On 2014/10/28 14:25:14, Dmitry Lomov (chromium) wrote:
Before this CL can land, I think we need a clear design for
implementation of
this feature.
In particular:
1. What the code generation would look like, on a high level?
2. How will we cache template call sites?
It is my guess that after we figure out 1 and 2, we will realize that we
do
not
actually need new AST nodes for template strings and we will be able to
desugar
at parse time into Javascript and some runtime calls, but I might be
mistaken.
So, there are two codegen paths that matter here, I think:
1) No tag:
1. Assert that cookedStrings.length === expressions.length + 1
2. If 0 expressions: plug the cooked string (TV) -> like string literal,
break.
3. setup scratch variable which contains a JSArray
4. Push cookedStrings(0) to scratch variable
5. For each index 0..N expression:
a. VisitForAccumulatorValue(expression i)
b. Push accumulator value to scratch variable
d. Push cookedStrings(++i) to scratch variable
6. __ Push(scratchVariable)
7. __ CallBuiltin(STRING_CONCAT, CALL_FUNCTION)
8. Plug result of STRING_CONCAT
The STRING_CONCAT builtin will receive a JSArray of cooked strings
interleaved
with expression values, and return the result of
`String.prototype.concat.apply('', interleavedElements)`
--- This works nicely, but I can't figure out how to set up a scratch
variable
with no binding, so that it doesn't leak into the programme.
2) With tag:
More complicated: Assert that cookedStrings.length == rawStrings.length,
assert that cookedStrings.length == expressions.length + 1
Setup 3 scratch variables, all JSArrays
Push each cookedString into first scratch variable
Push each rawString into second scratchVariable
For each expression, VisitForAccumulatorValue() and push result into third
scratch variable
__ Push(tagFn)
__ Push(scratchCookedStrings)
__ Push(scratchRawStrings)
__ Push(scratchExpressionValues)
__ CallBuiltin(CALL_TAGGED_TEMPLATE, CALL_FUNCTION)
Plug result of that builtin call
The builtin will A) call GetTemplateCallSite() with the first 2 scratch
variables, and B) apply tagFn with [callSite, ...scratchExpressionValues].
The callsite will be attached to the ASTNode so that it need not be
re-created
again. (steps 1 and 12 in GetTemplateCallSite())
---
So the challenge (for me) is figuring out how to setup each scratch
variable in
a way that doesn't leak into the programme or clobber any
current accumulator value. I haven't figured out how to set up scratch
variables
of a specific type, but they really need to be arrays, I think.
So, someone who knows the codegen side more than me could help figure out a
way
to make those algorithms a reality, or else help figure out a more efficient
algorithm, if one exists.
https://codereview.chromium.org/663683006/
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