Re: question on inline assembly and long long values

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2005-04-06 at 12:26 -0600, Chris Friesen wrote:
> I want to retrieve the msr (which is 64-bits) on a 970 when running in 
> 32-bit mode.  I have the following bit of code that seems to work, but 
> when looking at the code it always seems to use a suboptimal register 
> for the low word, and then it ends up having to copy it to the right 
> register to create a long long register pair.
> 
> static inline unsigned long long get_msr()
> {
> 	union {
> 		struct {
> 			unsigned long low;
> 			unsigned long high;
> 		} words;
> 		unsigned long long val;
> 	} val;
>          asm volatile( \
>                  "mfmsr  %0               \n\t" \
>                  "rldicl %1,%0,32,32       \n\t" \
>                  "rldicl %0,%0,0,32        \n\t" \
>                  : "=r" (val.words.low), "=r" (val.words.high));
> 	return val.val;
> }

Is this in kernel mode ? Why are you doing that ? 

> Using this code, the optimised assembly output of
> 
> 	unsigned long long a = asdf();
> 	unsigned long long b = asdf();
> 
> is
> 
> 	mfmsr  5
> 	rldicl 0,5,32,32
> 	rldicl 5,5,0,32
> 	mr 6,0
> 	mfmsr  7
> 	rldicl 9,7,32,32
> 	rldicl 7,7,0,32
> 	mr 8,9
> 
> I figure it should have been able to use registers 6/8 in the first 
> place, and save the extra moves.
> 
> Is there any way to help gcc optimise this?
> 
> 
> Chris
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-- 
Benjamin Herrenschmidt <benh@kernel.crashing.org>