On Mar 29, 2009 at 05:36PM, Guido van Rossum <guido@python.org> wrote:
- Issue #5593: code like 1e16+2.9999 is optimized away and its result stored as a constant (again), but the result can vary slightly depending on the internal FPU precision.
I would just not bother constant folding involving FP, or only if the values involved have an exact representation in IEEE binary FP format.
The Language Reference says nothing about the effects of code optimizations. I think it's a very good thing, because we can do some work here with constant folding. If someone wants to preserve precision with floats, it can always use a temporary variable, like in many other languages.
These problems have probably been there for a long time and almost no one seems to complain, but I thought I'd report them here just in case.
I would expect that constant folding isn't nearly effective in Python as in other (less dynamic) languages because it doesn't do anything for NAMED constants. E.g.
MINUTE = 60
def half_hour(): return MINUTE*30
This should be folded to "return 1800" but doesn't because the compiler doesn't know that MINUTE is a constant.
I completely agree. We can't say nothing about MINUTE at the time half_hour will be executed. The code here must never been changed.
Has anyone ever profiled the effectiveness of constant folding on real-world code? The only kind of constant folding that I expect to be making a diference is things like unary operators, since e.g. "x = -2" is technically an expression involving a unary minus.
At this time with Python 2.6.1 we have these results: def f(): return 1 + 2 * 3 + 4j dis(f) 1 0 LOAD_CONST 1 (1) 3 LOAD_CONST 5 (6) 6 BINARY_ADD 7 LOAD_CONST 4 (4j) 10 BINARY_ADD 11 RETURN_VALUE def f(): return ['a', ('b', 'c')] * (1 + 2 * 3) dis(f) 1 0 LOAD_CONST 1 ('a') 3 LOAD_CONST 7 (('b', 'c')) 6 BUILD_LIST 2 9 LOAD_CONST 4 (1) 12 LOAD_CONST 8 (6) 15 BINARY_ADD 16 BINARY_MULTIPLY 17 RETURN_VALUE With proper constant folding code, both functions can be reduced to a single LOAD_CONST and a RETURN_VALUE (or, definitely, by a single instruction at all with an advanced peephole optimizer). I'll show you it at PyCon in Florence, next month.
ISTM that historically, almost every time we attempted some new form of constant folding, we introduced a bug.
I found a very rich test battery with Python, which helped me a lot in my work of changing the ast, compiler, peephole, and VM. If they aren't enough, we can expand them to add more test cases. But, again, the Language Reference says nothing about optimizations. Cheers, Cesare