avr: inline assembly with lo8()/hi8() cuts symbols longer than 16 characters

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Since Nov 13, 2026.

  • #209 by @Firat17aze — open

Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
55/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
c
Domain
compilers, embedded-iot

Research direction

The issue provides a minimal C example using inline assembly and reproduces the symbol truncation with AVR target flags; start by reading that example and tracing how EmbCC parses or emits inline assembly containing lo8() or hi8(). Check the AVR port context in arch/avr/avr_context.c for the affected long symbols. Done means the example compiles with those symbols intact and the AVR RTOS library can build.

Written by the indexing model from the issue text.

Description

bug

On AVR, an inline-assembly template that uses lo8() or hi8() anywhere has every symbol longer than 16 characters cut to its first 16, and the assembler then refuses it. EmbCC 0.1.0 (macOS aarch64 release), --target=avr -mmcu=atmega328p -Os.

void abcdefghijklmnopqrst(void);
void g(void) { __asm__ __volatile__("ldi r18, lo8(2303)\n\tcall abcdefghijklmnopqrst" ::: "r18"); }

embcc: g.c:2: error: 'abcdefghijklmnop' is not a number, a register or an expression this assembler knows

What changes the result:

Template Result
ldi r18, lo8(2303) then call abcdefghijklmnopqrst error
ldi r18, hi8(2303) then call … error
call … first, then ldi r18, lo8(2303) error
ldi r18, lo8(2303), nop, then jmp … error
ldi r18, 7 then call abcdefghijklmnopqrst compiles
call abcdefghijklmnopqrst alone, with or without operands compiles
a 16-character name after lo8() compiles

So the cut follows the presence of lo8/hi8 in the block, not their position or the operands.

Where EmbLinkRTOS meets it: the AVR port's kernel start, arch/avr/avr_context.c, loads the stack pointer with lo8(%[top])/hi8(%[top]) and then calls emb_arch_switch_to and jumps to embk_thread_launch, both 18 characters. The port's library does not compile, so no Uno image links. Passing the two bytes as separate "i" operands avoids it; the RTOS keeps lo8/hi8, which avr-gcc and Clang accept.

Part of #184 (AVR).

Dominant language
C
Stars
1
Forks
2
Avg merge
20h 55m
Merged PRs (30d)
73

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