Re: [RFC] Proposal for tagged_by attribute to describe union discriminators

← All discussions

Project / Subsystem

gcc / gcc

Date

2026-07-18

Proposer

Eric Botcazou <botcazou@adacore.com>

Source type

public_inbox

Consensus

Proposed

Sentiment

—/10

Technical tradeoffs

• Enables better code generation and debugging for discriminated unions at the cost of increased compiler complexity.

View thread →

All attributes

project: gcc
subsystem: gcc
patch_id: —
discussion_id: 2417780.ElGaqSPkdT@fomalhaut
source_type: public_inbox
title: Re: [RFC] Proposal for tagged_by attribute to describe union discriminators
headline: Proposal for tagged_by attribute to describe union discriminators
tldr: Introduce tagged_by attribute for discriminated unions, impacting layout and debug info.
proposer: Eric Botcazou <botcazou@adacore.com>
consensus: Proposed
outcome: proposed
sentiment_score: —
technical_tradeoffs: • Enables better code generation and debugging for discriminated unions at the cost of increased compiler complexity.
series_id: —
series_role: standalone
series_parts: []
tags: • gcc
• attribute
• union
• type system
• debug info
url: https://inbox.sourceware.org/gcc/2417780.ElGaqSPkdT@fomalhaut
bugzilla_url: —
date: 2026-07-18T00:00:00.000Z

Re: [RFC] Proposal for tagged_by attribute to describe union discriminators

Eric Botcazou proposes the ‘tagged_by’ attribute to describe union discriminators in GCC. This attribute is used for driving layout and debug info generation, and it differs from UNION_TYPE by enabling runtime checks and having a dynamic size based on the active member. The predicate for this attribute references the tag via PLACEHOLDER_EXPRs.