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Widely available version of GCC and Clang beat our field asm on -O2. In particular, GCC 10.0.5, which is Bitcoin Core's current compiler for official x86_64 builds, produces code that is > 20% faster for fe_mul and > 10% faster for signature verification (see bitcoin-core#723). These are the alternatives to this PR: We could replace our current asm with the fastest compiler output that we can find. This is potentially faster, but it has multiple drawbacks: - It's more coding work because it needs detailed benchmarks (e.g., with many compiler/options). - It's more review work because we need to deal with inline asm (including clobbers etc.) and there's lack of experts reviewers in this area. - It's not unlikely that we'll fall behind again in a few compiler version, and then we have to deal with this again, i.e., redo the benchmarks. Given our history here, I doubt that we'll revolve this timely. We could change the default of the asm build option to off. But this will also disable the scalar asm, which is still faster. We could split the build option into two separate options for field and scalar asm and only disable the field asm by default. But this adds complexity to the build and to the test matrix. My conclusion is that this PR gets the low-hanging fruit in terms of performance. It simplifies our code significantly. It's clearly an improvement, and it's very easy to review. Whether re-introducing better asm (whether from a compiler or from CryptOpt) is worth the hassle can be evaluated separately, and should not hold up this improvement. Solves bitcoin-core#723.
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