672490 - Implicit coercion is a little faster than using "as" but they should be the same

Status: RESOLVED WONTFIX

(Tamarin Graveyard :: Virtual Machine, defect, P3)

Description

[Tommy Reilly]

http://jacksondunstan.com/articles/1305

Comment 1

[Tommy Reilly]

Attachment: test diff

Comment 2

[Tommy Reilly]

Thomas-Reillys-Mac-Pro:~ treilly$ ~/builds/tamarin-redux/release/shell/avmshell cast.abc
as: 63
impl: 170
Thomas-Reillys-Mac-Pro:~ treilly$ ~/builds/tamarin-redux/release64/shell/avmshell cast.abc
as: 60
impl: 158
Thomas-Reillys-Mac-Pro:~ treilly$ ~/builds/tamarin-redux/release64/shell/avmshell cast.abc
as: 59
impl: 157

Comment 3

[Tommy Reilly]

Attachment: test case 2, not skewed by DCE

Comment 4

[Tommy Reilly]

results with test 2

$ ~/builds/tamarin-redux/release/shell/avmshell cast.abc
as: 314
impl: 513
$ ~/builds/tamarin-redux/release/shell/avmshell cast.abc
as: 312
impl: 510
$ ~/builds/tamarin-redux/release-debugger/shell/avmshell cast.abc
as: 344
impl: 589
$ ~/builds/tamarin-redux/release64/shell/avmshell cast.abc
as: 239
impl: 399
$ ~/builds/tamarin-redux/release64/shell/avmshell cast.abc
as: 240
impl: 402

Comment 5

[Tommy Reilly]

Attachment: vector is even worse

Comment 6

[Tommy Reilly]

Okay so the executive summary is that you have to be careful about DCE eliding the astype_late if you never use the value (see test 2).   Werner surmises this is due to astype_late being pure.  

coerceobj_atom takes a Traits and doesn't have to call toClassITraits but it seems possibly branchier.   AvmCore::astype doesn't some nitfy stuff if Traits is a builtin type but the JIT knows that isn't the case so the as case could be even faster.   

Both operations could arguably do the atom testing in the JIT'd code and make a direct call to subtypeof for kObjectType atoms.   A good first step would be to try to make coerceobj_atom as fast as astype_late.  

And then for true glory the fast path of subtypeof could be inlined into the JIT'd code too.

Comment 7

[Tommy Reilly]

Attachment: test showing all 4 cases

previous tests/comments were bogus, as operator is slower than implicit cast, vector is even faster, and atom to atom is the fastest.  so no really big surprises here really.

Comment 8

[Tommy Reilly]

New results with fixed test.   So there is a bug that as operator probably slows things down more than it needs to, a specialization of astype for when we know the right hand side is a valid class is probably in order.   

$ ~/builds/tamarin-redux/release64/shell/avmshell cast.abc
as: 239
impl: 161
vec: 157
obj: 144

Comment 9

[Sylvestre Ledru [:Sylvestre]]

Tamarin isn't maintained anymore. WONTFIX remaining bugs.