[llvm] r234343 - Enable W4 warnings by default for MSVC builds

Mon Apr 13 12:25:01 PDT 2015

On Mon, Apr 13, 2015 at 3:21 PM, Zachary Turner <zturner at google.com> wrote:
> I think I figured all this out (mostly).  Basically it only happens with
> Ninja.  Ninja doesn't know what command line options actually mean, it just
> makes a big list of them and passes them through for each file.  The Visual
> Studio generator has to know, because to make a meaningful Visual Studio
> project it has to generate XML with semantic tags like
> <WarningLevel>Level4</WarningLevel>.  So I think what happens is the Visual
> Studio generator seems /W3 then later sees /W4 and only generates one tag in
> the vcxproj (I confirmed that even in my case where /W3 and /W4 are part of
> CMAKE_CXX_FLAGS, Only a single <WarningLevel>Level4</WarningLevel> goes into
> the vcxproj.
>
> There's still the question of why your CMAKE_CXX_FLAGS doesn't contain
> anything, and I'm not sure about that.  But I don't think it matters much,
> because you would be seeing the same behavior with Visual Studio either way
> (namely, that it would work).
>
> On the other hand, ninja just appends commands line and passes them all
> through without knowing what they mean, so it can't be made smart enough to
> know that /W3 and /W4 should be collapsed.  You can verify that running "cl
> /W3 /W4 foo.cpp" will reproduce the error, for example.
>
> The patch I included earlier removes /W3 from the command line, but only if
> it's about to append /W4.  I think this should cover everything?  Aaron
> raised a question on IRC about what happens if the user manually specifies a
> warning level on the command line.  I don't have a good answer to that,
> other than to say that if you manually write CMAKE_CXX_FLAGS:STRING="/W2" or
> something at CMake time, the current patch is already messed up as it will
> then append /W4 which in the best could would overwrite your /W2 setting,
> and in the worst case would give the warnings I'm seeing here.  So I feel
> like we should just be able to ignore this case, and if it becomes an issue
> for someone in the future, we can add something like -DMSVC_WARNING_LEVEL
> which will allow us a better means to be able to override the /W4 with
> something else.

If we're not regressing behavior with the patch you posted, then I
think that patch is good. We definitely don't want a ton of warnings
with Ninja.

~Aaron

>
> On Mon, Apr 13, 2015 at 12:11 PM Kaylor, Andrew <andrew.kaylor at intel.com>
> wrote:
>>
>> I’m not set up for Ninja, so I get this:
>>
>>
>>
>> CMake Error: CMake was unable to find a build program corresponding to
>> "Ninja".
>>
>>
>>
>>
>>
>> But really I don’t know how much time it’s worth spending to figure out
>> why it isn’t happening to me.  It is happening to you so we need to fix it.
>>
>>
>>
>>
>>
>> From: Zachary Turner [mailto:zturner at google.com]
>> Sent: Monday, April 13, 2015 12:02 PM
>>
>>
>> To: Kaylor, Andrew; Aaron Ballman
>> Cc: llvm-commits at cs.uiuc.edu
>> Subject: Re: [llvm] r234343 - Enable W4 warnings by default for MSVC
>> builds
>>
>>
>>
>> Can you try the ninja generator?  First run "vcvarsall amd64", then just
>> run "cmake -G Ninja f:\users\akaylor\llvm-s\llvm".  That's the only
>> difference I can see in what we're doing.
>>
>>
>>
>> On Mon, Apr 13, 2015 at 12:00 PM Kaylor, Andrew <andrew.kaylor at intel.com>
>> wrote:
>>
>> I added the line you suggested to CMakeLists.txt, and this is what I see:
>>
>>
>>
>> f:\users\akaylor\llvm-s\test>cmake -G "Visual Studio 12 2013 Win64"
>> f:\users\akaylor\llvm-s\llvm
>>
>> CMAKE_CXX_FLAGS =
>>
>> -- No build type selected, default to Debug
>>
>> -- The C compiler identification is MSVC 18.0.31101.0
>>
>> …
>>
>>
>>
>> Also, the /W3 flag isn’t being used during my build.
>>
>>
>>
>> -Andy
>>
>>
>>
>>
>>
>> From: Zachary Turner [mailto:zturner at google.com]
>> Sent: Monday, April 13, 2015 11:57 AM
>> To: Kaylor, Andrew; Aaron Ballman
>> Cc: llvm-commits at cs.uiuc.edu
>>
>>
>> Subject: Re: [llvm] r234343 - Enable W4 warnings by default for MSVC
>> builds
>>
>>
>>
>> No, CMake itself initializes CMAKE_CXX_FLAGS to include /W3.  By default,
>> if you do nothing, CMAKE_CXX_FLAGS will have /W3.  You can verify this by
>> printing the value of CMAKE_CXX_FLAGS in the first line of
>> llvm/CMakeLists.txt
>>
>>
>>
>> On Mon, Apr 13, 2015 at 11:55 AM Kaylor, Andrew <andrew.kaylor at intel.com>
>> wrote:
>>
>> I'm not sure I understand.  Are you saying this problem only happens if
>> you have CMAKE_CXX_FLAGS defined to include W3?
>>
>> I don't have CMAKE_CXX_FLAGS defined on my system and my version of CMake
>> (3.0.1) isn't defining it on its own, which would explain why I'm not seeing
>> the problem.  I guess your patch to strip W3 if it's there is OK, though it
>> should probably also look for W1 and W2.
>>
>> On the other hand, if there isn't some non-user reason that W3 would be
>> part of CMAKE_CXX_FLAGS I would say this fall under the "don't do that"
>> category of fix.
>>
>> -Andy
>>
>> -----Original Message-----
>> From: aaron.ballman at gmail.com [mailto:aaron.ballman at gmail.com] On Behalf
>> Of Aaron Ballman
>> Sent: Monday, April 13, 2015 11:44 AM
>> To: Zachary Turner
>> Cc: Kaylor, Andrew; llvm-commits at cs.uiuc.edu
>> Subject: Re: [llvm] r234343 - Enable W4 warnings by default for MSVC
>> builds
>>
>> When I do that, the first line of output shows /W3:
>>
>>
>> E:\llvm\x64>"D:\Program Files (x86)\Microsoft Visual Studio
>> 12.0\VC\vcvarsall.bat" amd64
>>
>> E:\llvm\x64>cmake -G "Visual Studio 12" ..\llvm CMAKE_CXX_FLAGS =  /DWIN32
>> /D_WINDOWS /W3 /GR /EHsc ....
>>
>> ~Aaron
>>
>> On Mon, Apr 13, 2015 at 2:37 PM, Zachary Turner <zturner at google.com>
>> wrote:
>> > Also what version of CMake are you using?  Maybe it's an issue with
>> > CMake >= 3.0?  Try putting the following line at the top of your
>> > CMakeLists.txt
>> >
>> > message("CMAKE_CXX_FLAGS = ${CMAKE_CXX_FLAGS}")
>> >
>> > If you don't see /W3 in there, you won't hit the problem.
>> >
>> > On Mon, Apr 13, 2015 at 11:36 AM Zachary Turner <zturner at google.com>
>> > wrote:
>> >>
>> >> Just realized maybe 2012 was a typo.  Are you actually still using
>> >> 2012?
>> >> I thought support was dropped.
>> >>
>> >> On Mon, Apr 13, 2015 at 11:33 AM Aaron Ballman
>> >> <aaron at aaronballman.com>
>> >> wrote:
>> >>>
>> >>> I do not get the warnings when building x64 or x86 on Windows 7,
>> >>> MSVC 2012, debug build. This is when building LLVM, Clang, lld, and
>> >>> clang tools extra
>> >>>
>> >>> ~Aaron
>> >>>
>> >>> On Mon, Apr 13, 2015 at 2:22 PM, Zachary Turner <zturner at google.com>
>> >>> wrote:
>> >>> > I'm building x64.  Can you try that?  Run "vcvarsall amd64" before
>> >>> > building LLVM
>> >>> >
>> >>> > On Mon, Apr 13, 2015 at 11:21 AM Kaylor, Andrew
>> >>> > <andrew.kaylor at intel.com>
>> >>> > wrote:
>> >>> >>
>> >>> >> I haven’t seen that.  I’m not seeing any warnings right now.
>> >>> >>
>> >>> >>
>> >>> >>
>> >>> >> From: Zachary Turner [mailto:zturner at google.com]
>> >>> >> Sent: Monday, April 13, 2015 11:17 AM
>> >>> >> To: Kaylor, Andrew; llvm-commits at cs.uiuc.edu
>> >>> >> Subject: Re: [llvm] r234343 - Enable W4 warnings by default for
>> >>> >> MSVC builds
>> >>> >>
>> >>> >>
>> >>> >>
>> >>> >> Hi Andy, this is causing a huge slew of warning spam.  Is this
>> >>> >> expected?
>> >>> >> CMake passes /W3 by default, so  every single translation unit I
>> >>> >> compile gives me the following warning:
>> >>> >>
>> >>> >>
>> >>> >>
>> >>> >> cl : Command line warning D9025 : overriding '/W3' with '/W4'
>> >>> >>
>> >>> >>
>> >>> >>
>> >>> >> Furthermore, since this is a command line warning and not a
>> >>> >> compiler warning, it cannot be suppressed.  Unless you have any
>> >>> >> better ideas on how to address this, I think this should be
>> >>> >> reverted.
>> >>> >>
>> >>> >>
>> >>> >>
>> >>> >> On Tue, Apr 7, 2015 at 12:07 PM Andrew Kaylor
>> >>> >> <andrew.kaylor at intel.com>
>> >>> >> wrote:
>> >>> >>
>> >>> >> Author: akaylor
>> >>> >> Date: Tue Apr  7 14:01:01 2015
>> >>> >> New Revision: 234343
>> >>> >>
>> >>> >> URL: http://llvm.org/viewvc/llvm-project?rev=234343&view=rev
>> >>> >> Log:
>> >>> >> Enable W4 warnings by default for MSVC builds
>> >>> >>
>> >>> >> Modified:
>> >>> >>     llvm/trunk/CMakeLists.txt
>> >>> >>
>> >>> >> Modified: llvm/trunk/CMakeLists.txt
>> >>> >> URL:
>> >>> >>
>> >>> >> http://llvm.org/viewvc/llvm-project/llvm/trunk/CMakeLists.txt?rev
>> >>> >> =234343&r1=234342&r2=234343&view=diff
>> >>> >>
>> >>> >>
>> >>> >> =================================================================
>> >>> >> =============
>> >>> >> --- llvm/trunk/CMakeLists.txt (original)
>> >>> >> +++ llvm/trunk/CMakeLists.txt Tue Apr  7 14:01:01 2015
>> >>> >> @@ -233,14 +233,7 @@ list(REMOVE_DUPLICATES LLVM_TARGETS_TO_B
>> >>> >>  include(AddLLVMDefinitions)
>> >>> >>
>> >>> >>  option(LLVM_ENABLE_PIC "Build Position-Independent Code" ON)
>> >>> >> -
>> >>> >> -# MSVC has a gazillion warnings with this.
>> >>> >> -if( MSVC )
>> >>> >> -  option(LLVM_ENABLE_WARNINGS "Enable compiler warnings." OFF)
>> >>> >> -else()
>> >>> >> -  option(LLVM_ENABLE_WARNINGS "Enable compiler warnings." ON)
>> >>> >> -endif()
>> >>> >> -
>> >>> >> +option(LLVM_ENABLE_WARNINGS "Enable compiler warnings." ON)
>> >>> >>  option(LLVM_ENABLE_MODULES "Compile with C++ modules enabled."
>> >>> >> OFF)  option(LLVM_ENABLE_CXX1Y "Compile with C++1y enabled." OFF)
>> >>> >> option(LLVM_ENABLE_LIBCXX "Use libc++ if available." OFF)
>> >>> >>
>> >>> >>
>> >>> >> _______________________________________________
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>> >>> >> llvm-commits at cs.uiuc.edu
>> >>> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>> >>> >
>> >>> >
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