Fix handling of negative bitpos in expand_debug_expr

[Richard Sandiford]

expand_debug_expr handled negative bit positions using:

            else if (bitpos < 0)
              {
                HOST_WIDE_INT units
                  = (-bitpos + BITS_PER_UNIT - 1) / BITS_PER_UNIT;
                op0 = adjust_address_nv (op0, mode1, units);
                bitpos += units * BITS_PER_UNIT;
              }

Here "units" is the negative of the (negative) byte offset, so I think
we should be offsetting OP0 by -units instead.  E.g. a bitpos of -17
would give units==3, so this code would move OP0 up by 3 bytes and set
bitpos to 7, giving a total bitpos of 31.

Just noticed by inspection.  An assert triggered for:

    gcc.target/i386/mpx/bitfields-1-lbv.c
    gcc.target/i386/mpx/field-addr-7-lbv.c
    gcc.target/i386/mpx/reference-3-lbv.cpp
    gcc.target/i386/mpx/reference-4-lbv.cpp

at -m32 but otherwise this case doesn't seem to trigger during a
bootstrap and regtest.

Tested on x86_64-linux-gnu.  OK to install?

Thanks,
Richard


gcc/
        * cfgexpand.c (expand_debug_expr): Fix address offset for negative
        bitpos.

diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index 21f21c9..77a1964 100644
--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -4473,7 +4473,7 @@ expand_debug_expr (tree exp)
              {
                HOST_WIDE_INT units
                  = (-bitpos + BITS_PER_UNIT - 1) / BITS_PER_UNIT;
-               op0 = adjust_address_nv (op0, mode1, units);
+               op0 = adjust_address_nv (op0, mode1, -units);
                bitpos += units * BITS_PER_UNIT;
              }
            else if (bitpos == 0 && bitsize == GET_MODE_BITSIZE (mode))