On 2018-01-24 11:25, Craig Howland wrote:
On 01/24/2018 11:46 AM, Andre Vieira (lists) wrote:
I looked a bit further into this and I can't reproduce this behavior for
the i386 backend. So I had a look through GCC's parser implementation for
_Generic and it seems GCC evaluates the expressions in each matching case,
that is to say it will also parse the default case. In arm, the type of
va_list is a struct, since it is invalid to cast structs to pointers, the
c-parser will error out when it parses this cast in the default case. For
an i386 gcc, and I'm guessing your case is the same, the type of va_list
can be cast to a pointer type, so it parses it without errors.
I don't know if parsing the expression of default if another case has
matched is intended behavior. I will investigate this further and if I can
convince myself this is wrong behavior I'll chase it with upstream gcc.
The final draft of the C11 standard, which I assume was not changed in the
final release, does say that only the chosen expression is evaluated. (Which
is as it must be. If it is choosing function names, for example, it can
hardly evaluate a non-chosen option, as that means calling the function. The
example they give is actually choosing a function.) A cast is an expression,
so it appears that you do need to chase it with upstream GCC. It obviously
has to parse each selection, but it must only evaluate one.
FYI for _Generic definition and issues see:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf [CD]
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1650.htm [DR original]
http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_423.htm [DR discussion]
https://gcc.gnu.org/ml/gcc-patches/2016-05/msg01054.html [GCC response]
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1930.htm [followup]
The official standard agrees with you at the end of #3, but see note (91 below;
selectively quoting from ISO/IEC 9899:2011[2012] thru TC3:
"
6 A generic selection is a primary expression. Its type and value depend on
the selected generic association, as detailed in the following subclause.
Forward references: declarations (6.7).
6.5.1.1 Generic selection
Syntax
1 generic-selection:
_Generic ( assignment-expression , generic-assoc-list )
generic-assoc-list:
generic-association
generic-assoc-list , generic-association
generic-association:
type-name : assignment-expression
default : assignment-expression
91) Thus, an undeclared identifier is a violation of the syntax.
Constraints
2 A generic selection shall have no more than one default generic association.
The type name in a generic association shall specify a complete object type
other than a variably modified type. No two generic associations in the same
generic selection shall specify compatible types. The controlling expression of
a generic selection shall have type compatible with at most one of the types
named in its generic association list. If a generic selection has no default
generic association, its controlling expression shall have type compatible with
exactly one of the types named in its generic association list.
Semantics
3 The controlling expression of a generic selection is not evaluated. If a
generic selection has a generic association with a type name that is compatible
with the type of the controlling expression, then the result expression of the
generic selection is the expression in that generic association. Otherwise, the
result expression of the generic selection is the expression in the default
generic association. None of the expressions from any other generic association
of the generic selection is evaluated.
4 The type and value of a generic selection are identical to those of its
result expression. It is an lvalue, a function designator, or a void expression
if its result expression is, respectively, an lvalue, a function designator, or
a void expression.
5 EXAMPLE The cbrt type-generic macro could be implemented as follows:
#define cbrt(X) _Generic((X), \
long double: cbrtl, \
default: cbrt, \
float: cbrtf \
)(X)
"