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

[Eli Friedman]

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).

-Eli