[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:
> 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.)


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  https://reviews.llvm.org/D136568/new/

https://reviews.llvm.org/D136568