Force specialization on the type argument of _similar_for

[martinholters]

Looking into the ["misc", "iterators", "zip(1:1, 1:1, 1:1, 1:1)"] regression reported in #30218 (comment), it looks like #28284 pushed the HasShape method of _similar_for beyond the inlining threshold for that case, causing the regression. Adding a @_meta_inline would directly combat that, but forcing specialization on the type argument is less invasive and still restores performance:

julia> for N in (1,1000), M in (1,2,3,4)
           @btime collect(zip($(Iterators.repeated(1:N, M)...,)...))
       end
# master
  30.213 ns (1 allocation: 96 bytes)
  32.706 ns (1 allocation: 96 bytes)
  38.512 ns (1 allocation: 112 bytes)
  130.325 ns (5 allocations: 272 bytes)
  1.069 μs (1 allocation: 7.94 KiB)
  1.753 μs (1 allocation: 15.75 KiB)
  2.367 μs (2 allocations: 23.52 KiB)
  3.187 μs (6 allocations: 31.48 KiB)
# this PR
  30.425 ns (1 allocation: 96 bytes)
  32.565 ns (1 allocation: 96 bytes)
  37.667 ns (1 allocation: 112 bytes)
  42.188 ns (1 allocation: 112 bytes)
  1.048 μs (1 allocation: 7.94 KiB)
  1.781 μs (1 allocation: 15.75 KiB)
  2.359 μs (2 allocations: 23.52 KiB)
  2.837 μs (2 allocations: 31.33 KiB)

I hope @nanosoldier runbenchmarks(ALL, vs = ":master") agrees. In that case, I guess we want this on 1.1 as well.

I vaguely remember having noticed this when working on #29238 but decided it was an orthogonal change to be done in a separate PR and then forgot about it...

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @ararslan

[martinholters]

OSX Travis ended with:

1788.795694  cd /Users/travis/build/JuliaLang/julia/base && if ! DYLD_FALLBACK_LIBRARY_PATH="/usr/local/lib:/lib:/usr/lib:/usr/local/opt/openblas-julia/lib:/usr/local/opt/suite-sparse-julia/lib" /Users/travis/build/JuliaLang/julia/usr/bin/julia-debug -O0 -C "generic;native" --output-o /Users/travis/build/JuliaLang/julia/usr/lib/julia/sys-debug-o.a.tmp  --startup-file=no --warn-overwrite=yes --sysimage /Users/travis/build/JuliaLang/julia/usr/lib/julia/sys.ji /Users/travis/build/JuliaLang/julia/contrib/generate_precompile.jl 1; then echo '*** This error is usually fixed by running `make clean`. If the error persists, try `make cleanall`. ***'; false; fi 
No output has been received in the last 30m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received
The build has been terminated

Restarted.

Benchmarks look good, but were run before I also changed the AbstractSet / AbstractDict methods, so better safe than sorry: @nanosoldier runbenchmarks(ALL, vs = ":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @ararslan

[martinholters]

Barring objections (or anyone beating me to it) I'll merge tomorrow.

[martinholters]

What's our policy for backporting performance enhancements to 1.0?

[StefanKarpinski]

If they're low risk then absolutely do it.