[JIT] Exit Transform Rewrite (#38282)
Summary:
After an early return, we conditionalize all further execution. This means that currently the pattern of
`if return elif return elif return` generates better code than `if return if return if return`. It's obviously not good to have semantically equivalent code generate worse IR, so we should rewrite the graph to handle this case. This came up in https://github.com/pytorch/pytorch/pull/37171
```
torch.jit.script
def test_foo(x: bool, y: bool):
if x:
return 1
return 2
print(test_foo.code)
```
generates:
```
def test_foo(x: bool,
y: bool) -> int:
_0 = uninitialized(int)
if x:
_1, _2 = True, 1
else:
_1, _2 = False, _0
if _1:
_3 = _2
else:
_3 = 2
return _3
```
while
```
torch.jit.script
def test_foo(x: bool, y: bool):
if x:
return 1
else:
return 2
print(test_foo.code)
```
generates:
```
def test_foo(x: bool,
y: bool) -> int:
if x:
_0 = 1
else:
_0 = 2
return _0
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38282
Differential Revision: D21576733
Pulled By: eellison
fbshipit-source-id: 80cf1ad7fbda6d8d58557abbfb21c90eafae7488
