fix some clang-cl warnings regarding unused parameters

[lemire]

They occur because we try to compile C code as C++17. The -std=c++17 flag should not be used when compiling C code. A quickfix is to set cflags_cc=[] in C dependencies. It clears the C++ flags.

Interestingly, it illustrates that the following configuration...

            ['target_arch in "ia32 x64" and OS!="ios"', {
              'defines': [ 'ADLER32_SIMD_SSSE3' ],
              'cflags': [ '-mssse3' ],
              'cflags_cc': [],
              'conditions': [

is not triggered. So you get both a build error because you are missing SSSE3, but also a visible warning.

I am sure that it is quite trivial.

[lemire]

@anonrig Would you happen to know why these variables (cflags, cflags_cpp) don't take hold here even though we are building under Windows with x64.

[targos]

I think it's a GYP issue. It was never meant to support Clang on Windows AFAIK and cflags are passed differently for it. See c1d1878

[targos]

Do these work? What are the fixed warnings?

[lemire]

Without these, you are passing-std=c++17 to clang-cl when compiling C code. The better solution would be for the build engine to recognize that it is compiling C code, and not pass the C++ flags.

[targos]

I'm surprised that it works because I don't see any occurence of cflags_cpp in the code base (including the source code of GYP)

[lemire]

You are correct. It should be cflags_cc.

[targos]

They occur because we try to compile C code as C++17.

I think this comes from common.gypi:

https://github.com/nodejs/node/blob/3f4ea7ad7f4a1cb058bdc5363311607d1793928e/common.gypi#L284-L289

[lemire]

I think this comes from common.gypi:

Yes. And that flag looks fine to me. The issue is that it gets applied when clang-cl builds C code. Node has several C dependencies and they are the ones that appear most broken right now.

Short of making the build engine smarter, I propose to clear the C++ flags when within a C dependency.

It does not quite work perfectly because, as we have observed, some flags do not get applied under clang-cl when it seems they should. This issue leads to breakage having to do with SIMD instructions.

The better solution would be to fix the build engine so that it actually supports clang-cl as a first class compiler.

[lemire]

Basically this PR gets you closer to building under clang-cl without warnings and errors because it manually removes the unnecessary C++ flags within C projects. I think that you pretty much need to do this if fixing GYP is not an option.

[targos]

I understand what you're trying to do but what I understand of GYP is that it ignores cflags and related options for the Visual studio generator.

[lemire]

I understand what you're trying to do but what I understand of GYP is that it ignores cflags and related options for the Visual studio generator.

Checkout my PR and run it. It will take you a minute.

It works! It drastically reduces the volume of warnings.

It is a verifiable statement, you can check.

[lemire]

To put it another way, GYP does not ignore all of these changes, just a couple.

[targos]

First warning I see (clean clone with this patch applied):

clang-cl : warning : argument unused during compilation: '-std:c++17' [-Wunused-command-line-argument] [D:\Git\nodejs\node\deps\histogram\histogram.vcxproj]

Then a lot more in uvwasi and zlib

[lemire]

First warning I see (clean clone with this patch applied):

With a fresh build? That's intriguing.

[lemire]

Please see nodejs#35433 (comment)

I think that there is a fundamental problem that should be solved and that what I am proposing here is wrong.

Closing.

[lemire]

@targos Note that the approach illustrated by this PR does work. You do get rid of the warnings when setting cflags_cc to the empty array. (An earlier commit had cflags_cpp which is the wrong variable.)

It is a hack, however. I don't recommend it.