hcat broken in Enzyme?

[StephenVavasis]

See the MWE below. The first test run simulates hcat by storing entries in a matrix in consecutive column numbers, and this works. The second test run actually uses hcat and breaks Enzyme.

Source code:

module enzyme_test

using Enzyme

function use_hcat(x)
    w = zeros(length(x), length(x))
    for i = 1 : length(x)
        w[:,i] .= x
    end
    p = hcat(w, x)
    return sum(p)
end

function dont_use_hcat(x)
    w = zeros(length(x), length(x))
    for i = 1 : length(x)
        w[:,i] .= x
    end
    p = zeros(length(x), length(x) + 1)
    p[:, 1:length(x)] .= w
    p[:, length(x) + 1] .= x
    return sum(p)
end

function deriv1(use_hcat_p)
    x = [1.0, 2.0, 4.0]
    dx = [1.0, 0.0, 0.0]
    if use_hcat_p
        println("function val using hcat = ", use_hcat(x))
        s = autodiff(Forward,
                     use_hcat,
                     Duplicated(x, dx))
    else
        println("function val not using hcat = ", dont_use_hcat(x))
        s = autodiff(Forward,
                     dont_use_hcat,
                     Duplicated(x, dx))
    end
    return s[1]
end
end

Test runs:

julia> include("enzymetest.jl")
WARNING: replacing module enzyme_test.
Main.enzyme_test

julia> enzyme_test.deriv1(false)
function val not using hcat = 28.0
4.0

julia> enzyme_test.deriv1(true)
function val using hcat = 28.0
ERROR: Enzyme compilation failed due to an internal error.
 Please open an issue with the code to reproduce and full error log on github.com/EnzymeAD/Enzyme.jl
 To toggle more information for debugging (needed for bug reports), set Enzyme.Compiler.VERBOSE_ERRORS[] = true (default false)

Stacktrace:
 [1] length
   @ .\essentials.jl:12
 [2] _typed_hcat
   @ .\abstractarray.jl:1702

Stacktrace:
  [1] julia_error(msg::String, val::Ptr{…}, errtype::Enzyme.API.ErrorType, data::Ptr{…}, data2::Ptr{…}, B::Ptr{…})
    @ Enzyme.Compiler C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\errors.jl:461
  [2] julia_error(cstr::Cstring, val::Ptr{…}, errtype::Enzyme.API.ErrorType, data::Ptr{…}, data2::Ptr{…}, B::Ptr{…})
    @ Enzyme.Compiler C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\errors.jl:299
  [3] EnzymeCreateForwardDiff(logic::Enzyme.Logic, todiff::LLVM.Function, retType::Enzyme.API.CDIFFE_TYPE, constant_args::Vector{…}, TA::Enzyme.TypeAnalysis, returnValue::Bool, mode::Enzyme.API.CDerivativeMode, runtimeActivity::Bool, strongZero::Bool, width::Int64, additionalArg::Ptr{…}, typeInfo::Enzyme.FnTypeInfo, uncacheable_args::Vector{…})
    @ Enzyme.API C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\api.jl:342
  [4] enzyme!(job::GPUCompiler.CompilerJob{…}, mod::LLVM.Module, primalf::LLVM.Function, TT::Type, mode::Enzyme.API.CDerivativeMode, width::Int64, parallel::Bool, actualRetType::Type, wrap::Bool, modifiedBetween::NTuple{…} where N, returnPrimal::Bool, expectedTapeType::Type, loweredArgs::Set{…}, boxedArgs::Set{…})
    @ Enzyme.Compiler C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\compiler.jl:2514
  [5] compile_unhooked(output::Symbol, job::GPUCompiler.CompilerJob{…})
    @ Enzyme.Compiler C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\compiler.jl:4834
  [6] compile(target::Symbol, job::GPUCompiler.CompilerJob; kwargs::@Kwargs{})
    @ GPUCompiler C:\Users\vavasis\.julia\packages\GPUCompiler\Ecaql\src\driver.jl:67
  [7] compile
    @ C:\Users\vavasis\.julia\packages\GPUCompiler\Ecaql\src\driver.jl:55 [inlined]
  [8] _thunk(job::GPUCompiler.CompilerJob{…}, postopt::Bool)
    @ Enzyme.Compiler C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\compiler.jl:5697
  [9] _thunk
    @ C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\compiler.jl:5695 [inlined]
 [10] cached_compilation
    @ C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\compiler.jl:5749 [inlined]
 [11] thunkbase(mi::Core.MethodInstance, World::UInt64, FA::Type{…}, A::Type{…}, TT::Type, Mode::Enzyme.API.CDerivativeMode, width::Int64, ModifiedBetween::NTuple{…} where N, ReturnPrimal::Bool, ShadowInit::Bool, ABI::Type, ErrIfFuncWritten::Bool, RuntimeActivity::Bool, StrongZero::Bool, edges::Vector{…})
    @ Enzyme.Compiler C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\compiler.jl:5863
 [12] thunk_generator(world::UInt64, source::Union{…}, FA::Type, A::Type, TT::Type, Mode::Enzyme.API.CDerivativeMode, Width::Int64, ModifiedBetween::NTuple{…} where N, ReturnPrimal::Bool, ShadowInit::Bool, ABI::Type, ErrIfFuncWritten::Bool, RuntimeActivity::Bool, StrongZero::Bool, self::Any, fakeworld::Any, fa::Type, a::Type, tt::Type, mode::Type, width::Type, modifiedbetween::Type, returnprimal::Type, shadowinit::Type, abi::Type, erriffuncwritten::Type, runtimeactivity::Type, strongzero::Type)
    @ Enzyme.Compiler C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\compiler.jl:6056
 [13] autodiff
    @ C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\Enzyme.jl:654 [inlined]
 [14] autodiff
    @ C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\Enzyme.jl:558 [inlined]
 [15] autodiff
    @ C:\Users\vavasis\.julia\packages\Enzyme\LMVya\src\Enzyme.jl:530 [inlined]
 [16] deriv1(use_hcat_p::Bool)
    @ Main.enzyme_test c:\Users\vavasis\OneDrive - University of Waterloo\Katerina\myosin\enzymetest.jl:30
 [17] top-level scope
    @ REPL[161]:1
Some type information was truncated. Use `show(err)` to see complete types.

[StephenVavasis]

I forgot to say: Julia version is 1.11 and Enzyme version is 0.13.66.