Re: supression of 'matching constraint does not allow a register' warning

[Daniel Jacobowitz <drow at false dot org>]

On Fri, Apr 30, 2004 at 11:29:07PM -0700, Jim Wilson wrote:
> Daniel Jacobowitz wrote:
> >Er, except I'm not sure that's right; something was said about operands
> >unreferenced in the asm text being a bad idea.
> 
> The optimizer never looks at the asm text, so whether an operand is used 
> has no effect on how the compiler will optimize.  The asm text is only 
> used when we emit assembly to the .s file.
> 
> If this really matters, one could always add "; %2" assuming ; is the 
> assembler comment character, but I doubt that will do anything useful.
> 
> The only thing I can think of is that if an operand has side effects, 
> and the compiler makes assuptions about those side-effects, then whether 
> an operand is used could make a difference.  However, there are so many 
> other issues here, the operand could be used multiple times, the operand 
> might not actually be used by the instruction, or we might be using the 
> operand in a special way.  The optimizer really can't make any 
> assumptions about what is happening here, so it shouldn't make any 
> difference whether or not the operand is used.
> 
> The only thing I can immediately think of that has side-effects is 
> auto-inc addresses, and you could avoid them if necessary by using an o 
> constraint instead of a m constraint.
> 
> In the absence of an actual bug report, I don't think there is anything 
> wrong with unreferenced operands.

Thanks - that does make sense to me.  I thought I remembered Richard
saying something to the contrary, but he denies it, so I have no idea
what I was thinking!

-- 
Daniel Jacobowitz