Fixed a build error when using CMake 3.15.1 + NDK-R20

[azhuang]

Summary

std::decay_t used by llvm/utils/benchmark/include/benchmark/benchmark.h is a c++14 feature, but the CMakelist uses c++11, it's the root-cause of build error.

There are two options to fix the error.
1) change the CMakelist to support c++14.
2) change std::decay_t to std::decay, it's what the patch done.

This bug can only be reproduced by CMake 3.15, we didn't observer the bug with CMake 3.16. But based on the code's logic, it's an obvious bug of LLVM.

[lebedev.ri]

Please

1. Proceed with 2) change std::decay_t to std::decay fix instead
2. Please submit it upstream first

[azhuang]

Changed change std::decay_t to std::decay as per review.

[azhuang]

Update the patch.

[lebedev.ri]

... will you please also submit this upstream?

[azhuang]

I cannot push to the github.

haz20@hazpc:~/nvme0/git/llvm-project$ git push origin HEAD:main
Username for 'https://github.com': AnzhongHuang
Password for 'https://[e-mail]':
remote: Permission to llvm/llvm-project.git denied to AnzhongHuang.
fatal: unable to access 'https://github.com/llvm/llvm-project.git/': The requested URL returned error: 403

[azhuang]

@lebedev.ri Can you tell me how to "submit this upstream"?

[lebedev.ri]

In D93794#2472424, @azhuang wrote:

@lebedev.ri Can you tell me how to "submit this upstream"?

Err, by upstream i meant submit a PR to the https://github.com/google/benchmark

[azhuang]

It looks like we only need to upgrade the file, it was corrected there: https://github.com/google/benchmark/blob/master/include/benchmark/benchmark.h

[lebedev.ri]

Ah, so it's 1bd6123b781120c9190b9ba58b900cdcb718cdd1 that broke this.
Sorry for the noise, let's just go with this fix as is.
Please specify the name [e-mail] to be used for commit's Author: field.

[azhuang]

Author: AnZhong Huang <[e-mail]>

[danilaml]

llvm/utils/benchmark/include/benchmark/benchmark.h

993 ↗

(On Diff #313830)

Shouldn't it be typename std::decay<Lambda>::type ?

[lebedev.ri]

@azhuang i guess i should have asked, can you show the build error that happens without this patch?

llvm/utils/benchmark/include/benchmark/benchmark.h

993 ↗

(On Diff #313830)

Hm, indeed. I'm surprised no bots complained yet.

[lebedev.ri]

In D93794#2472651, @lebedev.ri wrote:

@azhuang i guess i should have asked, can you show the build error that happens without this patch?

I have temporarily reverted it for now.

[azhuang]

using BenchType =
    internal::LambdaBenchmark<typename std::decay<Lambda>::type>;

should be correct, I didn't see the original change, but my patch just passed the build, so I thought it's a correct fix.

The build error message without the patch.

00:32:57  /jenkins/workspace/DevOpsTest/Hongmei/vulkan/android_xgl_llpc-psdb/driver_build/drivers/xgl/icd/imported/llvm-project/llvm/utils/benchmark/include/benchmark/benchmark.h:993:52: error: no template named 'decay_t' in namespace 'std'; did you mean 'decay'?
00:32:57    using BenchType = internal::LambdaBenchmark<std::decay_t<Lambda>>;
00:32:57                                                ~~~~~^~~~~~~
00:32:57                                                     decay
00:32:57  /experiments/ndk/android-ndk-r20/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/include/c++/v1/type_traits:1363:29: note: 'decay' declared here
00:32:57  struct _LIBCPP_TEMPLATE_VIS decay
00:32:57

[azhuang]

Change the patch based on comments.

using BenchType = internal::LambdaBenchmark<typename std::decay<Lambda>::type>;
Should be a correct fix, but the previous change can also pass compiling :(

[lebedev.ri]

Interesting. But during compiling *which* binary does the error manifest? benchmark's tests ?

[azhuang]

You can check the amd vulkan driver for the reference, the benchmark folder will be built by default,

00:32:57 [ 1%] Built target CREATE_LLVM_NATIVE
00:32:57 Scanning dependencies of target benchmark

compiler/llpc/llvm/utils/benchmark/src/CMakeFiles/benchmark.dir/benchmark.cc.o

[azhuang]

Any further comments on this patch?

[lebedev.ri]

I'm being dense here, sorry. I thought benchmark is usually built with -std=c++14,
but as per llvm-project/llvm/utils/benchmark/CMakeLists.txt, that is not the case.
So i'll just reland this.

llvm/utils/benchmark/CMakeLists.txt

if (BENCHMARK_ENABLE_LTO)
     set(CMAKE_EXE_LINKER_FLAGS_RELWITHDEBINFO "${CMAKE_EXE_LINKER_FLAGS_RELWITHDEBINFO} /LTCG")
 
     set(CMAKE_CXX_FLAGS_MINSIZEREL "${CMAKE_CXX_FLAGS_MINSIZEREL} /GL")
     set(CMAKE_STATIC_LINKER_FLAGS_MINSIZEREL "${CMAKE_STATIC_LINKER_FLAGS_MINSIZEREL} /LTCG")
     set(CMAKE_SHARED_LINKER_FLAGS_MINSIZEREL "${CMAKE_SHARED_LINKER_FLAGS_MINSIZEREL} /LTCG")
     set(CMAKE_EXE_LINKER_FLAGS_MINSIZEREL "${CMAKE_EXE_LINKER_FLAGS_MINSIZEREL} /LTCG")
   endif()
 else()
-  # Try and enable C++11. Don't use C++14 because it doesn't work in some
-  # configurations.
-  add_cxx_compiler_flag(-std=c++11)
-  if (NOT HAVE_CXX_FLAG_STD_CXX11)
-    add_cxx_compiler_flag(-std=c++0x)
-  endif()
+  add_cxx_compiler_flag(-std=c++14)
 
   # Turn compiler warnings up to 11
   add_cxx_compiler_flag(-Wall)
 
   add_cxx_compiler_flag(-Wextra)
   add_cxx_compiler_flag(-Wshadow)
   # FIXME(kbobyrev): Document this change.
   # add_cxx_compiler_flag(-Werror RELEASE)