JuliaLang / julia / #37030
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optimizer: eliminate safe `typeassert` calls (#42706)

Adds a very simple optimization pass to eliminate `typeassert` calls.
The motivation is, when SROA replaces `getfield` calls with scalar values,
then we can often prove `typeassert` whose first operand is a replaced
value is no-op:
```julia
julia> struct Foo; x; end

julia> code_typed((Int,)) do a
                   x1 = Foo(a)
                   x2 = Foo(x1)
                   typeassert(x2.x, Foo).x
               end |> only |> first
CodeInfo(
1 ─ %1 = Main.Foo::Type{Foo}
│   %2 = %new(%1, a)::Foo
│        Main.typeassert(%2, Main.Foo)::Foo # can be nullified
└──      return a
)
```

Nullifying `typeassert` helps succeeding (simple) DCE to eliminate dead
allocations, and also allows LLVM to do more aggressive DCE to emit simpler code.

Here is a simple benchmarking:
> sample target code:
```julia
julia> function compute(T, n)
           r = 0
           for i in 1:n
               x1 = T(i)
               x2 = T(x1)
               r += (x2.x::T).x::Int
           end
           r
       end
compute (generic function with 1 method)

julia> struct Foo; x; end

julia> mutable struct Bar; x; end
```

> on master
```julia
julia> @benchmark compute(Foo, 1000)
BenchmarkTools.Trial: 10000 samples with 8 evaluations.
 Range (min … max):  3.263 μs … 145.828 μs  ┊ GC (min … max): 0.00% … 97.14%
 Time  (median):     3.516 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   4.015 μs ±   3.726 μs  ┊ GC (mean ± σ):  3.16% ±  3.46%

   ▇█▆▄▅▄▄▃▂▁▂▁                                               ▂
  ▇███████████████▇██▇▇█▇▇▆▇▇▇▇▅▆▅▇▇▅██▇▇▆▇▇▇█▇█▇▇▅▆▆▆▆▅▅▅▅▄▄ █
  3.26 μs      Histogram: log(frequency) by time      8.52 μs <

 Memory estimate: 7.64 KiB, allocs estimate: 489.

julia> @benchmark compute(Bar, 1000)
BenchmarkTools.Trial: 10000 samples with 4 evaluations.
 Range (min … max):  6.990 μs … 288.079 μs  ┊ GC (min … max): 0.00% … 97.03%
 Ti... (continued)

71177 of 79680 relevant lines covered (89.33%)

14615023.88 hits per line

Jobs
ID Job ID Ran Files Coverage
1 #37030.1 0
89.33
Source Files on build #37030
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