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even though NullSink is no-ops, it causes llvm to not inline this function, for a net perf reduction
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these instructions had memory sizes reported for the operand, if it was
a memory operand, but for versions with non-memory operands the decoded
`Instruction` would imply that non memory access would happen at all.
now, decoded instructions in these cases will report a more useful
memory size.
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while x86 branches of immediates are all relative to PC, other
architectures may have absolute branches to immediate addresses, leaving
this syntax ambiguous and potentially confusing. yaxpeax prefers to
write relative offsets `$+...` as a rule, so uphold that here.
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and ip/flags
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not that xop will ever be wanted, rip
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this profiles slightly better? not entirely sure why...
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the evex route would allow "valid" instructions that have the opcode
`invalid`. this is.. not correct.
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at least on my zen2.
when reading prefixes, optimize for the likely case of reading an
instruction rather than an invalid run of prefixes. checking if we've
exceeded the x86 length bound immediately after reading the byte is only
a benefit if we'd otherwise read an impossibly-long instruction; in this
case we can exit exactly at prefix byte 15 rather than potentially later
at byte 16 (assuming a one-byte instruction like `c3`), or byte ~24 (a
more complex store with immediate and displacement).
these casese are extremely unlikely in practice. more likely is that
reading a prefix byte is one of the first two or three bytes in an
instruction, and we will never benefit from checking the x86 length
bound at this point. instead, only check length bounds after decoding
the entire instruction. this penalizes the slowest path through the
decoder but speeds up the likely path about 5% on my zen2 processor.
additionally, begin reading instruction bytes as soon as we enter the
decoder, and before initial clearing of instruction data. again, this is
for zen2 pipeline reasons. reading the first byte and corresponding
`OPCODES` entry improves the odds that this data is available by the
time we check for `Interpretation::Prefix` in the opcode scanning
loop. then, if we did *not* load an instruction, we immediately know
another byte must be read; begin reading this byte before applying `rex`
prefixes, and as soon as a prefix is known to not be one of the
escape-code prefix byte (c5, c4, 62, 0f). this clocked in at another ~5%
in total.
i've found that `read_volatile` is necessary to force rust to begin the
loadwhere it's written, rather than reordering it over other data. i'm
not committed to this being a guaranteed truth.
also, don't bother checking for `Invalid`. again, `Opcode::Invalid` is a
relatively unlikely path through the decoder and `Nothing` is already
optiimized for `None` cases. this appears to be another small improvement
in throughput but i wouldn't want to give it a number - it was
relatively small and may not be attributable to this effect.
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this measures a bit faster. it doesn't seem like it should be. the rex
prefix checks compile identically but move a lea for a later expression
up and pipelines better?
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also remove redundant assignments of operand_count and some OperandSpec,
bulk-assign all registers and operands on entry to `read_instr`. this
all, taken together, shaves off about 7 cycles per decode.
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vex decoding is really intended to avoid explosions in code size more than anything...
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