Re: Reordered class fields?
On 2012-10-31 02:53, Peter Summerland wrote: The order of the fields is rearranged for packing. Does that affect the tupleof property? The example in http://dlang.org/class.html for Class properties tulpleof seems to implie that the the fields the returned Expression Tuple are arranged in lexical order (i.e., as defined by the programmer in the class definition). Is this always true for classes? What about structs? I don't know. But I would not count on the order of tupleof. I would consider that implementation defined. -- /Jacob Carlborg
Re: Reordered class fields?
On Wednesday, 31 October 2012 at 07:19:19 UTC, Jacob Carlborg
wrote:
On 2012-10-31 02:53, Peter Summerland wrote:
The order of the fields is rearranged for packing. Does that
affect the
tupleof property? The example in http://dlang.org/class.html
for Class
properties tulpleof seems to implie that the the fields the
returned
Expression Tuple are arranged in lexical order (i.e., as
defined by the
programmer in the class definition). Is this always true for
classes?
What about structs?
I don't know. But I would not count on the order of tupleof. I
would consider that implementation defined.
Thanks for the help.
Should the the following example, taken from the D Language
Reference, be considered incorrect or at least misleading? It
clearly depends on lexical ordering of the returned fields:
Class Properties
The .tupleof property returns an ExpressionTuple of all the
fields in the class, excluding the hidden fields and the fields
in the base class.
class Foo { int x; long y; }
void test(Foo foo) {
foo.tupleof[0] = 1; // set foo.x to 1
foo.tupleof[1] = 2; // set foo.y to 2
foreach (x; foo.tupleof)
writef(x);// prints 12
}
It would be nice if the Language Reference was specific on this
point. I am aware that the order of the members returned by
__traits(allMembers, D) is not defined (per the LR). But that is
a larger, more complex list.
I am just beginning with D, but I think having the tupleof
property for classes and structs return their fields in lexical
order might be useful.
E.g., I have written a scan function to load up a struct or
class from a row returned by a database query. I have, say,
scan!x,y,z(obj) to load data into fields x,y,z of obj. Based on
the example above and by experimenting, it appears that at least
for dmd running on Linux, that the fields returned by tupleof are
indeed in lexical order. That allowed me to have a default
for the string of field names which indicates that all the fields
should be loaded. I.e., I have scan!(obj), which can be written
scan(obj). Again, it seems to work for simple classes and structs
with Dmd on Ubuntu.
Don't get me wrong -- I'll be happy without the default version
if that is the answer. I'm not suggesting any changes.
Re: Reordered class fields?
On 2012-10-31 09:14, Peter Summerland wrote:
Thanks for the help.
Should the the following example, taken from the D Language Reference,
be considered incorrect or at least misleading? It clearly depends on
lexical ordering of the returned fields:
Class Properties
The .tupleof property returns an ExpressionTuple of all the fields in
the class, excluding the hidden fields and the fields in the base class.
class Foo { int x; long y; }
void test(Foo foo) {
foo.tupleof[0] = 1; // set foo.x to 1
foo.tupleof[1] = 2; // set foo.y to 2
foreach (x; foo.tupleof)
writef(x);// prints 12
}
I would at least consider it bad practice. Even if the compiler
guarantees a specific order it's quite easy for the developer to change
the order of the fields in Foo and then have test break.
I would suggest one always access the fields by name. Example:
https://github.com/jacob-carlborg/orange/blob/master/orange/serialization/Serializer.d#L1448
https://github.com/jacob-carlborg/orange/blob/master/orange/util/Reflection.d#L260
It would be nice if the Language Reference was specific on this point.
I am aware that the order of the members returned by
__traits(allMembers, D) is not defined (per the LR). But that is a
larger, more complex list.
I am just beginning with D, but I think having the tupleof property for
classes and structs return their fields in lexical order might be useful.
1. It wouldn't hurt if it was clearly specified in the language
specification.
2. You can always sort the list
3. I wouldn't count on the order regardless, see above
E.g., I have written a scan function to load up a struct or class from
a row returned by a database query. I have, say, scan!x,y,z(obj) to
load data into fields x,y,z of obj. Based on the example above and by
experimenting, it appears that at least for dmd running on Linux, that
the fields returned by tupleof are indeed in lexical order. That
allowed me to have a default for the string of field names which
indicates that all the fields should be loaded. I.e., I have
scan!(obj), which can be written scan(obj). Again, it seems to work
for simple classes and structs with Dmd on Ubuntu.
Indeed, DMD seems to return them in lexical order, but again, I wouldn't
rely on the, see above.
Don't get me wrong -- I'll be happy without the default version if that
is the answer. I'm not suggesting any changes.
--
/Jacob Carlborg
Re: Reordered class fields?
Peter Summerland: The order of the fields is rearranged for packing. Does that affect the tupleof property? The example in http://dlang.org/class.html for Class properties tulpleof seems to implie that the the fields the returned Expression Tuple are arranged in lexical order (i.e., as defined by the programmer in the class definition). Is this always true for classes? I have added your question here: http://d.puremagic.com/issues/show_bug.cgi?id=8873 What about structs? D structs are almost PODs (if they are nested inside a function and they are not defined as static they have one more field at the beginning), so tupleof for structs gives the fields in definition order. Bye, bearophile
Re: Reordered class fields?
On Wednesday, 31 October 2012 at 08:23:26 UTC, Jacob Carlborg
wrote:
On 2012-10-31 09:14, Peter Summerland wrote:
Thanks for the help.
Should the the following example, taken from the D Language
Reference,
be considered incorrect or at least misleading? It clearly
depends on
lexical ordering of the returned fields:
Class Properties
The .tupleof property returns an ExpressionTuple of all the
fields in
the class, excluding the hidden fields and the fields in the
base class.
class Foo { int x; long y; }
void test(Foo foo) {
foo.tupleof[0] = 1; // set foo.x to 1
foo.tupleof[1] = 2; // set foo.y to 2
foreach (x; foo.tupleof)
writef(x);// prints 12
}
I would at least consider it bad practice. Even if the compiler
guarantees a specific order it's quite easy for the developer
to change the order of the fields in Foo and then have test
break.
I would suggest one always access the fields by name. Example:
https://github.com/jacob-carlborg/orange/blob/master/orange/serialization/Serializer.d#L1448
https://github.com/jacob-carlborg/orange/blob/master/orange/util/Reflection.d#L260
Thanks for the links. I can probably use some of the Reflection
module in my code. I am making a very minimal sqlite3 library,
which mostly just manages connections and statements to ensure
they are closed and finalized automatically. I did add bind, step
and scan methods to the Sqlite3Stmt struct but that's about it.
Other than that you have to use the native c interface.
More importantly, I think that it is really very instructive to
read over your code.
Re breaking test. You are right of course. But in my case the POD
object is meant only to be used to package a row of data. The
client cannot change it. Here is an example of how I might make a
function to query a database
// POD structure to package rows returned by query
struct Employee
{
int id_num;
string name;
int rank;
}
// returns an array of Employee objs for employees with given rank
Employee[] getEmployees(Sqlite3 db, int rank)
{
static string query =
select e_num, e_name, e_rank from employee where e_rank = ?;
auto app = appender!(Employee[]);
auto stmt = db.prepare(query);
stmt.bind(rank);
stmt.scanFields!(id_num,name,rank)(app); // SAFE
//stmt.scanFields(app); // WORKS FOR POD STRUCTS LIKE EMPLOYEE
return app.data;
}
It's not the greatests: You have to change the query if the
database changes or if you want to change Employee, etc. But at
least the query and Employee are defined locally in your code.
It would be nice if the Language Reference was specific on
this point.
I am aware that the order of the members returned by
__traits(allMembers, D) is not defined (per the LR). But that
is a
larger, more complex list.
I am just beginning with D, but I think having the tupleof
property for
classes and structs return their fields in lexical order might
be useful.
1. It wouldn't hurt if it was clearly specified in the language
specification.
2. You can always sort the list
3. I wouldn't count on the order regardless, see above
E.g., I have written a scan function to load up a struct or
class from
a row returned by a database query. I have, say,
scan!x,y,z(obj) to
load data into fields x,y,z of obj. Based on the example above
and by
experimenting, it appears that at least for dmd running on
Linux, that
the fields returned by tupleof are indeed in lexical order.
That
allowed me to have a default for the string of field names
which
indicates that all the fields should be loaded. I.e., I have
scan!(obj), which can be written scan(obj). Again, it seems
to work
for simple classes and structs with Dmd on Ubuntu.
Indeed, DMD seems to return them in lexical order, but again, I
wouldn't rely on the, see above.
Don't get me wrong -- I'll be happy without the default
version if that
is the answer. I'm not suggesting any changes.
Re: Reordered class fields?
On Monday, 22 October 2012 at 11:06:50 UTC, Jacob Carlborg wrote: On 2012-10-22 10:48, bearophile wrote: This page says: http://dlang.org/class.html The D compiler is free to rearrange the order of fields in a class to optimally pack them in an implementation-defined manner. Consider the fields much like the local variables in a function - the compiler assigns some to registers and shuffles others around all to get the optimal stack frame layout. This frees the code designer to organize the fields in a manner that makes the code more readable rather than being forced to organize it according to machine optimization rules. Explicit control of field layout is provided by struct/union types, not classes. Ok, I didn't know that. The order of the fields is rearranged for packing. Does that affect the tupleof property? The example in http://dlang.org/class.html for Class properties tulpleof seems to implie that the the fields the returned Expression Tuple are arranged in lexical order (i.e., as defined by the programmer in the class definition). Is this always true for classes? What about structs?
Re: Reordered class fields?
On 2012-36-22 01:10, bearophile bearophileh...@lycos.com wrote: This benchmark shows that if you allocate the class instances on the heap one at a time the total amount of memory used is the same for the various Bar (maybe because of the GC), so that optimization is useful for emplace() only and similar in-place allocations The current GC always allocates a power of two, with a minimum of 16 bytes. You should see an effect if you make a class that will be above such a threshold without reordering, and below with. So is such class field reordering worth an enhancement request in Bugzilla? Nothing bad can come of it. -- Simen
Re: Reordered class fields?
On 2012-10-22 01:36, bearophile wrote: D classes are free to reorder their fields, so maybe Bar1 should reorder its fields as Bar3, to save 4 bytes for each instance on 32 bit systems: Are D allowed to reorder the class fields? What happens then to binary compatibility? -- /Jacob Carlborg
Re: Reordered class fields?
Simen Kjaeraas: The current GC always allocates a power of two, with a minimum of 16 bytes. You should see an effect if you make a class that will be above such a threshold without reordering, and below with. Right. Nothing bad can come of it. OK :-) -- Jacob Carlborg: Are D allowed to reorder the class fields? This page says: http://dlang.org/class.html The D compiler is free to rearrange the order of fields in a class to optimally pack them in an implementation-defined manner. Consider the fields much like the local variables in a function - the compiler assigns some to registers and shuffles others around all to get the optimal stack frame layout. This frees the code designer to organize the fields in a manner that makes the code more readable rather than being forced to organize it according to machine optimization rules. Explicit control of field layout is provided by struct/union types, not classes. Bye, bearophile
Re: Reordered class fields?
On 2012-10-22 10:48, bearophile wrote: This page says: http://dlang.org/class.html The D compiler is free to rearrange the order of fields in a class to optimally pack them in an implementation-defined manner. Consider the fields much like the local variables in a function - the compiler assigns some to registers and shuffles others around all to get the optimal stack frame layout. This frees the code designer to organize the fields in a manner that makes the code more readable rather than being forced to organize it according to machine optimization rules. Explicit control of field layout is provided by struct/union types, not classes. Ok, I didn't know that. -- /Jacob Carlborg
Reordered class fields?
D classes are free to reorder their fields, so maybe Bar1 should
reorder its fields as Bar3, to save 4 bytes for each instance on
32 bit systems:
class Bar1 {
void Hello() {}
float f;
double d;
}
class Bar2 {
void Hello() {}
align(4) float f;
align(4) double d;
}
class Bar3 {
void Hello() {}
double d;
float f;
}
void main() {
pragma(msg, __traits(classInstanceSize, Bar1)); // 24
pragma(msg, __traits(classInstanceSize, Bar2)); // 20
pragma(msg, __traits(classInstanceSize, Bar3)); // 20
}
This benchmark shows that if you allocate the class instances on
the heap one at a time the total amount of memory used is the
same for the various Bar (maybe because of the GC), so that
optimization is useful for emplace() only and similar in-place
allocations:
class Bar1 {
void Hello() {}
float f;
double d;
}
class Bar2 {
void Hello() {}
align(4) float f;
align(4) double d;
}
class Bar3 {
void Hello() {}
double d;
float f;
}
int main() {
pragma(msg, __traits(classInstanceSize, Bar1)); // 24
pragma(msg, __traits(classInstanceSize, Bar2)); // 20
pragma(msg, __traits(classInstanceSize, Bar3)); // 20
//--
//auto arr = new Bar1[1_000_000]; // 38.2 MB
//auto arr = new Bar2[1_000_000]; // 38.2 MB
auto arr = new Bar3[1_000_000]; // 38.2 MB
foreach (ref a; arr)
a = new typeof(arr[0])();
int count;
foreach (i; 0 .. 500_000_000) count++;
return count;
}
So is such class field reordering worth an enhancement request in
Bugzilla?
Bye,
bearophile
