hubert.reinterpretcast added inline comments.
================
Comment at: clang/test/Sema/constant-builtins-ilogb.cpp:1
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// expected-no-diagnostics
----------------
aaron.ballman wrote:
> hubert.reinterpretcast wrote:
> > There seems to be no C language test in the patch (although the builtin
> > presumably is okay at least as part of arithmetic constant expressions).
> >
> > @aaron.ballman, what are your thoughts re: integer constant expression
> > contexts? For example:
> > ```
> > struct C { int x : __builtin_ilogb(1. + 1.); };
> > ```
> WG14 adopted https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2713.htm for
> C2x to clarify that implementations are not allowed to extend what they
> consider to be an integer constant expression. The operand in this case is a
> function call expression, which is not one of the permissible things in an
> ICE, so the standard doesn't want us to make it one.
>
> I believe that Clang's response to that paper is to implement it to the
> letter rather than to the intent. e.g., issue a warning that a constant
> expression is being folded into an ICE but otherwise accept the code. We have
> too many situations already in which we fold a constant expression to an ICE
> and changing that behavior would be observable (and a performance
> regression). So I think it's fine for us to treat the builtin call as an ICE
> so long as we issue a (pedantic) warning about folding it.
>
> (FWIW, I don't think it qualifies as an arithmetic constant expression
> because that also doesn't allow function call expressions. But we can extend
> the definition of an arithmetic constant expression.)
Thanks Aaron. I think a C language test for this patch may be a good way to
explore the extension space (and any pedantic conformance diagnostics).
Particular to the example above and ICEs is that the argument involves
floating-point operations. Would the desired model be that the call is
considered a literal for the purposes of the language requirements?
It seems that built-ins that are accepted in constant expressions in C are a
more general issue though (and I am not sure if, for this patch, we defer the
issue for some general approach that would take care of the class of problems
as a whole).
Repository:
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https://reviews.llvm.org/D136568/new/
https://reviews.llvm.org/D136568
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