Q for inline arm assembly: 'M' constraint?

[Bryce Schober <bryceman at dpzone dot com>]

Ok guys, I'm trying to work up some fixed-point multiplication routines 
optimized in assembly for arm7.  Here are two different functions.

The first does 16.16 multiply with 64-bit intermediate precision and the 
shifting / rounding for a 16.16 result.

[code]
inline int fixp_mul_32s_n16( int a, int b ) {
	int res, tmp;
	__asm__ __volatile__ (
		"smull	%0, %1, %2, %3			\n\t"
		"movs	%0, %0, lsr #16			\n\t"
		"adc	%1, %0, %1, lsl #(32-16)	\n\t"
		: "=&r" (res), "=&r" (tmp)
		: "r" (a), "r" (b)
	);
	return res;
}
[/code]

The other function was supposed to take a const argument for the "n" 
part of the Qm.n fixed-point format.

[code]
inline int fixp_mul_32s_nX( int a, int b, char n ) {
	int res, tmp;
	__asm__ __volatile__ (
		"smull	%0, %1, %2, %3			\n\t"
		"movs	%0, %0, lsr #%4			\n\t"
		"adc	%1, %0, %1, lsl #(32-%4)	\n\t"
		: "=&r" (res), "=&r" (tmp)
		: "r" (a), "r" (b), "M" (n)
	);
	return res;
}
[/code]

But the assembler complains with the below error output:

[console]
arm-elf-gcc -g -c -I ../../include -save-temps -O0 -MMD -MP -MF 
dep/test_math.d -o obj/test_math.o test_math.c
In file included from test_math.c:8:
fixp.h: In function `fixp_mul_32s_nX':
fixp.h:26: warning: asm operand 4 probably doesn't match constraints
fixp.h:26: error: impossible constraint in `asm'
[/console]

I assume that the problem is something to do with the "M" constraint, as 
I can use the first function without issue.  So what is the correct way 
to use the "M" constraint?  Or do I even need the "M" constraint?  Or is 
 there a better way to do this altogether?

Thanks in advance for your help,
- Bryce Schober

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