JuliaLang/julia

Bug in heapsnapshot generation for arrays of inlined objects

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#61396 opened on Mar 23, 2026

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 (5 comments) (0 reactions) (0 assignees)Julia (48,709 stars) (5,773 forks)batch import
GCgood first issueobservability

Description

Given an array like this:

julia> m = Memory{Pair{Any,Any}}([Pair{Any,Any}(1, 2), Pair{Any,Any}(3, 4)])
2-element Memory{Pair{Any, Any}}:
 1 => 2
 3 => 4

We will end up generating multiple lines with duplicate keys, which is not correct:

Src, Idx, Dst
  m,   1,   1
  m,   1,   2
  m,   2,   3
  m,   2,   4

Probably the right fix is to generate something like this instead:

Src,  Name, Dst
  m, [1].x,   1
  m, [1].y,   2
  m, [2].x,   3
  m, [2].y,   4

This would be fixed in the C++ code for generating the heapsnapshot, https://github.com/JuliaLang/julia/blob/8493b13f44455a5f16f1f830c223845a3dacf2c8/src/gc-heap-snapshot.cpp#L533-L537, and in the GC code that marks those edges, e.g. here: https://github.com/JuliaLang/julia/blob/8493b13f44455a5f16f1f830c223845a3dacf2c8/src/gc-stock.c#L2865-L2869

Contributor guide

Tech stack
cppc
Domain
toolingobservability
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic C++ knowledgeFamiliarity with Julia runtime
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Review the code in gc heap snapshot.cpp (lines 533-537) and gc stock.c (lines 2865-2869) to understand the current edge naming for arrays. Implement the suggested change to use indexed keys like [1].x and [1].y instead of duplicate integer ids. Test the change with the provided example.