Raymond Hettinger <[email protected]> added the comment:
Thanks for the link. This is a worthwhile experiment. However, the potential
gains will be hard to come by.
The workload of LOAD_CONST is very small. After paying for the usual dispatch
logic overhead, all it does is an index into a struct member and an incref.
Both the co_consts table and the popular constant objects are already likely to
be in the L1 data cache.
##DEBUG_LABEL: TARGET_LOAD_CONST
movslq %r15d, %rax ## OpArg fetch, typically a zero code
register rename
movq -368(%rbp), %rcx ## 8-byte Reload to access co_consts
movq 24(%rcx,%rax,8), %rax ## The actual indexing operation (3
cycles)
incq (%rax) ## The incref
A specialized opcode for a specific constant like None can 1) eliminate the
oparg fetch (likely saving nothing), and 2) eliminate the two sequentially
dependent memory access (this is a win):
##DEBUG_LABEL: TARGET_LOAD_NONE
movq __Py_NoneStruct@GOTPCREL(%rip), rax
incq (%rax) ## The incref
Any more general opcode for loading small ints would still need the oparg fetch
and the incref. To win, it would need to convert the oparg into an int more
efficiently than the two movq steps. If the small int table is in a fixed
location (not per-subinterpreter), then you can save 2 cycles with the simpler
address computation:
##DEBUG_LABEL: TARGET_SMALLINT
movslq %r15d, %rax ## OpArg fetch, typically a zero code
register rename
movq __Py_SmallInt@GOTPCREL(%rip), %rcx ## Find an array of
ints
movq (%rcx,%rax,8), %rax ## Cheaper address computation takes 1
cycle
incq (%rax) ## The incref
The 2 cycle win (intel-only) will be partially offset by the additional
pressure on the L1 data cache. Right now, the co_consts is almost certainly in
cache, holding only the constants that actually get used (at a rate of 8 per
cache line). Accesses into a small_int array will push other data out of L1.
IIRC, Serhiy already experimented with a LOAD_NONE opcode and couldn't get a
measurable win.
----------
nosy: +serhiy.storchaka
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<https://bugs.python.org/issue45152>
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