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supranational / blst

Multilingual BLS12-381 signature library
Apache License 2.0
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Runtime detection bug on macOS #172

Closed jtraglia closed 1 year ago

jtraglia commented 1 year ago

Hi there, I just noticed that runtime detection doesn't appear to be working on my macOS computer with the portable build. I believe I have identified the issue. On macOS, BSD systems, and maybe Windows, an underscore (_) is prepended to all symbols, so the runtime detection assembly code is looking for the wrong symbol/variable. This applies to both x86/arm64.

See the mach-o ARM assembly below, which is looking for __blst_platform_cap (two underscores):

https://github.com/supranational/blst/blob/5358144578b7b8b38e9b8d1da4f085057bbf1125/build/mach-o/sha256-armv8.S#L18

If you list symbols in libblst, you'll see that there are two different symbols:

$ nm libblst.a | ag cap
0000000000000004 C __blst_platform_cap
000000000001e828 D ___blst_platform_cap

The first symbol is from the assembly above, which creates a common variable __blst_platform_cap. The second (data) symbol ___blst_platform_cap (with an extra underscore) was created below. We care about the second one.

https://github.com/supranational/blst/blob/5358144578b7b8b38e9b8d1da4f085057bbf1125/src/cpuid.c#L7

If I replace __blst_platform_cap with ___blst_platform_cap in sha256-armv8.S, my benchmark is noticeably faster.

Not sure if this triage is 100% correct, but I think it's close enough.

dot-asm commented 1 year ago

Fixed in asm/arm-xltate.pl. Thanks!