[patch, ARM] Add mulhi3 expander.

[Richard Earnshaw <rearnsha at arm dot com>]

When we have the DSP multiply extension (armv5e and up for ARM or T2), 
then a 16-bit multiply is best done with smulbb rather than mul.  This 
saves us doing redundant extends and may even be a faster instruction on 
some cores.  Unfortunately, expand does not try this operation by 
default; but it's trivial to add to the back-end.  Since the value 
produced is extended, we represent this by expanding into the widening 
mul version and then subreg-ing the result.

	* arm.md (mulhi3): New expand pattern.

R.

Attachment: mulhi.patch
Description: Text document