Leif wrote:
> Any reasonable compiler does constant folding and loop-invariant code
> motion; even Python's byte-code compiler should do that.
Almost none is done. Only the most trivial cases are handled. For
example, addition:
sage: preparser(False)
sage: type(2)
<type 'int'>
sage: def f():
....: x = 2+3
....:
sage: dis.dis(f)
2 0 LOAD_CONST 3 (5)
3 STORE_FAST 0 (x)
6 LOAD_CONST 0 (None)
9 RETURN_VALUE
But not division:
sage: def g():
....: x = 2/3
....:
sage: dis.dis(g)
2 0 LOAD_CONST 1 (2)
3 LOAD_CONST 2 (3)
6 BINARY_DIVIDE
7 STORE_FAST 0 (x)
10 LOAD_CONST 0 (None)
13 RETURN_VALUE
As for code motion:
sage: def h():
....: for i in range(10):
....: if x < (2/3):
....: x += 1
....:
sage: dis.dis(h)
2 0 SETUP_LOOP 51 (to 54)
3 LOAD_GLOBAL 0 (range)
6 LOAD_CONST 1 (10)
9 CALL_FUNCTION 1
12 GET_ITER
>> 13 FOR_ITER 37 (to 53)
16 STORE_FAST 0 (i)
3 19 LOAD_FAST 1 (x)
22 LOAD_CONST 2 (2)
25 LOAD_CONST 3 (3)
28 BINARY_DIVIDE
29 COMPARE_OP 0 (<)
32 JUMP_IF_FALSE 14 (to 49)
35 POP_TOP
4 36 LOAD_FAST 1 (x)
39 LOAD_CONST 4 (1)
42 INPLACE_ADD
43 STORE_FAST 1 (x)
46 JUMP_ABSOLUTE 13
>> 49 POP_TOP
50 JUMP_ABSOLUTE 13
>> 53 POP_BLOCK
>> 54 LOAD_CONST 0 (None)
57 RETURN_VALUE
Simon wrote:
> Since we talk about a spyx-file (not pyx), the Sage preparser is
> applied to 2/3, if I am not mistaken. Hence, 2/3 is not 0 but the
> rational number 2/3.
I think you're mistaken. That's why I brought up the missing decimal
point in the first place, because the comparison was always failing so
it was always taking the maximum number of loops.
sage: cat "spre2.spyx"
print 2, type(2)
print 2/3, type(2/3)
print 2./3, type(2./3)
sage: !touch spre2.spyx
sage: load "spre2.spyx"
Compiling ./spre2.spyx...
2 <type 'int'>
0 <type 'int'>
0.666666666667 <type 'float'>
"NO Sage preparsing is applied to spyx files, e.g., 1/3 will result in
0 in a spyx file instead of the rational number 1/3."
Doug
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