[gcc r13-2289] expmed: Fix store_bit_field_1 subreg offset

[Richard Sandiford <rsandifo@gcc.gnu.org>]

https://gcc.gnu.org/g:1025025b612d632920fe710bb58d36e4d43f3220

commit r13-2289-g1025025b612d632920fe710bb58d36e4d43f3220
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Tue Aug 30 15:43:48 2022 +0100

    expmed: Fix store_bit_field_1 subreg offset
    
    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.

Diff:
---
 gcc/expmed.cc | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/gcc/expmed.cc b/gcc/expmed.cc
index cdc0adb3892..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 = 0;
+      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)