Illegal instruction 4 with old Mac

[ntustison]

Just in case anybody else runs across this issue--

I have an old Mac Pro (Early 2009). Any binary from compiling the latest ANTs gives me the error "Illegal instruction: 4". I traced it to this change. Using the previous version of the file corrects this problem. Based on the commit message, I've tried changing the optimization flags to

  -DANTs_C_OPTIMIZATION_FLAGS:STRING="-march=native"
  -DANTs_CXX_OPTIMIZATION_FLAGS:STRING="-march=native"
  -DANTs_C_WARNING_FLAGS:STRING="-Wall -Werror"
  -DANTs_CXX_WARNING_FLAGS:STRING="-Wall -Werror"

and

  -DANTs_C_OPTIMIZATION_FLAGS:STRING="-march=native"
  -DANTs_CXX_OPTIMIZATION_FLAGS:STRING="-march=native"

but neither attempt caused things to work.

[cookpa]

This has come up a couple of times for users lately. I think it may indirectly depend on the OS because different compilers might make a different guess about the optimizer flags, but the underlying issue is wrong optimization flags being set for the CPU.

I don't think the command line options will work because the error arises in ITK, and command line CMake options don't get passed through to the external projects in a Superbuild.

There is apparently a way to fix this

https://cmake.org/pipermail/cmake/2018-January/067002.html

[gdevenyi]

To be clear, the devs believe the issue is that your CPU is older than corei7, which is what our default "arch" setting is:
/CMake/ITKSetStandardCompilerFlags.cmake@master#L245

You can adjust this by adding "-march=native" to your compile options.

[cookpa]

Do we think any of the recent CMake changes have fixed this?

[gdevenyi]

No, the issue is that a commit from @hjmjohnson set the lowest level of instruction sets to "new" CPUs:

ANTs/CMake/ITKSetStandardCompilerFlags.cmake

Lines 241 to 247 in bcab744

	set(InstructionSetOptimizationFlags
	# https://gcc.gnu.org/onlinedocs/gcc-4.8.0/gcc/i386-and-x86_002d64-Options.html
	# NOTE the corei7 release date was 2008
	-mtune=native # Tune the code for the computer used compile ITK, but allow running on generic cpu archetectures
	-march=corei7 # Use ABI settings to support corei7 (circa 2008 ABI feature sets, core-avx circa 2013)
	)
	endif()

The solution is the user can override these settings.

[cookpa]

I found a machine that produces this error, so I looked into it a bit. Adding

  -DSuperBuild_ANTS_C_OPTIMIZATION_FLAGS="-mtune=native -march=native" \
  -DSuperBuild_ANTS_CXX_OPTIMIZATION_FLAGS="-mtune=native -march=native" 

to the cmake call worked (using GCC 4.9.1 on Linux). I guess for portable builds one would want to replace "native" with "x86-64".

It looks like "corei7" has been removed from the list of GCC supported architectures:

https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html

So I'm not sure what a modern GCC will do with "-march=corei7".

[cookpa]

Updated the Wiki, going to close this and related issues

https://github.com/ANTsX/ANTs/wiki/Compiling-ANTs-on-Linux-and-Mac-OS#compilation-fails-or-run-time-errors-occur-with-illegal-instruction-errors