[PATCH] Fix PRs 52080, 52097 and 48124, rewrite bitfield expansion, enable the C++ memory model wrt bitfields everywhere

[Richard Guenther]

On Fri, 30 Mar 2012, Eric Botcazou wrote:

> > > May I apply the patch I posted?  It boostrapped/regtested fine on
> > > x86-64/Linux.
> >
> > Yes.
> 
> Thanks.  Unfortunately, while this was the last identified problem on x86, 
> another issue is visible on x86-64 as a miscompilation of XML/Ada at -O0.
> Reduced testcase attached:
> 
>  gnat.dg/pack18.adb
>  gnat.dg/pack18_pkg.ads
> 
> The executable segfaults because it attempts a read at 0x2000000000000000.
> 
> The scenario is a follows: Rec is packed record so its fields are bit fields, N 
> being at bit offset 129.  The representative is at offset 0.
> 
> get_bit_range is invoked on N with a bitpos of 1, because there is variable 
> offset and its DECL_FIELD_OFFFSET is added to it instead of bitpos.  Hence
> 
>   bitpos - bitoffset
> 
> is (unsigned HOST_WIDE_INT) -128.  This value enters unchanged the new code in 
> store_bit_field and the division:
> 
>      offset = bitregion_start / BITS_PER_UNIT;
> 
> yields the problematic big number.
> 
> It would therefore appear that bitstart and bitend need to be signed offsets, 
> at least until they are adjusted by store_bit_field.

Hmm, yeah.  Or something like

Index: expr.c
===================================================================
--- expr.c      (revision 186082)
+++ expr.c      (working copy)
@@ -4490,8 +4490,8 @@ get_bit_range (unsigned HOST_WIDE_INT *b
   bitoffset += (tree_low_cst (DECL_FIELD_BIT_OFFSET (field), 1)
                - tree_low_cst (DECL_FIELD_BIT_OFFSET (repr), 1));
 
-  *bitstart = bitpos - bitoffset;
-  *bitend = *bitstart + tree_low_cst (DECL_SIZE (repr), 1) - 1;
+  *bitstart = bitpos < (HOST_WIDE_INT) bitoffset ? 0 : bitpos - 
bitoffset;
+  *bitend = bitpos + tree_low_cst (DECL_SIZE (repr), 1) - bitoffset - 1;
 }
 
 /* Returns true if the MEM_REF REF refers to an object that does not

which conservatively truncates the bitrange.

Richard.