Re: [PATCH][committed]middle-end intialize regnum in store_bit_field_1

[Richard Sandiford via Gcc-patches]

Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> On Tue, 30 Aug 2022, Tamar Christina wrote:
>
>> Hi All,
>> 
>> This initializes regnum to 0 for when undefined_p.
>> 0 is the right default as it's supposed to get the lowpart
>> when undefined.
>> 
>> Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
>> 
>> Ok for master?
>
> OK.

I'm not sure this is the right fix.  We were still supposed to use the
correct byte offset in the undefined case.

Is the attached OK?  Tested on aarch64-linux-gnu.

Thanks,
Richard

----------------------

store_bit_field_1 tries to convert a field assignment into a subreg
assignment.  Normally it must check that the field occupies a full
word (or more specifically, a full REGMODE_NATURAL_SIZE chunk),
so that writing to the subreg doesn't clobber any other fields.
But it can skip that check if the structure is known to be in
an undefined state.

The idea was that, in the undefined case, we could rely on
simplify_gen_subreg to do the check for a valid subreg, rather
than having to repeat the required endianness logic in the caller.

Before the addition of the undefined case, the code could use
regnum * regsize to get the byte offset, where regnum came from
checking that the start was word-aligned.  In the undefined case
we need to calculate the byte offset explicitly.

gcc/
        * expmed.cc (store_bit_field_1): Fix byte offset calculation
        for undefined structures.
---
 gcc/expmed.cc | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/gcc/expmed.cc b/gcc/expmed.cc
index 8d7418be418..6c02c3bb850 100644
--- a/gcc/expmed.cc
+++ b/gcc/expmed.cc
@@ -794,7 +794,7 @@ store_bit_field_1 (rtx str_rtx, poly_uint64 bitsize, 
poly_uint64 bitnum,
         words or to cope with mode punning between equal-sized modes.
         In the latter case, use subreg on the rhs side, not lhs.  */
       rtx sub;
-      HOST_WIDE_INT regnum;
+      poly_uint64 bytenum;
       poly_uint64 regsize = REGMODE_NATURAL_SIZE (GET_MODE (op0));
       if (known_eq (bitnum, 0U)
          && known_eq (bitsize, GET_MODE_BITSIZE (GET_MODE (op0))))
@@ -808,13 +808,13 @@ store_bit_field_1 (rtx str_rtx, poly_uint64 bitsize, 
poly_uint64 bitnum,
              return true;
            }
        }
-      else if (((constant_multiple_p (bitnum, regsize * BITS_PER_UNIT, &regnum)
-                && multiple_p (bitsize, regsize * BITS_PER_UNIT))
-               || undefined_p)
+      else if (multiple_p (bitnum, BITS_PER_UNIT, &bytenum)
+              && (undefined_p
+                  || (multiple_p (bitnum, regsize * BITS_PER_UNIT)
+                      && multiple_p (bitsize, regsize * BITS_PER_UNIT)))
               && known_ge (GET_MODE_BITSIZE (GET_MODE (op0)), bitsize))
        {
-         sub = simplify_gen_subreg (fieldmode, op0, GET_MODE (op0),
-                                    regnum * regsize);
+         sub = simplify_gen_subreg (fieldmode, op0, GET_MODE (op0), bytenum);
          if (sub)
            {
              if (reverse)
-- 
2.25.1