[cfe-commits] [patch] Add codegen for the __noop intrinsic

[Nico Weber]

On Wed, Sep 26, 2012 at 4:37 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Wed, Sep 26, 2012 at 12:29 AM, Nico Weber <thakis at chromium.org> wrote:
>> On Wed, Sep 26, 2012 at 4:24 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>> On Tue, Sep 25, 2012 at 11:36 PM, Nico Weber <thakis at chromium.org> wrote:
>>>> On Wed, Sep 26, 2012 at 3:06 PM, Nico Weber <thakis at chromium.org> wrote:
>>>>> On Wed, Sep 26, 2012 at 2:49 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>>>>> On Tue, Sep 25, 2012 at 10:05 PM, Nico Weber <thakis at chromium.org> wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> similar to the __debugbreak patch, this adds codegen support for __noop.
>>>>>>
>>>>>> You don't actually have to call llvm.donothing... like the name says,
>>>>>> it doesn't do anything. :)  "return RValue::get(0);" should be
>>>>>> sufficient.
>>>>>
>>>>> My patch has nicer -S -emit-llvm output :-) But sure, I'll change it.
>>>>>
>>>>>>
>>>>>> Do we actually perform semantic analysis correctly for __noop?
>>>>>
>>>>> Not completely.
>>>>>
>>>>>> In MSVC, are the arguments unevaluated in the semantic analysis sense?
>>>>>
>>>>> cl seems to do sema for __noop, __noop(3 + someclassinstanc) results
>>>>> in a syntax error. So does wrapping a statement in __noop.
>>>
>>> How about something like the following?
>>>
>>> template<typename T> static void f() { T x = 10; }
>>> void g() { __noop(f<void>()); }
>>>
>>> (By "unevaluated', I mean in the same sense that the operand of sizeof
>>> is unevaluated.)
>>
>> This compiles fine with cl but currently errors out with clang:
>>
>> C:\src\chrome\src>type test.cc
>> template<typename T> static void f() { T x = 10; }
>> void g() { __noop(f<void>()); }
>>
>> C:\src\chrome\src>cl /c test.cc
>> Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.30319.01 for 80x86
>> Copyright (C) Microsoft Corporation.  All rights reserved.
>>
>> test.cc
>>
>> C:\src\chrome\src>..\..\llvm-ninja\bin\clang++.exe -c test.cc
>> test.cc:2:19: error: argument type 'void' is incomplete
>> void g() { __noop(f<void>()); }
>>                   ^~~~~~~~~
>> test.cc:1:42: error: variable has incomplete type 'void'
>> template<typename T> static void f() { T x = 10; }
>>                                          ^
>> test.cc:2:19: note: in instantiation of function template
>> specialization 'f<void>' requested here
>> void g() { __noop(f<void>()); }
>>                   ^
>> 2 errors generated.
>
> That's what I was afraid of.  To get it right, you'll need to make the
> parser recognize __noop as a keyword and parse it similarly to sizeof
> (using EnterExpressionEvaluationContext to push an Unevaluated
> context).

The attached patch contains adds noop as UnaryExprOrTypeTraitExpr
variant. That way it gets the unevaluated context for free. Is this an
ok approach? (I still need to check how cl handles some things in the
included test, so it's not a final patch, but it's pretty close.)

Nico
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