Re: Rationale for an old TRUNCATE patch

[Adam Nemet <anemet@caviumnetworks.com>]

Jeff Law writes:
> Ian Lance Taylor wrote:
> > Adam Nemet <anemet@caviumnetworks.com> writes:
> >
> >   
> >> I am trying to understand the checkin by Jeff Law from about 11 years ago:
> >>
> >>    r19204 | law | 1998-04-14 01:04:21 -0700 (Tue, 14 Apr 1998) | 4 lines
> >>    
> >>    
> >>            * combine.c (simplify_rtx, case TRUNCATE): Respect value of
> >>            TRULY_NOOP_TRUNCATION.
> >>    
> >>    
> >>    Index: combine.c
> >>    ===================================================================
> >>    --- combine.c	(revision 19018)
> >>    +++ combine.c	(revision 19204)
> >>    @@ -3736,7 +3736,9 @@ simplify_rtx (x, op0_mode, last, in_dest
> >>           if (GET_MODE_CLASS (mode) == MODE_PARTIAL_INT)
> >>     	break;
> >>     
> >>    -      if (GET_MODE_BITSIZE (mode) <= HOST_BITS_PER_WIDE_INT)
> >>    +      if (GET_MODE_BITSIZE (mode) <= HOST_BITS_PER_WIDE_INT
> >>    +	  && TRULY_NOOP_TRUNCATION (GET_MODE_BITSIZE (mode),
> >>    +				    GET_MODE_BITSIZE (GET_MODE (XEXP (x, 0)))))
> >>     	SUBST (XEXP (x, 0),
> >>     	       force_to_mode (XEXP (x, 0), GET_MODE (XEXP (x, 0)),
> >>     			      GET_MODE_MASK (mode), NULL_RTX, 0));
> >>
> >> This optimization simplifies the input to a truncate by only computing bits
> >> that won't be eliminated by the truncation.  Normally these are the bits in
> >> the output mode mask.  Note that the optimization does not change the truncate
> >> into a low-part subreg, which would pretty automatically warrant the
> >> TRULY_NOOP_TRUNCATION check.
> >>     
> >
> > I agree that this patch looks wrong in todays compiler.  There should be
> > no need to call TRULY_NOOP_TRUNCATION if you are in a TRUNCATE anyhow.
> >   
> Based on reviewing my old notes, we do have to ensure that combine 
> doesn't replace a TRUNCATE with a SUBREG as that can result in having a 
> 32bit value that isn't sign-extended, which clearly causes MIPS64 ports 
> grief.

Thanks for the long information in your other reply.

As I said in the original email, we are not turning a TRUNCATE into a SUBREG
in this transformation.  We're just simplifying the input expression to
truncate with the knowledge that only the truncated mode bits are relevant
from the input.  At the end we are still left with a truncate expression but
possibly with a simpler operand.

Adam