[x86] [sysv] returned structures more than register width should be passed in rdi

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#8,426 opened on Aug 18, 2026 13 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 4/5
Estimated time: 3-5 days
Newbie friendliness: 50/100

Issue type: Bug
Clarity: Mostly clear
Activity status: Quiet
Tech stack: cpp
Domain: compilers, reverse-engineering

Research direction

Start by locating the x86 SysV ABI handling for returned structures and trace how the method signature is represented. Compare the behavior for structures at and above register width, then verify that oversized returns are represented with the return reference in rdi before this; done means the displayed signature matches the requested form.

Written by the indexing model from the issue text.

Description

Core: Calling Convention

Struct `Class::method`(class Class* `this`)

where sizeof(Struct) > REG_SIZE must be handled as

void `Class::method`(Struct& ret, class Class* `this`)

Dominant language: C++
Stars: 1.3k
Forks: 298
Avg merge: 4d 13h
Merged PRs (30d): 20

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