Re: [PATCH v2] PR rtl-optimization/83565: Fix 32-bit rotate on ia64

[Jim Wilson <jimw at sifive dot com>]

On 12/23/2017 04:36 PM, James Clarke wrote:
> 	PR rtl-optimization/83565
> 	* gcc/config/ia64/ia64.md ("*rotrsi3_internal"): Mask out higher 32
> 	bits from the shift result.
> 	("*rotlsi3_internal"): Likewise

This doesn't look right to me.  On ia64, the upper 32-bits of a 32-bit 
value in a 64-bit register are garbage bits.  So there should be no need 
to clear them here after the operation.

Note for instance that lshrsi3 clears the upper 32-bits before shifting 
right, because they are garbage bits.  And note for instance that 
ashlsi3 just shifts left, and doesn't care that it is putting garbage in 
the upper 32-bits.

I think either nonzero bits is broken in the SUBREG case, or ia64 
perhaps should not be setting WORD_REGISTER_OPERATIONS.

Jim