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From: Grant Edwards <invalid@invalid.invalid>
Newsgroups: comp.arch.embedded
Subject: Re: gcc arm inline asm: how to output value for .set directive?
Date: Thu, 3 Apr 2025 23:06:25 -0000 (UTC)
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On 2025-04-03, Grant Edwards <invalid@invalid.invalid> wrote:
> How do I convince ARM GCC's extended asm() to emit a value that can be
> used in a .set directive?  Here's a simplified example:

>       asm("\t.set foo_offset, %[off]" : : [off] "i" ( __builtin_offsetof(shm_t, foo) ) : );

That didn't work, because gcc emits #40 instead of 40:

>   35              			.set foo_offset, #40

I finally stumbled across some example code that showed me the
answer. It's not the _constraint_ in the input operand list (the "i"
above) that matters (I had tried all upper/lower ascii letters).

You need a modifier in the _template_ string that references that
input operand:

  asm("\t.set foo_offset, %c[off]" : : [off] "i" ( __builtin_offsetof(shm_t, foo) ) : );
                           
The secret is the 'c' in "%c[off]"

Now that I know what to look for, I found it in the manual

https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Generic-Operand-Modifiers

  6.11.2.8 Generic Operand Modifiers

I had completely missed the difference between a qualifier and a
modifier...