compiler: Optimize map comprehensions to use maps:from_keys/2

[michalmuskala]

Map comprehensions with constant values that don't depend on generator
variables are now compiled to use maps:from_keys/2 instead of
maps:from_list/1. This avoids creating intermediate {Key, Value} tuples.

Before: #{K => Value || K <- List} -> maps:from_list([{K, Value} || K <- List])
After: #{K => Value || K <- List} -> maps:from_keys([K || K <- List], Value)

The optimization applies when the value expression:

• Is safe (cannot fail at runtime)
• Has no pre-expressions (is a simple value)
• Does not reference any generator-bound variables
• For multi-valued comprehensions all values are the same

Examples that are optimized:

• #{K => 42 || K <- List} % literal
• #{K => X || K <- List} % outer variable
• #{K => {X, Y} || K <- List} % tuple of outer vars
• #{K => X, {x,K} => X || K <- List} % all values the same

Examples that are NOT optimized:

• #{K => V || {K, V} <- List} % value from generator
• #{K => K * 2 || K <- List} % value depends on key
• #{K => f(X) || K <- List} % function call (not safe)
• #{K => a, {x,K} => b || K <- List} % different values

[bjorng]

Please rebase on the latest master.

[github-actions]

CT Test Results

    2 files    335 suites   9m 6s ⏱️
  865 tests   861 ✅ 4 💤 0 ❌
5 701 runs  5 697 ✅ 4 💤 0 ❌

Results for commit 7a16eba.

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[michalmuskala]

Sorry about that, should be correctly rebased on master now

[bjorng]

Multiple test suite modules fail to compile (try_catch_SUITE, bs_bincomp_SUITE, and so on). Also, Elixir.Base fails to compile.

[michalmuskala]

This should be now resolved, I successfully run the whole compile test suite

[bjorng]

I've force-pushed a rebased and slightly updated version of this branch. Here is what I did:

1. I added a few more tests to mc_SUITE to ensure that all of v3_core:is_safe_same/2 is covered.
2. Removed the clause that handled nomatch in v3_core:add_pre_bound_vars/2. As far as I understand, that clause can never be executed because matching in a filter is an error in OTP 29. If the new compr_assign feature is enabled, a match will handled as a generator. I also added a simple smoke-test of assignment for a map comprehension in lc_SUITE.
3. Two test cases in beam_debug_info_SUITE failed for me, so I've added a separate commit to properly handle multi comprehensions.

I've added this PR along with an updated primary bootstrap to our daily builds.

[michalmuskala]

Amazing, thank you for fixing this up

[bjorng]

Thanks for your pull request.