Re: Q for inline arm assembly: 'M' constraint?

[Doug Evans <dje at transmeta dot com>]

Bryce Schober writes:
 > Richard Earnshaw wrote:
 > 
 > > inline int fixp_mul_32s_nX( int a, int b, char n ) {
 > > 	int res, tmp;
 > > 	int p = 32-n;
 > > 	__asm__ __volatile__ (
 > > 		"smull	%0, %1, %2, %3			\n\t"
 > > 		"movs	%0, %0, lsr %4			\n\t"
 > > 		"adc	%1, %0, %1, lsl %5		\n\t"
 > > 		: "=&r" (res), "=&r" (tmp)
 > > 		: "r" (a), "r" (b), "rM" (n), "rM" (p)
 > > 	);
 > > 	return res;
 > > }
 > > 
 > > This allows the compiler to use a register or a constant in the range 
 > > 0-31.  But it's quite possible that you will always get a register passed 
 > > in this case.
 > 
 > Ok, I've tried this, and you're right, I always get a register, but why? 
 >   Is there some way I can define this function so as to only allow 
 > constants for "n"?  What further baffles me:
 > 
 > inline int fixp_mul_32s_nX( int a, int b ) {
 > 	int res, tmp;
 > 	const char n = 8;
 > 	const char m = 32 - n;
 > 	__asm__ __volatile__ (
 > 		"smull	%0, %1, %2, %3				\n\t"
 > 		"movs	%0, %0, lsr %4				\n\t"
 > 		"adc	%1, %0, %1, lsl %5			\n\t"
 > 		: "=&r" (tmp), "=&r" (res)
 > 		: "r" (a), "r" (b), "M" (n), "M" (m)
 > 	);
 > 	return res;
 > }
 > 
 > This function has the same problem.  Is the compiler that stupid?  Do I 
 > really have to put the constant directly in the asm like "M" (24)?  Is 
 > it just me, or does the "M" constraint seem useless?  Am I expecting too 
 > much?  Is there some other way to do what I'm trying to?

Experiment:
Try defining fixp_mul_32s_nX as a macro instead of an inline,
and don't put values intended to be constants in temps (e.g. `p').
[gcc's "statement expressions" come in handy here]

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