Check before removing march=native

[xhochy]

If you set HNSWLIB_NO_NATIVE and build on Apple M1 the removal of march=native fails because it isn't in the list of arguments.

[yurymalkov]

Hi @xhochy,
Can you check if #461 resolved the issue? It is merged to develop branch

[xhochy]

From my understanding this is even worse as it removes the option overall to omit -march=native from the build.

[dyashuni]

@xhochy
#461 should fix the issues with -march=native. It automatically checks if the flag is available on the platform. If the flag is not available it is removed. No need to set HNSWLIB_NO_NATIVE for macOS.
Do you think that we still need HNSWLIB_NO_NATIVE to manually control -march=native on macOS ?

[xhochy]

We need a general switch to disable march=native as otherwise the binaries are no longer redistributable. The linked PR removes that possibility completely. The PR here only fixed this option for the corner case of Apple Silicon.

[yurymalkov]

We need a general switch to disable march=native as otherwise the binaries are no longer redistributable

@xhochy Can you provide more details about this problem?
Context: at some point we implemented runtime checks for AVX and AVX512 (b801080), so for x64 I expected it to run on different (older) machines without problems.

[xhochy]

These runtime checks are useful for code where you explicitly use AVX & co. The problem with march=native is that it instructs the compiler to use all instructions that are available on the build machine anywhere in the executable. Thus, yiu cannot control the usage of these instruction sets at runtime.

In the case of conda-forge, we build packages on Azure Pipelines where normally all the latest instruction sets are available but the users of the packages have older CPUs and not necessarily access to the latest AVX512 features. Thus binaries compiled with march=native may fail at arbitrary points in the execution.

[dyashuni]

@xhochy thank you, it makes sense
Could you please change the setup.py file from the develop branch to add back the environment variable HNSWLIB_NO_NATIVE to skip -march=native and -mcpu=apple-m1 ?

I think we can add
this

hnswlib/setup.py

Lines 81 to 82 in 6f2c82d

	if not os.environ.get("HNSWLIB_NO_NATIVE"):
	c_opts['unix'].append('-march=native')

and a check if not os.environ.get("HNSWLIB_NO_NATIVE"): here
https://github.com/nmslib/hnswlib/blob/develop/setup.py#L100-L114