> If the
> (and:SI (subreg:SI (reg:HI xxx) 0) (const_int 0x84c))
> to
> (subreg:SI (and:HI (reg:HI xxx) (const_int 0x84c)) 0)
> transformation is kosher for WORD_REGISTER_OPERATIONS, then I guess the
> invalid operation is then in
> simplify_context::simplify_binary_operation_1
> case AND:
> ...
> if (HWI_COMPUTABLE_MODE_P (mode))
> {
> HOST_WIDE_INT nzop0 = nonzero_bits (trueop0, mode);
> HOST_WIDE_INT nzop1;
> if (CONST_INT_P (trueop1))
> {
> HOST_WIDE_INT val1 = INTVAL (trueop1);
> /* If we are turning off bits already known off in OP0, we
> need not do an AND. */
> if ((nzop0 & ~val1) == 0)
> return op0;
> }
> We have there op0==trueop0 (reg:HI 175) and op1==trueop1 (const_int 2124
> [0x84c]).
> We then for integral? modes smaller than word_mode would then need to
> actually check nonzero_bits in the word_mode (on paradoxical subreg of
> trueop0?). If INTVAL (trueop1) is >= 0, then I think just doing
> nonzero_bits in the wider mode would be all we need (although the
> subsequent (nzop1 & nzop0) == 0 case probably wants to have the current
> nonzero_bits calls), not really sure what for WORD_REGISTER_OPERATIONS
> means AND with a constant which has the most significant bit set for the
> upper bits.
Yes, I agree that there is a tension between this AND case and the swapping
done in the combiner for WORD_REGISTER_OPERATIONS. I also agree that it would
make sense do call nonzero_bits on word_mode instead of mode here in this case
because AND is a word_register_operation_p.
> So, perhaps just in the return op0; case add further code for
> WORD_REGISTER_OPERATIONS and sub-word modes which will call nonzero_bits
> again for the word mode and decide if it is still safe.
Does it work to just replace mode by word_mode in the calls to nonzero_bits?
> That patch doesn't change anything at all on the testcase, it is still
> miscompiled.
OK, too bad, thanks for trying it!
--
Eric Botcazou
