On Tuesday, 1 April 2014 at 11:50:51 UTC, Johannes Pfau wrote:
I started fixing GDC bug #8 (*) which is basically that array op
evaluation order currently depends on the target architecture.
Consider
this example:
a()[] = b()[] + c()[];
The order in which c,a,b are called is currently architecture
specific.
As stated in that bug report by Andrei we want this to evaluate
LTR, so
a() first, then b(), then c().
These operations are actually rewritten to calls to extern(C)
functions. Arguments to C function should be evaluated LTR as
well, but
dmd currently evaluates them RTL (GDC: architecture dependent).
In order
to fix the array op bug in gdc we have to define the evaluation
order
for extern(C) function parameters.
So I've changed extern(C) functions to evaluate LTR in GDC and
then had
to change the array op code, cause that assumed extern(C)
function
evaluate RTL. Now I'd like to push these array op changes into
dmd as we
want to keep as few gdc specific changes as possible and dmd
(and ldc)
will need these changes anyway as soon as they implement
extern(C)
functions as LTR. This is required by dmd issue #6620 (**) and
the
language spec (***).
However, if we apply only these changes the array op order
reverses for
DMD as it evaluates extern(C) function arguments RTL.
So I need someone with dmd backend knowledge to fix the
evaluation
order of extern(C) function parameters to be LTR.
Evaluation order of assignments should also be fixed to be LTR
in the
dmd backend. Although not strictly required for the array op
changes
it'd be inconsistent to have array op assignments execute LTR
but
normal assignments RTL:
a()[] = b()[] + c()[]; //Array op assignment
a() = b() + c(); //Normal assignment
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1 2 3
The frontend changes for dmd are here:
https://github.com/jpf91/dmd/tree/fixOrder
Frontend:
https://github.com/jpf91/dmd/commit/5d61b812977dbdc1f99100e2fbaf1f45e9d25b03
Test cases:
https://github.com/jpf91/dmd/commit/82bffe0862b272f02c27cc428b22a7dd113b4a07
Druntime changes (need to be applied at the same time as dmd
changes)
https://github.com/jpf91/druntime/tree/fixOrder
https://github.com/jpf91/druntime/commit/f3f6f49c595d4fb25fb298e435ad1874abac516d
(*) http://bugzilla.gdcproject.org/show_bug.cgi?id=8
(**) https://d.puremagic.com/issues/show_bug.cgi?id=6620
(***) https://github.com/D-Programming-Language/dlang.org/pull/6
The evaluation order of assign operators should not be LTR as
they have right associativity. In "a = b = c", c has to be
evaluated first, then b and then a. Similarly, in "a = b + c",
"b+c" has to be evaluated first before a is evaluated. Otherwise
it will be very confusing, that in some cases it is LTR and in
some it is RTL.
Other binary operators like "+" have left associativity, and
hence evaluation for these should be LTR as mentioned in D spec.
The C spec requires that the function arguments are to be pushed
in RTL order.
The DMD codegen uses pushl x86 instructions for pushing args. If
the frontend changes the func args evaluation order to LTR, then
the backend has to be modified to use mov x86 instructions as is
done by gcc codegen.
- Sarath
