2015-11-19 22:29 GMT+01:00 stepharo <stepharo@free.fr>:

SSA form?

and for method arguments? foo: count | sum | sum := 0. 1 to: count do: [ :ku | sum := sum + ku ]. ^ sum transforms to: "foo: arg1 | tmp1 tmp2 | tmp1 := 0. tmp2 := arg1. 1 to: tmp2 do: [ :tmp3 | tmp1 := tmp1 + tmp3 ]. ^ tmp1"

Le 19/11/15 22:14, Nicolai Hess a écrit :

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 ?

You can use whileTrue: if it's a problem for you not to evaluate the limit at each iteration of the loop. 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 ?

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? (* 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 ?

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