Hi Chapel Developers --
Here's a second opportunity for comment. A few of us have been frustrated
for awhile about the way references are represented in the Chapel
compiler, both in terms of making the IR more complex and making the code
harder to reason about.
In particular, for code like this:
proc foo(ref x: int) { ... }
var a = 1;
foo(a);
fairly early in compilation (function resolution-ish), the IR is modified
to more-or-less represent:
proc foo(ref x: ref(int)) { ... }
var a = 1;
var ref_tmp = &a;
foo(ref_tmp);
It seems regrettable to introduce these explicit reference types and their
corresponding temps so early in the compilation process. An extreme
argument against this might be "What if we were code generating to a
back-end language that had different argument-passing semantics than C,
where arguments are passed by ref by default and therefore such rewriting
isn't necessary and may even represent a step backwards?" A simpler
argument might just be "Is there any value in making the IR this different
from Chapel so early (rather than closer to codegen, and dependent on
targeting C or a C-like back-end, say)?"
I've asked Kyle and Vass to explore whether we can put off transformations
like this until a later pass in the compiler, but wanted to do a quick
check to see if anyone had strong objections or gotchas in mind before
they set off.
Thanks,
-Brad
[For context, one motivation for this effort is to optimize such ref_tmps
out of inlined procedures that take ref arguments. E.g., if the body of
foo() above was:
x = 0;
I'd like the IR and generated code to say:
a = 0;
rather than:
ref_tmp = &a;
*_ref_tmp = 0;
It's been argued that propagation optimizations could squash the _ref_tmp
out of this to generate the simpler code, and I don't disagree, but my
feeling is "Why inesrt a pointless temp in the first place?" Today the
answer is "because the argument expects a ref(int) type and so the actual
must match the formal." But if we were to put off the introduction of
ref(int) types until a later pass, we could avoid the temp altogether.]
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