CANONICAL of structs stays the same with the transition
> > from TYPE_STRUCT_EQUALITY to TYPE_CANONICAL but all the derived types
> > remain stable.
> > 
> > Martin
> > 
> > > 
> > > BTW, the idea of lazily updating TYPE_CANONICAL is basically what I've
> > > described as the option to update all the derived types where it would
> > > pretty much do that for all TYPE_STRUCTURAL_EQUALITY_P types in the
> > > hash table (see if they are derived from the type in question and recompute
> > > the TYPE_CANONICAL after recomputing all the TYPE_CANONICAL of its base
> > > types), except perhaps even more costly (if the trigger would be some
> > > build_array_type/build_function_type/... function is called and found
> > > a cached TYPE_STRUCTURAL_EQUALITY_P type).  Note also that
> > > TYPE_STRUCTURAL_EQUALITY_P isn't the case just for the C23 types which
> > > are marked that way when incomplete and later completed, but by various
> > > other cases for types which will be permanently like that, so doing
> > > expensive checks each time some build*_type* is called that refers
> > > to those would be expensive.
> > > 
> > >    Jakub
> > > 
> >