Landing: f84dfd830596

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Project / Subsystem

gcc / avr

Date

2026-07-03

Author

Georg-Johann Lay

Commit

f84dfd8305963f124c624570bb6eb0c84149f67a

Source

github

Perf win

Yes

Breaking

No

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All attributes

project: gcc
subsystem: avr
patch_id: —
commit_hash: f84dfd8305963f124c624570bb6eb0c84149f67a
source_type: github
headline: AVR: Adding +/-1 to a lower reg doesn't need a scratch.
tldr: AVR target code generation for adding/subtracting 1 is optimized to avoid using a scratch register.
author: Georg-Johann Lay
outcome: committed
performance_win: true
breaking_change: false
series_id: —
series_parts: []
tags: • avr
• code-generation
• optimization
• register-pressure
discussion_id_link: —
bugzilla_pr: —
url: https://github.com/gcc-mirror/gcc/commit/f84dfd8305963f124c624570bb6eb0c84149f67a
date: 2026-07-03T00:00:00.000Z

The GCC AVR backend has been optimized to eliminate the need for a scratch register when adding or subtracting 1 from lower-numbered registers. Previously, such operations required a temporary register, increasing register pressure. The new implementation uses sequences of sec (set carry flag) with adc (add with carry) or sbc (subtract with carry) instructions, which achieve the +/-1 operation without spilling to a scratch register, thus reducing register pressure while maintaining code size.