tree a79641f1ef7f49531f8b4efb84f04c0633a01881
parent 31ece98da1b52769e24871066962990d28245f64
author David Benjamin <davidben@google.com> 1645457435 -0500
committer Adam Langley <agl@google.com> 1645643848 +0000

Check static CPU capabilities on x86.

On Arm, our CRYPTO_is_*_capable functions check the corresponding
preprocessor symbol. This allows us to automatically drop dynamic checks
and fallback code when some capability is always avilable.

This CL does the same on x86, as well as consolidates our
OPENSSL_ia32cap_P checks in one place. Since this abstraction is
incompatible with some optimizations we do around OPENSSL_ia32cap_get()
in the FIPS module, I've marked the symbol __attribute__((const)), which
is enough to make GCC and Clang do the optimizations for us. (We already
do the same to DEFINE_BSS_GET.)

Most x86 platforms support a much wider range of capabilities, so this
is usually a no-op. But, notably, all x86_64 Mac hardware has SSSE3
available, so this allows us to statically drop an AES implementation.
(On macOS with -Wl,-dead_strip, this seems to trim 35080 bytes from the
bssl binary.) Configs like -march=native can also drop a bunch of code.

Update-Note: This CL may break build environments that incorrectly mark
some instruction as statically available. This is unlikely to happen
with vector instructions like AVX, where the compiler could freely emit
them anyway. However, instructions like AES-NI might be set incorrectly.

Change-Id: I44fd715c9887d3fda7cb4519c03bee4d4f2c7ea6
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/51548
Reviewed-by: Adam Langley <agl@google.com>