Preliminary deforestation implementation for hkmc2 #289
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| object B | ||
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| // TODO: should fuse `A` and `B` with the pattern match in `f` |
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Any idea why this doesn't happen?
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When I was looking into the explanation for this I realized that this may not be the best example to illustrate the issue here. I think a better one looks like this:
fun f1(x) = if x is A then 1 else 2
fun f2(x) = if x is A then 2 else 3
fun f3(x) = if x is A then 3 else 4
let p = if true then AA(A) else BB(B)
if p is
AA(x) then f1(x)
BB(x) then f2(x) + f3(x)
The fusion doesn't happen now (but can happen some time before) is because now the symbol for the first pattern variable (x in the source program) in both branches (is AA(x) then ...; is BB(x) then ...) is shared, which means that the program after lowering looks like:
if p is
AA then
param0 = p.x
x1 = param0
f1(x1)
BB then
param0 = p.x
x2 = param0
f2(x2) + f3(x2)
The shared param0 in both branches leads to both A (in AA(A)) and B (in BB(B)) flowing into all f1, f2 and f3, causing clash. Before, param0 is not shared (the first branch uses param0 and the second branch uses param1), so that A in AA(A) could be fused, and only B in BB(B) couldn't get fused.
I said this program in the difftest file is not a good example because the fusion can also happen if we do not distinguish two call sites for the consumer f.
I am trying to address this issue by generating different type variables for the shared param0 in different branches.
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| // TODO: fusion opportunity is lost | ||
| // when instantiation ids of dtors are also tracked |
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I don't understand the comment. Care to explain?
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Sure, here I mean that if we do not distinguish different call sites for consumers, we may be able to fuse again. Now this cannot fuse because there are two call sites for the inner function (which turned out to be only a consumer), so two different inner functions are considered to be called at these two call sites, causing strategy clash. This may also be related to the merging of consumers (in the next program in new-todo.mls).
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Superseded by #335 |
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