On Thu, May 02, 2013 at 06:53:31PM +0100, Richard Sandiford wrote:
> Jakub Jelinek <ja...@redhat.com> writes:
> > When working on PR57130, I've wondered why we don't simplify it much earlier
> > and end up with creating such weirdness.
> >
> > The following patch fixes that, by using nonzero_bits to see if all the low
> > bits must be zero and in that case just return zero.
> >
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> >
> > 2013-05-02 Jakub Jelinek <ja...@redhat.com>
> >
> > * simplify-rtx.c (simplify_truncation): If nonzero_bits
> > on op shows all bits zero in mode, return zero.
>
> I'm probably wrong, but just in case: it looks like this might
> fall foul of:
>
> /* Try to simplify a MODE truncation of OP, which has OP_MODE.
> Only handle cases where the truncated value is inherently an rvalue.
>
> If op is a register, it might be being used as an lvalue instead of an rvalue.
> We don't want to simplify (subreg (reg ..)) to (const_int 0) in that case.
>
> It should be OK to put this under the TRUNCATE case in
> simplify_unary_operation_1, but I don't know if that'd be good
> enough for your testcase.
I understand, if simplify_subreg would be called on LHS, that would be a bad
transformation. Unfortunately, simplify_rtx isn't called on the testcase
and there isn't a TRUNCATE, just a SUBREG, so simplify_unary_operation_1
wouldn't be the right spot for it anyway.
What about this? Is combine_simplify_rtx only called on rvalues?
This improves the *.combine dump on the testcase too.
Bootstrapped/regtested on x86_64-linux and i686-linux.
2013-05-03 Jakub Jelinek <ja...@redhat.com>
* combine.c (combine_simplify_rtx) <case SUBREG>: If nonzero_bits
on op shows all bits zero in mode of a lowpart subreg, return zero.
--- gcc/combine.c.jj 2013-05-03 14:55:32.000000000 +0200
+++ gcc/combine.c 2013-05-03 15:46:31.923323029 +0200
@@ -5417,6 +5417,16 @@ combine_simplify_rtx (rtx x, enum machin
SUBREG_BYTE (x));
if (temp)
return temp;
+
+ /* If op is known to have all lower bits zero, the result is zero. */
+ if (SCALAR_INT_MODE_P (mode)
+ && SCALAR_INT_MODE_P (op0_mode)
+ && GET_MODE_PRECISION (mode) < GET_MODE_PRECISION (op0_mode)
+ && subreg_lowpart_offset (mode, op0_mode) == SUBREG_BYTE (x)
+ && HWI_COMPUTABLE_MODE_P (op0_mode)
+ && (nonzero_bits (SUBREG_REG (x), op0_mode)
+ & GET_MODE_MASK (mode)) == 0)
+ return CONST0_RTX (mode);
}
/* Don't change the mode of the MEM if that would change the meaning
Jakub
