[PATCH] D136568: [Clang] Support constexpr builtin ilogb

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: clang/test/Sema/constant-builtins-ilogb.cpp:1
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// expected-no-diagnostics
----------------
hubert.reinterpretcast wrote:
> 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).
> 
> 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).

+1, especially because C2x has support for constexpr object definitions (which we've not implemented in Clang yet).

> 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?

I think that model makes the most sense to me. If we can compute the correct answer at compile time, it seems a bit silly to defer until runtime just because the builtin looks like a function call expression.

> 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).

Yeah, I think it is a more general issue with builtins in C (and how we handle constant expression evaluation in C). I don't think this patch will make the problem significantly *worse*, so I think I'd be fine not addressing the specific concern for this builtin in this patch. But it would probably make a lot of sense for us to fix up the builtin situation before converting a ton of math library functions to builtins since all of those are going to have the same class of concern.


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https://reviews.llvm.org/D136568