[cfe-dev] clang builtin function attributes

Aaron Ballman via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 5 12:04:01 PDT 2016

On Tue, Apr 5, 2016 at 2:56 PM, Taewook Oh via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> Thank you for your reply. I wonder if clang does not support "pure"
> attribute at all, or has a different name for it.

Clang does support __attribute__((pure)); see Attr.td:1309, and it
affecting codegen in CGCall.cpp:1639 for more details.


> Thanks,
> Taewook
> From: <metafoo at gmail.com> on behalf of Richard Smith <richard at metafoo.co.uk>
> Date: Tuesday, April 5, 2016 at 11:48 AM
> To: Taewook Oh <twoh at fb.com>
> Cc: via cfe-dev <cfe-dev at lists.llvm.org>
> Subject: Re: [cfe-dev] clang builtin function attributes
> On Tue, Apr 5, 2016 at 11:28 AM, Taewook Oh via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
>> Hello,
>> Unlike gcc, clang does not provide builtin functions for functions in
>> ctype.h, such as tolower. To generated better optimized code, I'm planning
>> to add these functions as builtins with attributes. Regarding this, I have a
>> few questions:
>> In "Attributes in Clang" page
>> (http://clang.llvm.org/docs/AttributeReference.html), "nothrow" and "const"
>> are not in the list while they are still used in Builtins.def. Are these
>> attributes deprecated?
> No, they're just not documented yet. (We'd appreciate patches to
> include/clang/AttrDocs.td and include/clang/Attr.td if you're feeling
> enthusiastic, but note that this needs to be original documentation -- it
> wouldn't be acceptable to copy documentation from GCC due to license
> differences.)
>> (Assuming they are not deprecated) Is there a place that I can find
>> definitions for attributes used in builtins.def? In particular, I wonder if
>> "const" attributes of clang has same meaning with gcc's "const" attribute,
>> as it seems that clang does not support "pure" attribute.
> Yes, the attributes listed in clang's Builtins.def are the
> __attribute__((X)) form, which generally mean the same thing to Clang as
> they do to GCC (with rare exceptions).
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