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%
Time (median): 7.657 μs ┊ GC (median): 0.00%
Time (mean ± σ): 9.019 μs ± 9.710 μs ┊ GC (mean ± σ): 4.59% ± 4.28%
▆█▆▄▃▂▂▁▂▃▂▁ ▁ ▁
██████████████████████▇▇▇▇▇▆██████▇▇█▇▇▇▆▆▆▆▅▆▅▄▄▄▅▄▄▃▄▄▂▄▅ █
6.99 μs Histogram: log(frequency) by time 20.7 μs <
Memory estimate: 23.27 KiB, allocs estimate: 1489.
```
> on this branch
```julia
julia> @benchmark compute(Foo, 1000)
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
Range (min … max): 1.234 ns … 116.188 ns ┊ GC (min … max): 0.00% … 0.00%
Time (median): 1.246 ns ┊ GC (median): 0.00%
Time (mean ± σ): 1.307 ns ± 1.444 ns ┊ GC (mean ± σ): 0.00% ± 0.00%
█▇ ▂▂▁ ▂ ▁
██████▇█▇▅▄▆▇▆▁▃▄▁▁▁▁▁▃▁▃▁▁▄▇▅▃▃▃▁▃▄▁▃▃▁▃▁▁▃▁▁▁▄▃▁▃▇███▇▇▇▆ █
1.23 ns Histogram: log(frequency) by time 1.94 ns <
Memory estimate: 0 bytes, allocs estimate: 0.
julia> @benchmark compute(Bar, 1000)
BenchmarkTools.Trial: 10000 samples with 1000 evaluations.
Range (min … max): 1.234 ns … 33.790 ns ┊ GC (min … max): 0.00% … 0.00%
Time (median): 1.245 ns ┊ GC (median): 0.00%
Time (mean ± σ): 1.297 ns ± 0.677 ns ┊ GC (mean ± σ): 0.00% ± 0.00%
█▇ ▃▂▁ ▁
██████▆▆▅▁▄▅▅▄▁▄▄▄▃▄▃▁▃▁▃▄▃▁▃▁▃▁▁▁▃▃▁▃▃▁▁▁▁▁▁▁▃▁▄█████▇▇▇▇ █
1.23 ns Histogram: log(frequency) by time 1.96 ns <
Memory estimate: 0 bytes, allocs estimate: 0.
```
This `typeassert` elimination would be much more effective if we implement
more aggressive SROA based on strong [alias analysis](https://github.com/aviatesk/EscapeAnalysis.jl)
and/or [more aggressive Julia-level DCE](https://github.com/JuliaLang/julia/issues/27547).
But this change is so simple that I don't think it hurts anything to have
it for now.
