libbb/sha1: add a comment
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
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@ -6,6 +6,28 @@
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# also contains the diff of the generated file.
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exec >hash_md5_sha_x86-64.S
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# There is a way to use XMM registers (which always exist for x86-64!) for W[]
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# For example, if we load W as follows:
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# %xmm0: w[0x0] w[0x1] w[0x2] w[0x3]
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# %xmm4: w[0x4] w[0x5] w[0x6] w[0x7]
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# %xmm8: w[0x8] w[0x9] w[0xa] w[0xb]
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# %xmm12: w[0xc] w[0xd] w[0xe] w[0xf]
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# then the xor'ing operation to generate next W[0..3] is:
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# movaps %xmm0, %xmmT2
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# palignr $0x8, %xmm4, %xmmT2 # form (w[0x2],w[0x3],w[0x4],w[0x5])
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# # Right-shifts xmm4:xmmT2 by 8 bytes. Writes shifted result to xmmT2. SSSE3 insn.
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# movaps %xmm0, %xmmT13
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# palignr $0x4,%xmm0,%xmmT13 # form (w[0xd],w[0xe],w[0xf],w[0x0])
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# xmm0 = xmm0 ^ t2 ^ xmm8 ^ t13
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# xmm0 = rol32(xmm0,1) # no such insn, have to use pslld+psrld+or
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# and then results can be extracted for use:
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# movd %xmm0, %esi # new W[0]
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# pextrd $1, %xmm0, %esi # new W[1]
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# # SSE4.1 insn. Can use EXTRACTPS (also SSE4.1)
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# pextrd $2, %xmm0, %esi # new W[2]
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# pextrd $3, %xmm0, %esi # new W[3]
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# ... but this requires SSE4.1 and SSSE3, which are not universally available on x86-64.
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echo \
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'### Generated by hash_md5_sha_x86-64.S.sh ###
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