Re: Rationale for an old TRUNCATE patch

[Adam Nemet <anemet at caviumnetworks dot com>]

Ian Lance Taylor writes:
> truncate has a machine independent meaning.

Yes, I guess with your definition below it does.  It's interesting though that
Jim had said the opposite in the excerpts posted by Jeff:

  And a later message from Jim:
  
  Truncate converts a value from a larger to a smaller mode in a machine
  dependent way.
  
  A subreg just chops off the high bits.  A truncate does not.  The name might
  be a little confusing, but the whole point of truncate is to have something
  that operates differently than a subreg.
  
  Combine is clearly making an invalid transformation.
 
> Yes.  The bits in Nmode's mask are determined by the truncate.  The
> other bits are don't-care.  If the result of the truncate happens to
> wind up in a register, then in some cases PROMOTE_MODE will apply.

And IIUC this don't-care nature of the other bits that allows backends to
define the upper bits.  For example to have sign-bit copies there in registers
to enforce the MIPS64 SI mode representation.  And treating the don't care
bits outside SI mode in this way is true for any other SI-mode operations
performed on registers not just truncate, right?  Hmm, nice.

Adam