[Bug tree-optimization/40210] New: gcc needs byte swap builtins

["eric-bugs at omnifarious dot org" <gcc-bugzilla at gcc dot gnu dot org>]

gcc needs some built in functions for byte swapping.

I've been experimenting with the various versions of byte swapping functions
out there, and they either result in code that's opaque to the optimizer (i.e.
swapping something twice is not considered a null operation) or the optimizer
doesn't recognize that a byte swap is what's happening and renders it as a
complex series of shift, and and or instructions.

I know very little about the internals of gcc, but my ignorant preference would
be to make tree-ssa recognize that code like this:

inline uint64_t byteswap_64(const uint64_t x)
{
   return
      ((((x) & 0xff00000000000000ull) >> 56)
      | (((x) & 0x00ff000000000000ull) >> 40)
      | (((x) & 0x0000ff0000000000ull) >> 24)
      | (((x) & 0x000000ff00000000ull) >> 8)
      | (((x) & 0x00000000ff000000ull) << 8)
      | (((x) & 0x0000000000ff0000ull) << 24)
      | (((x) & 0x000000000000ff00ull) << 40)
      | (((x) & 0x00000000000000ffull) << 56));
}

is a byte swap and optimize appropriately.  If this were being done to an
entire array, it might even be possible to vectorize it efficiently.

This would also mean that code to pull specific bits out of a pre or post swap
value could be moved around and fiddled to get the value out of a different
place if it made for more efficient register usage.


-- 
           Summary: gcc needs byte swap builtins
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: eric-bugs at omnifarious dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40210