[Python-Dev] [ANN] superinstructions (VPython 0.1)

Antoine Pitrou solipsis at pitrou.net
Thu Oct 23 15:28:15 CEST 2008


J. Sievers <cadr4u <at> gmail.com> writes:
> A sequence of code such as LOAD_CONST LOAD_FAST BINARY_ADD will, in
> CPython, push some constant onto the stack, push some local onto the
> stack, then pop both off the stack, add them and push the result back
> onto the stack.
> Turning this into a superinstruction means inlining LOAD_CONST and
> LOAD_FAST, modifying them to store the values they'd otherwise push
> onto the stack in local variables and adding a version of BINARY_ADD
> which reads its arguments from those local variables rather than the
> stack (this reduces dispatch time in addition to pops and pushes).

The problem is that this only optimizes code like "x + 1" but not "1 + x" or "x
+ y". To make this generic a first step would be to try to fuse LOAD_CONST and
LOAD_FAST into a single opcode (and check it doesn't slow down the VM). This
could be possible by copying the constants table into the start of the frame's
variables array when the frame is created, so that the LOAD_FAST code still does
a single indexed array dereference. Since constants are constants, they don't
need to be copied again when the frame is re-used by a subsequent call of the
same function (but this would slow done recursive functions a bit, since those
have to create new frames each time they are called).

Then fusing e.g. LOAD_FAST LOAD_FAST BINARY_ADD into ADD_FAST_FAST would cover
many more cases than the optimization you are writing about, without any
explosion in the number of opcodes.



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