First of all, scrap my other message...
There was a debate several months ago on this issue: how much should the
df scanner be a theorem prover with respect to how many bits survive an
operation.
For instance, I could easily add to your list, anding and oring
operations which also preserve bits.
The rules should be simple. Bits of the dest reg survive only if one of
the following is true.
- there is a STRICT_LOW_PART (of a SUBREG)
- there is a ZERO_EXTRACT (not necessarily of a SUBREG!)
- the subreg is part of a multiword subreg
The last point is decided in other parts of the code than the one Roman
is touching. So, Roman's change to df_read_modify_subreg_p is wrong;
other subregs, in particular (subreg:HI (reg:SI 123) 2), should not be
affected. At most, we might want there the more pedantic
return (isize > osize
&& isize > REGMODE_NATURAL_SIZE (GET_MODE (isize));
It seems to me that all is missing is setting DF_REF_PARTIAL for
ZERO_EXTRACT. That is
- if (GET_CODE (dst) == STRICT_LOW_PART)
- dst_in_strict_lowpart = true;
+ if (GET_CODE (dst) == STRICT_LOW_PART
+ || GET_CODE (dst) == ZERO_EXTRACT)
+ flags |= DF_REF_PARTIAL;
Paolo
