Add a new option for requiring BMI2 intrinsics#171
Conversation
If in an envioronment where the desired output code will be run on a broad set of x86 CPUs, BMI2 isn't guaranteed to be available. When compiling against MSVC, `bzhi` will still be inserted into the instruction stream because the `SNAPPY_HAVE_BMI2` check will succeeed (it compiles without issue), but cannot be run by Sandy Bridge and early CPUs.
|
Hello, I can confirm that this real issue. snappy have special flags for AVX(2), and you expect that if it's off, you will not get any modern AVX(2) or BMI2 instruction in output binary. Unfortunately this is not true. in RocksDB we have this bug. So far we are going to fix it with From my side, please merge this PR, it will help and it fix real issue. Radek |
|
Thanks, do we need to disable BMI2 for MSVC? Otherwise we cannot guarantee that it is present. Maybe set SNAPPY_REQUIRE_BMI2 to OFF if the compiler matches MSVC? |
|
Hello @danilak-G Please correct me if I'm wrong, but I think by default, MSVC doesn't use AVX2 or BMI2. Only when you specify I think we should little refactor build system. The way how we detect BMI2/AVX2 is little non standard. We should create two CMake properties, one for AVX2, which at the moment will be only compiler flag, and second for BMI2(Imply AVX2) which will setup compiler flags and also enable BMI2 intrinsic implementation. This should give us good flexibility and make the CPU flags cleaner for other developers. Radek |
4 participants
If in an envioronment where the desired output code will be run on a broad set of x86 CPUs, BMI2 isn't guaranteed to be available. When compiling against MSVC,
bzhiwill still be inserted into the instruction stream because theSNAPPY_HAVE_BMI2check will succeeed (MSVC compiles without issue), but cannot be run by Sandy Bridge and earlier CPU generations.