Patches item #1624059, was opened at 2006-12-29 07:01
Message generated for change (Comment added) made by loewis
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Category: Core (C code)
Group: Python 2.6
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Neal Norwitz (nnorwitz)
Assigned to: Guido van Rossum (gvanrossum)
Summary: fast subclasses of builtin types
Initial Comment:
This is similar to a patch posted on python-dev a few months ago (or more). I
modified it to also handle subclassing exceptions which should speed up
exception handling a bit. (This was proposed by Guido based on the original
patch.) I also dropped an extra bit that was going to indicate if it was a
builtin type or a subclass of a builtin type.
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>Comment By: Martin v. Löwis (loewis)
Date: 2007-01-06 15:24
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I made a couple of assembler experiments (see attached a.c), with gcc 4.1
on x86.
A "bit mask enumeration" test (f) compiles into four instructions:
movl 8(%eax), %eax
andl $-268435456, %eax
cmpl $1879048192, %eax
je .L18
(fall-through being the else case)
A single bit test of a flag (g) compiles to two instructions:
testl $-1073741824, 8(%eax)
je .L9
(fall-through being the if case)
Adding an identity test (comparison with the address of a global),
followed by a bit mask test (h), compiles into six instructions:
cmpl $int_type, %eax
je .L2
movl 8(%eax), %eax
andl $-268435456, %eax
cmpl $1879048192, %eax
je .L2
(fall-through being the else case)
In the common case, only two of these instructions are executed.
So all-in-all, I would agree with Guido that adding bit flags is more
efficient. However, existing bits cannot be recycled: in existing
binary extension modules, these flags are set, so if the modules don't
get recompiled, the type check would believe that the types are
subtypes.
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Comment By: Guido van Rossum (gvanrossum)
Date: 2007-01-04 04:59
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This looks fine, but I have some questions about alternative
implementations:
- Why does the typical PyFoo_Check() macro first call PyFoo_CheckExact()
before calling the fast bit checking macro? Did you measure that this is
in fact faster? True, it means always a pointer deref, so maybe it is --
but OTOH it is more instructions.
- Why not have a separate bit for each type? Then you could make the fast
macro test for (flags & mask) != 0 instead of testing for (flag & mask) ==
value. It would use up all the remaining bits, but I suspect there are
some unused (or reusable) bits in lower positions: 1L<<2 is unused (was
GC), and 1L<<11 also seems unused. And bits 18 through 23! And I'm
guessing that INPLACEOPS (1L<<3) isn't all that interesting any more they
were introduced in 2.0... So it really looks like you have plenty of bits.
Of course I don't know if it matters; would be worth it perhaps to look at
the machine code.
- Oops, it looks like your comment is off. You claim to be using bits
24-27, leaving 28-31 free, but in fact you're using bits 28-31!
BTW You're inroducing quite a few lines over 80 chars. Perhaps cut back a
bit?
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Comment By: Neal Norwitz (nnorwitz)
Date: 2006-12-29 07:04
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I forgot to mention this patch works by using unused bits in tp_flags.
This saves a function call when checking for a subclass of a builtin type.
There's one funky thing about this patch, the change to
Objects/exceptions.c. I didn't investigate why this was necessary, or more
likely I did why when I added it and forgot. I know that without adding
BASE_EXC_SUBCLASS to tp_flags, test_exceptions fails.
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