Summary:
In preparing to utilize a new Intel instruction extension, I
noticed problems with the existing build script in regard to the
existing utilized extensions, either with USE_SSE or PORTABLE flags.
* PORTABLE=0 was interpreted the same as PORTABLE=1. Now empty and 0
mean the same. (I guess you were not supposed to set PORTABLE= if you
wanted non-portable--except that...)
* The Facebook build script extensions would set PORTABLE=1 even if
it's already set in a make var or environment. Now it does not override
a non-empty setting, so use PORTABLE=0 for fully optimized build,
overriding Facebook environment default.
* Put in an explanation of the USE_SSE flag where it's used by
build_detect_platform, and cleaned up some confusing/redundant
associated logic.
* If USE_SSE was set and expected intrinsics were not available,
build_detect_platform would exit early but build would proceed with
broken, incomplete configuration. Now warning is gracefully recovered.
* If USE_SSE was set and expected intrinsics were not available,
build would still try to use flags like -msse4.2 etc. which could lead
to unexpected compilation failure or binary incompatibility. Now those
flags are not used if the warning is issued.
This should not break or change existing, valid build scripts.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5800
Test Plan: manual case testing
Differential Revision: D17369543
Pulled By: pdillinger
fbshipit-source-id: 4ee244911680ae71144d272c40aceea548e3ce88
6a171724b7