Hi Nicolai, On Sat, Nov 21, 2015 at 10:40 AM, Nicolai Hess <nicolaihess@gmail.com> wrote:
2015-11-20 9:58 GMT+01:00 Eliot Miranda <eliot.miranda@gmail.com>:
Hi Nicolai,
On Nov 20, 2015, at 12:33 AM, Nicolai Hess <nicolaihess@gmail.com> wrote:
Thanks eliot, Clement,
but if the argument for to:do: is not an expression but one of the method argument, this transformation is not needed. Or still?
That's the corollary to what I said. Since method (*) arguments are read-only it is safe to use them directly and avoid the transformation. One could implement this quickly in the Squeak compiler too :-). How many cases have you identified where this matters? Is it very common?
I don't know, but I don't think so. Recompiling all ~90000 compiled methods, once with opal and once with the other compiler and comparing the numbers of tempVars gives a difference of about 400 methods. Not all 400 differences came from the extra tempvars for the to:do: transform, and sometimes opal generates code with less temp vars.
Well it's a nice surprise to find out that the Squeak compiler does use a variable directly if it's not assigned to in the block. Thanks to your observation I've simplified the test a little, not scanning for assignments if the variable is a method argument or a block argument and the assignment to block arg preference is not in effect. Anyway, adding extra tempvars for accessing the method arguments seems to
be a bug.
Agreed ;-) (* and with the right preference, block)
_,,,^..^,,,_ (phone)
2015-11-20 8:31 GMT+01:00 Clément Bera <bera.clement@gmail.com>:
You can use whileTrue: if it's a problem for you not to evaluate the limit at each iteration of the loop.
I don't need this. I just compared some compiled methods bytecodes after the latest fix for the wrong frame size calculation. For this, I compared opal compilers result with that of the old compiler. And I noticed that some methods compiled with opal had some "hidden" temps and couldn't explain way:)
If you look at OrderedCollection for example, there are implemented differently in Pharo/VW and Squeak. Pharo and VW use to:do: whereas Squeak uses whileTrue: to iterate over the collection. Hence, if you remove elements from the OrderedCollection while iterating over it, the behavior is different.
2015-11-20 1:34 GMT+01:00 Eliot Miranda <eliot.miranda@gmail.com>:
Because were to:do: not inlined the expression passed as the to: actual argument would be evaluated exactly once (bound to the formal argument for to:) and so assigning to a temporary preserves the semantics in the most direct way. If the expression has side effects then those side effects must occur only once.
Note that were the to: expression merely another variable then the compiler could only substitute the original variable if it's value could not change, e.g. in
exampleToDoArgumentLimitIsExpression | count sum | count := 10. sum := 0. 1 to: count do: [ :each | sum := sum + each]. ^sum
It is ok to use count directly as the limit variable, but in this it is not:
exampleToDoArgumentLimitIsExpression | count sum | count := 10. sum := 0. 1 to: count do: [ :each | sum := sum + each. count := #somethingElseEntirely]. ^sum
_,,,^..^,,,_ (phone)
On Nov 19, 2015, at 1:14 PM, Nicolai Hess <nicolaihess@gmail.com> wrote:
What is the purpose of replacing expressions, used as limits in a to:do: call with by a new temporary variable?
For example:
OCOpalExamples>>#exampleToDoArgumentLimitIsExpression the code is
exampleToDoArgumentLimitIsExpression | count sum | count := 10. sum := 0. 1 to: count - 1 do: [ :each | sum := sum + each]. ^sum
the decompiled bytecode :
exampleToDoArgumentLimitIsExpression | tmp1 tmp2 tmp3 | tmp1 := 10. tmp2 := 0. tmp3 := tmp1 - 1. 1 to: tmp3 do: [ :tmp4 | tmp2 := tmp2 + tmp4 ]. ^ tmp2
So, the expression (count - 1) in the to:do: loop is replaced by a new temporay (tmp3).
Ok, but why ?
-- _,,,^..^,,,_ best, Eliot