int main()
{
asm volatile (
"clr r25\n"
"ldi r24, %0\n"
:: "M" ((unsigned char)137)
);
}asm.c:4:5: warning: asm operand 0 probably doesn't match constraints
asm volatile (
^
asm.c:4:5: error: impossible constraint in 'asm'
Tested with avr-gcc 5.4.0 and 8.4.0. If I make the constant less than 128, there is no error. The docs for the "M" constraint say:
| 8-bit integer constant | 0 to 255 |
https://www.nongnu.org/avr-libc/...
There are a few asm operations that only allow 7 bit constants. Not sure if that is a factor, here.
Jim
I'd say bug. I suspect -128 would work.
"i" should work as a constraint.
'Tis part of the generic portion,
so I'd expect it to be better tested and
complaints about it more likely to be heard.
That's a bug for sure, if you cast to any form of char (signed, unsigned or unqualified) this error occurs. If you cast to int types there are no errors.
"n" works for me. From gas manual:
Using "n" instead of "M":
000000b8 <main>: b8: 99 27 eor r25, r25 ba: 89 e8 ldi r24, 0x89 ; 137 bc: 90 e0 ldi r25, 0x00 ; 0 be: 80 e0 ldi r24, 0x00 ; 0 c0: 08 95 ret
That's a bug for sure, if you cast to any form of char (signed, unsigned or unqualified) this error occurs. If you cast to int types there are no errors.
Thanks. I never would've thought of trying an int cast. That works perfectly, and the 0-255 range constraint gets properly enforced. If I with 257 I get an error as expedted:
asm.c:3:5: warning: asm operand 0 probably doesn't match constraints
asm volatile (
^~~
asm.c:3:5: error: impossible constraint in 'asm'
I'd say bug. I suspect -128 would work.
"i" should work as a constraint.
It's a workaround, but if the constant evaluates to >255, the error isn't caught until the assembler stage, making it hard to track back to the C source:
# avr-gcc -mmcu=attiny85 asm.c
C:\msys64\tmp\ccVmqeIB.s: Assembler messages:
C:\msys64\tmp\ccVmqeIB.s:22: Warning: constant out of 8-bit range: 256
C:\msys64\tmp\ccVmqeIB.s:22: Error: operand out of range: 256
"n" works for me. From gas manual:
Using "n" instead of "M":
000000b8 <main>: b8: 99 27 eor r25, r25 ba: 89 e8 ldi r24, 0x89 ; 137 bc: 90 e0 ldi r25, 0x00 ; 0 be: 80 e0 ldi r24, 0x00 ; 0 c0: 08 95 ret
I like #4 better. With "M" and an int16 cast, when the constant evaluates to >255 I get a constraint error as I showed in #6. With "n" and an int16 cast, I don't get the error until the assembler stage, like in #7. If I use "n" and an char cast, although it works as desired with values up to 255, if the expression evaluates to >255, the char cast wraps, and I don't get any error indicating the constant was larger than 8 bits.
