Slightly refactor compile.jl #1
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jpsamaroo
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LGTM. Any ideas you have for better testing would be great. |
Keno
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Jan 17, 2020
…#32605) The bug here is a bit subtle, but perhaps best illustrated with the included test case: ``` function f32579(x::Int64, b::Bool) if b x = nothing end if isa(x, Int64) y = x else y = x end if isa(y, Nothing) z = y else z = y end return z === nothing end ``` The code just after SSA conversion looks like: ``` 2 1 ─ goto JuliaLang#3 if not _3 3 2 ─ %2 = Main.nothing::Core.Compiler.Const(nothing, false) 5 3 ┄ %3 = φ (#2 => %2, #1 => _2)::Union{Nothing, Int64} │ %4 = (%3 isa Main.Int64)::Bool └── goto JuliaLang#5 if not %4 6 4 ─ %6 = π (%3, Int64) └── goto JuliaLang#6 8 5 ─ %8 = π (%3, Nothing) 10 6 ┄ %9 = φ (JuliaLang#4 => %6, JuliaLang#5 => %8)::Union{Nothing, Int64} │ %10 = (%9 isa Main.Nothing)::Bool └── goto JuliaLang#8 if not %10 11 7 ─ %12 = π (%9, Nothing) └── goto JuliaLang#9 13 8 ─ %14 = π (%9, Int64) 15 9 ┄ %15 = φ (JuliaLang#7 => %12, JuliaLang#8 => %14)::Union{Nothing, Int64} │ %16 = (%15 === Main.nothing)::Bool └── return %16 ``` Now, we have special code in SROA (despite it not really being an SROA transform) that looks at `===` and replaces it by a nest of phis of booleans. The reasoning for this transform is that it eliminates a use of a value where we only care about the type and not the content, thus making it more likely that the value will subsequently be eligible for SROA. In addition, while it goes along resolving which values feed into any particular phi, it accumulates and type conditions it encounters along the way. Thus in the example above, something like the following happens: - We look at %14, which πs to %9 with an Int64 constraint, so we only consider the JuliaLang#4 predecessor for %9 (due to the constraint), until we get to %3, where we again only consider the #1 predecessor, where we find the argument (of type Int64) and conclude the result is always false - Now we pop the next item of the stack from our original phi, look at %12, which πs to %9 with a Nothing constraint. At this point we used to terminate the search because we already looked at %9. However, crucially, we looked at %9 only with an Int64 constraint, so we missed the fact that `nothing` was in fact a possible value for this phi. The result was a missing entry in the generated phi node: ``` 1 ─ goto JuliaLang#3 if not b 2 ─ %2 = Main.nothing::Core.Compiler.Const(nothing, false) 3 ┄ %3 = φ (#1 => false)::Bool │ %4 = φ (#2 => %2, #1 => _2)::Union{Nothing, Int64} │ %5 = (%4 isa Main.Int64)::Bool └── goto JuliaLang#5 if not %5 4 ─ %7 = π (%4, Int64) └── goto JuliaLang#6 5 ─ %9 = π (%4, Nothing) 6 ┄ %10 = φ (JuliaLang#4 => %3, JuliaLang#5 => %3)::Bool │ %11 = φ (JuliaLang#4 => %7, JuliaLang#5 => %9)::Union{Nothing, Int64} │ %12 = (%11 isa Main.Nothing)::Bool └── goto JuliaLang#8 if not %12 7 ─ goto JuliaLang#9 8 ─ nothing::Nothing 9 ┄ %16 = φ (JuliaLang#7 => %10, JuliaLang#8 => %10)::Bool └── return %16 ``` (note the missing #2 predecessor in phi node %3), which would result in an undefined value at runtime, though in this case LLVM would have taken advantage of that to just return 0: ``` define i8 @julia_f32579_16051(i64, i8) { top: ; @ REPL[1]:15 within `f32579' ret i8 0 } ``` Compare this now to the optimized IR with this patch: ``` 1 ─ goto JuliaLang#3 if not b 2 ─ %2 = Main.nothing::Core.Compiler.Const(nothing, false) 3 ┄ %3 = φ (#2 => true, #1 => false)::Bool │ %4 = φ (#2 => %2, #1 => _2)::Union{Nothing, Int64} │ %5 = (%4 isa Main.Int64)::Bool └── goto JuliaLang#5 if not %5 4 ─ %7 = π (%4, Int64) └── goto JuliaLang#6 5 ─ %9 = π (%4, Nothing) 6 ┄ %10 = φ (JuliaLang#4 => %3, JuliaLang#5 => %3)::Bool │ %11 = φ (JuliaLang#4 => %7, JuliaLang#5 => %9)::Union{Nothing, Int64} │ %12 = (%11 isa Main.Nothing)::Bool └── goto JuliaLang#8 if not %12 7 ─ goto JuliaLang#9 8 ─ nothing::Nothing 9 ┄ %16 = φ (JuliaLang#7 => %10, JuliaLang#8 => %10)::Bool └── return %16 ``` The %3 phi node has its missing entry and the generated LLVM code correctly returns `b`: ``` define i8 @julia_f32579_16112(i64, i8) { top: %2 = and i8 %1, 1 ; @ REPL[1]:15 within `f32579' ret i8 %2 } ```
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Alternatively, we could refactor
runtests.jlto compile each function in it into a native .so and executable, which would make it possible to use Julia's testing infrastructure. Let me know if you'd prefer I do that instead.