Adam Nemet <ane...@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. > Specifically I am curious if this was prompted by MIPS (or SH) or maybe some > other target that have a notion of truncate different from MIPS. It pretty much has to have been MIPS or SH since I don't think any other target back then defined TRULY_NOOP_TRUNCATION as anything other than 1. Ian
