------- Additional Comments From dberlin at gcc dot gnu dot org 2005-04-24
15:27 -------
Subject: Re: [4.0/4.1 regression] miscompiled
pointer subtraction broke Linux kernel
> tree-ssa-pre uses force_gimple_operand with SIMPLE==false, so if
> expr is already a valid rhs, force_gimple_operand should do nothing.
> If it does, I consider that to be a bug.
>
> Your patch to use unshare_expr is IMHO unnecessarily expensive.
>
>
>
Here's the gdb trace, btw:
Breakpoint 3, create_expression_by_pieces (block=0x400c4870,
expr=0x88403f0, stmts=0x400c6510) at tree-ssa-pre.c:1322
expr came from a node in ANTIC_IN, we *can't* modify it, because it's
not a copy.
(gdb) p debug_generic_stmt (expr)
(charD.3 *) &0B->typeD.1681;
...
1388 newexpr = force_gimple_operand (folded,
(gdb) n
1390 if (forced_stmts)
(gdb) p debug_generic_stmt (expr)
(charD.3 *) &D.1708_3->typeD.1681;
tada!
This is the reason we have to unshare it before passing it in.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21173
