Commits on Jun 6, 2020

1. Inline singleton splats …

   As noted in #36087 and #29114, splatting integers currently has a
   performance penalty that is unexpected. For tuples and SimpleVectors,
   we have special purpose inliners that will simply inline the
   tuple/SimpleVector into the call being splatted. However, for
   everything else we'd have to run the iteration protocol to find
   out what the values to substitute are. This change does just that,
   limited to the case of length-1 (and empty) iterables. Benchmark:
   
   ```
   f(x) = (x...,)
   @code_typed f(1)
   @benchmark f(1)
   ```
   
   Before:
   ```
   julia> @code_typed f(1)
   CodeInfo(
   1 ─ %1 = Core._apply_iterate(Base.iterate, Core.tuple, x)::Tuple{Int64}
   └──      return %1
   ) => Tuple{Int64}
   
   julia> @benchmark f(1)
   BenchmarkTools.Trial:
     memory estimate:  32 bytes
     allocs estimate:  2
     --------------
     minimum time:     209.357 ns (0.00% GC)
     median time:      213.404 ns (0.00% GC)
     mean time:        218.674 ns (0.16% GC)
     maximum time:     1.922 μs (0.00% GC)
     --------------
     samples:          10000
     evals/sample:     540
   ```
   
   After:
   ```
   julia> @code_typed f(1)
   CodeInfo(
   1 ─ %1 = invoke Base.iterate(_2::Int64)::Tuple{Int64,Nothing}
   │   %2 = (getfield)(%1, 1)::Int64
   │   %3 = (getfield)(%1, 2)::Nothing
   │        invoke Base.iterate(_2::Int64, %3::Nothing)::Nothing
   │   %5 = Core.tuple(%2)::Tuple{Int64}
   └──      return %5
   ) => Tuple{Int64}
   
   julia> @benchmark f(1)
   BenchmarkTools.Trial:
     memory estimate:  0 bytes
     allocs estimate:  0
     --------------
     minimum time:     3.044 ns (0.00% GC)
     median time:      3.047 ns (0.00% GC)
     mean time:        3.049 ns (0.00% GC)
     maximum time:     7.700 ns (0.00% GC)
     --------------
     samples:          10000
     evals/sample:     1000
   ```
   
   Obviously this isn't 100% optimal yet, because the `iterate` calls themselves
   don't get inlined, but it's a lot better. Inlining the `iterate` calls is
   left for a follow up commit.

   

   Keno committed Jun 6, 2020
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