compilation error with arm neon and gcc 11.2.1

[lmarz]

When trying to build embree under aarch64, cmake doesn't detect arm neon and enables the -msse2 flag instead, which causes a compiler error.
gcc does support arm neon though with the header file "arm_neon.h" and the macro __ARM_NEON.

Manually enabling EMBREE_ARM in cmake gives the following error output for the first 4 files:
build-embree.log

embree compiles successfully, when using clang

[svenwoop]

Could you please check if this worked with Embree 3.13.3?

[luyatshimbalanga]

I confirm the issue with version 3.13.4 on aarch64 architecture which suggests a regression compared to the previous version with GCC compiler.
For references, the listed parameters on the %build section:

export CXXFLAGS="%{optflags} -Wl,--as-needed"
%cmake \
	-DCMAKE_BUILD_TYPE=Release \
        -DCMAKE_CXX_STANDARD=17 \
	-DCMAKE_INSTALL_LIBDIR=%{_libdir} \
	-DCMAKE_INSTALL_PREFIX=%{_prefix} \
	-DEMBREE_COMPACT_POLYS=ON \
	-DEMBREE_IGNORE_CMAKE_CXX_FLAGS=OFF \
%if %{with ispc}
        -DEMBREE_ISPC_SUPPORT=ON \
%else
        -DEMBREE_ISPC_SUPPORT=OFF \
%endif
        -DEMBREE_MAX_ISA=DEFAULT \
	-DEMBREE_TUTORIALS=OFF 

The macros in %{...} are listed under the Fedora Packaging Guidelines.

The same codes lines works fine on v3.13.3 hence this log which potentially shows a regression on v3.13.4.

[ppisar]

I also recommend updating README.md which reads:

  Embree requires at least an x86 CPU with support for
  SSE2 or an Apple M1 CPU.

That's misleading because people think that they need Appli M1 CPU, while a generic AArch64 CPU is enough.

[StefanBruens]

The problem is caused by erroneous changes for the Apple silicon support.

The flags are set in e.g. gnu.cmake, included here:
https://github.com/embree/embree/blob/489b746c0d5010e0da10345e9dc96768bec9a037/CMakeLists.txt#L255

But for anything but APPLE, EMBREE_ARM is only set here:
https://github.com/embree/embree/blob/489b746c0d5010e0da10345e9dc96768bec9a037/CMakeLists.txt#L282-L285

I.e. all conditionals depending on EMBREE_ARM in gnu.cmake are wrong.

[akien-mga]

I can confirm the issue in 3.13.5 on Mageia Cauldron aarch64, while 3.13.3 built fine.

I believe this was fixed by 82ca6b5, but 3.13.6 was never released with the fix (and now 4.0.0 is a compat breaking update).

[svenwoop]

Please just stay on Embree 3.13.3 when you compat with Embree 3 is important. Otherwise switching to Embree 4 should be straight forward.

[StefanBruens]

Please just stay on Embree 3.13.3 when you compat with Embree 3 is important. Otherwise switching to Embree 4 should be straight forward.

The correct answer is "Use Embree 3.13.5 and cherry-pick 82ca6b5". Otherwise, one misses the fixes in 3.13.4/3.13.5.

[StefanBruens]

Or even simpler, pass -DEMBREE_ARM=ON to cmake.