altanh commented on pull request #10026:
URL: https://github.com/apache/tvm/pull/10026#issuecomment-1027242901
I wrote this as a comment in the code, but reposting here for future
reference:
```
Current Limitations
-------------------
1. For simplicity, we only insert kills when visiting Let bindings, and
always emit the kill as a
single subsequent binding. This is slightly inaccurate; for example, if the
condition of an If
is dead after the test, we can immediately kill the condition in each branch:
let %x = if (%dead_cond) {
let %_0 = memory.kill(%dead_cond);
...
} else {
let %_1 = memory.kill(%dead_cond);
...
}
as opposed to:
let %x = if (%dead_cond) ...
let %_0 = memory.kill(%dead_cond);
2. Killed variables are calculated as live in - live out, which misses
variables that are
actually dead but not in live in. Examples include: when the last use of a
var is the result
expr of an If branch; when bound vars (i.e. function inputs, pattern matched
vars, dead
bindings) are never used.
3. When the result expr of an If branch is a variable, and this expr is the
last use of the
var, we cannot "kill" the var since it is being returned. The VM compiler
also emits a Move
instruction to merge the branch results, which creates another ObjectRef to
the Object held
by the var. The var is also not in the subsequent live-in (since it is
indeed dead by this
point, see above), so it won't be killed.
However, these limitations are unlikely to cause large leaks in practice.
```
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