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Fix Julia 1.13 segfault on undef/partially-initialized struct return (#3087) #3264

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Fix Julia 1.13 segfault on undef/partially-initialized struct return (#3087) #3264

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25 changes: 24 additions & 1 deletion src/llvm/transforms.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -1901,6 +1901,11 @@ function remove_readonly_unused_calls!(fn::LLVM.Function, next::Set{String})
end

function propagate_returned!(mod::LLVM.Module)
sretkind = LLVM.kind(if LLVM.version().major >= 12
LLVM.TypeAttribute("sret", LLVM.Int32Type())
else
LLVM.EnumAttribute("sret")
end)
globs = LLVM.GlobalVariable[]
for g in globals(mod)
if linkage(g) == LLVM.API.LLVMInternalLinkage ||
Expand Down Expand Up @@ -2133,6 +2138,24 @@ function propagate_returned!(mod::LLVM.Module)
illegalUse = true
break
end
# If every caller passes undef/poison for this argument (val === nothing),
# the interprocedural const-prop below would replace the argument with undef
# inside the body. That is only sound for arguments used purely as values.
# For an sret-like output pointer (`sret`/`enzyme_sret`/`enzyme_sret_v`) the
# body *stores into* the argument, so replacing it with undef turns those
# stores into stores-to-undef. Downstream InstCombine/SROA then treat that as
# UB and collapse the whole function to `unreachable`/`poison`, miscompiling
# the augmented forward pass (segfault, issue #3087). Skip the replacement for
# such write-only output pointers when there is no concrete value to propagate.
if !illegalUse && val === nothing && any(
(
kind(attr) == sretkind ||
kind(attr) == kind(StringAttribute("enzyme_sret")) ||
kind(attr) == kind(StringAttribute("enzyme_sret_v"))
) for attr in collect(parameter_attributes(fn, i))
)
illegalUse = true
end
if !illegalUse
if val === nothing
val = LLVM.UndefValue(value_type(arg))
Expand Down Expand Up @@ -2332,7 +2355,7 @@ function delete_writes_into_removed_args(fn::LLVM.Function, toremove::Vector{Int
cur, cval = pop!(todorep)
if isa(cur, LLVM.StoreInst)
if operands(cur)[2] == cval
LLVM.API.LLVMInstructionEraseFromParent(nphi)

@vchuravy vchuravy Jun 29, 2026

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There is no nphi defined so that looks real

@wsmoses wsmoses Jul 5, 2026

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yeah so this part should be extracted to a separate pr and is useful, other parts more unclear

LLVM.API.LLVMInstructionEraseFromParent(cur)
continue
end
end
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