[Bug target/56897] New: unaligned memory access on alpha

[martynas at venck dot us]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56897

             Bug #: 56897
           Summary: unaligned memory access on alpha
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: martynas@venck.us


Hi,

After looking at the RTL dumps here and there, I've noticed that
GCC generates unaligned loads in the following manner:

foo = ([reg:X & -8] << (64 - (((reg:FP+reg:Y) & 0x7) << 3))) >> 56

Obviously, we're losing every eighth byte here (consider FP-0,FP-8).
Since FP is aligned, GCC optimizes this to foo = 0.  The right way
to do this is:

foo = ([reg:X & -8] << (56 - (((reg:FP+reg:Y-1) & 0x7) << 3))) >> 56

To reproduce, try this out:

unsigned long foo = 0x010203040a0b0c0d;
printf("%02x",   *((char *)&foo + 7));

With -O (and onwards) it will turn out to be zero;  at every &X+15,
&X+23, etc.  Depending on the offset from the frame pointer.

Attached the diff against trunk.

Martynas.