feat: add parametrized labels to LLL

[charles-cooper]

What I did

change labels in LLL to be parameterized. previously, stack items were implicitly consumed from jumpdests using pass, liveness analysis for stack items is impossible in the general case. now, labels in LLL are very similar to subroutines since they take parameters and are properly terminated. fixes #2511.

this also enables translation to certain LLVM backends since implicit stack items cannot be handled in translation to a register machine.

I also added an exit_to instruction for LLL. This compiles the same as goto -- to a JUMP instruction -- but has different scoping semantics. exit_to terminates a subroutine while goto expects control flow to be returned. this is an important hint for liveness analysis, if you hit an exit_to, all stack items in scope must be consumed, whereas if you hit a goto the same requirement does not hold.

How I did it

change the codegen everywhere that stack items are consumed implicitly. in practice, this happens in private functions, and in function common cleanup blocks.

How to verify it

see that tests pass. the assembly output for all contracts i looked at were byte-for-byte the same except for those which return straight out of a loop (using the LLL cleanup_repeat instruction), so i added a couple more tests to test returns out of loops (in the void return and external cases)

Commit message

Change labels in LLL to be parameterized. Previously, stack items were
implicitly consumed from jumpdests using `pass`, which made liveness
analysis for stack items impossible in general. Since labels now take
parameters, they are very similar to subroutines so we could use that
terminology interchangeably.
 
This change also enables translation to other architectures such as LLVM
backends since implicit stack items cannot be handled in translation to
a register machine.
 
Lastly, this commit also added an `exit_to` instruction for LLL. This
compiles the same as `goto` -- to a JUMP instruction -- but has
different scoping semantics. `exit_to` terminates a subroutine while
`goto` expects control flow to be returned. This is an important hint
for liveness analysis. If you hit an `exit_to`, all stack items in scope
must be consumed, whereas if you hit a `goto` the same requirement does
not hold.
 
In order to accomodate the semantic change, this commit changes the
codegen everywhere that stack items are consumed implicitly. In
practice, this happens in private functions, and in function common
cleanup blocks.

Note: the assembly output for all contracts I looked at were
byte-for-byte the same with the exception of those which returned
straight out of a loop (using the LLL `cleanup_repeat` instruction), so
this commit adds a couple more tests to test returns out of loops (in
the void return and external cases)

Description for the changelog

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[lgtm-com]

This pull request introduces 1 alert when merging 2ea434d into 0004e0c - view on LGTM.com

new alerts:

• 1 for Unused import

[lgtm-com]

This pull request introduces 1 alert when merging 5d177b2 into e480e91 - view on LGTM.com

new alerts:

• 1 for Unused import

[lgtm-com]

This pull request introduces 1 alert when merging 685696a into e480e91 - view on LGTM.com

new alerts:

• 1 for Unused import

[codecov-commenter]

Codecov Report

Merging #2598 (64bc1c5) into master (e480e91) will increase coverage by 0.02%.
The diff coverage is 94.36%.

❗ Current head 64bc1c5 differs from pull request most recent head 1b1ec32. Consider uploading reports for the commit 1b1ec32 to get more accurate results

@@            Coverage Diff             @@
##           master    #2598      +/-   ##
==========================================
+ Coverage   86.26%   86.29%   +0.02%     
==========================================
  Files          91       91              
  Lines        9646     9739      +93     
  Branches     2437     2466      +29     
==========================================
+ Hits         8321     8404      +83     
- Misses        828      833       +5     
- Partials      497      502       +5     

Impacted Files	Coverage Δ
vyper/codegen/module.py	91.15% <ø> (ø)
vyper/utils.py	87.14% <ø> (ø)
vyper/codegen/lll_node.py	92.74% <76.47%> (-1.28%)	⬇️
vyper/lll/compile_lll.py	93.42% <92.72%> (-0.20%)	⬇️
.../codegen/function_definitions/external_function.py	98.09% <100.00%> (+0.13%)	⬆️
.../codegen/function_definitions/internal_function.py	100.00% <100.00%> (ø)
vyper/codegen/return_.py	94.11% <100.00%> (+0.36%)	⬆️
vyper/codegen/self_call.py	95.55% <100.00%> (+0.81%)	⬆️
vyper/codegen/stmt.py	88.62% <100.00%> (-0.36%)	⬇️
vyper/compiler/output.py	93.82% <100.00%> (+0.17%)	⬆️
... and 4 more

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[lgtm-com]

This pull request introduces 1 alert when merging 93fe033 into 14cbedb - view on LGTM.com

new alerts:

• 1 for Unused local variable