On Tuesday, 1 April 2014 at 22:04:43 UTC, Timon Gehr wrote:
On 04/01/2014 08:40 PM, Sarath Kodali wrote:
...
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.
Note that this is after a paragraph that suggests to make
evaluation in some cases LTR and in some RTL.
There are 2 evaluation orders that need to be considered while
evaluating expressions - the evaluation order of operators and
the the evaluation order of operands of an operator. The
evaluation order of operators is well defined and is done
according to its precedence and associativity. However the
evaluation order of operands for some of the binary operators is
not defined. D left it undefined for assign operator. So in
"a=b", the compiler can choose to evaluate a first and then b.
However in "a=b=c", "b=c" has to be evaluated first due to right
associativity of '=' operator. Similarly in "a=b+c", "b+c" has to
be evaluated first due to higher precedence of + operator over =
operator. In both these cases, the right operand of = operator
is evaluated first and then the left operand. So it naturally
follows that even in the unspecified case (a=b), the right
operand should be evaluated first so that it is consistent with
other cases of = operator. All this means, the evaluation order
of operands also should be according to the associativity of its
operator. You can test this with other right or left associative
binary operators.
Other binary operators like "+" have left associativity, and
hence
evaluation for these should be LTR as mentioned in D spec.
...
What's the presumed relation between associativity and
evaluation order?
In particular, the ternary operator ?: is right associative.
How on earth are you going to evaluate it right to left?
The C spec requires that the function arguments are to be
pushed in RTL
order.
[citation needed]
You can get that info from any C ABI doc from Intel or AMD or
some other arch.
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
The backend does not necessarily have to be modified to achieve
this.
Can you please explain how you are going to do that without
modifying the backend?
- Sarath
