On Tue, Feb 7, 2012 at 10:50 AM, Eric Botcazou <ebotca...@adacore.com> wrote: > Hi, > > this is a regression present on mainline and 4.6 branch, apparently a fallout > of the MEM_REF introduction. get_inner_reference can be called on MEM_REFs > whose base has been shifted to the left > > char *output = buf; > > output += a; > output -= 1; > > output[0]; > > and, since the constant negative offset is merged into the MEM_REF, it will be > returned as the BITPOS by get_inner_reference, which wreaks havoc later in the > bitfield manipulation routines which expect non-negative bit positions. > > Clearly nothing says that the returned BITPOS should be non-negative but, on > the other hand, it de facto was in the pre-MEM_REF world for valid programs > (except maybe in a very specific case in Ada). The attached patch attempts to > patch things up to bring us back to the previous de facto situation. > > Bootstrapped/regtested on x86_64-suse-linux, OK for mainline and 4.6 branch?
Ok. Note that with your patch we can still get negative bitpos for invalid code like char *output = buf; output[-1]; but I suppose we don't need to worry about this case too much (if we do we'd need to adjust the TREE_CODE (offset) == INTEGER_CST case as well). Thanks, Richard. > > 2012-02-07 Eric Botcazou <ebotca...@adacore.com> > > PR middle-end/51994 > * expr.c (get_inner_reference): If there is an offset, add a negative > bit position to it (if any). > > > 2012-02-07 Eric Botcazou <ebotca...@adacore.com> > > * gcc.c-torture/execute/20120207-1.c: New test. > > > -- > Eric Botcazou
