VIEW_CONVERT_EXPR slots for strict-align targets (PR 83884)

VIEW_CONVERT_EXPR slots for strict-align targets (PR 83884)

This PR is about a case in which we VIEW_CONVERT a variable-sized
unaligned record:

 <record_type 0x7ffff6d92888 check_displace_generation__T245b sizes-gimplified type_7 BLK
    size <var_decl 0x7ffff6846510 D.3499 ...>
    unit-size <var_decl 0x7ffff68465a0 D.3500 ...>
    align:8 ...>

to an aligned 32-bit integer.  The strict-alignment handling of
this case creates an aligned temporary slot, moves the operand
into the slot in the operand's original mode, then accesses the
slot in the more-aligned result mode.

Previously the size of the temporary slot was calculated using:

                  HOST_WIDE_INT temp_size
                    = MAX (int_size_in_bytes (inner_type),
                           (HOST_WIDE_INT) GET_MODE_SIZE (mode));

int_size_in_bytes would return -1 for the variable-length type,
so we'd use the size of the result mode for the slot.  r256152 replaced
int_size_in_bytes with tree_to_poly_uint64, which triggered an ICE.

If op0 has BLKmode we do a block copy of GET_MODE_SIZE (mode) bytes
and then convert the slot to "mode":

                  poly_uint64 mode_size = GET_MODE_SIZE (mode);
                  ...
                  if (GET_MODE (op0) == BLKmode)
                    {
                      rtx size_rtx = gen_int_mode (mode_size, Pmode);
                      emit_block_move (new_with_op0_mode, op0, size_rtx,
                                       (modifier == EXPAND_STACK_PARM
                                        ? BLOCK_OP_CALL_PARM
                                        : BLOCK_OP_NORMAL));
                    }
                  else
                    ...

                  op0 = new_rtx;
                }
            }

          op0 = adjust_address (op0, mode, 0);

so I think in that case just the size of "mode" is enough, even if op0
is a fixed-size type.  For non-BLKmode op0 we first move in op0's mode
and then convert the slot to "mode":

                    emit_move_insn (new_with_op0_mode, op0);

                  op0 = new_rtx;
                }
            }

          op0 = adjust_address (op0, mode, 0);

so I think we want the maximum of the two mode sizes in that case.

2018-01-17  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR middle-end/83884
	* expr.c (expand_expr_real_1): Use the size of GET_MODE (op0)
	rather than the size of inner_type to determine the stack slot size
	when handling VIEW_CONVERT_EXPRs on strict-alignment targets.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256779 138bc75d-0d04-0410-961f-82ee72b054a4