[Bug target/44278] New: Use ubfx to extract unsigned bit fields at the low end

["carrot at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Compile the following code with options -march=armv7-a -mthumb -O2

unsigned int foo6(unsigned u)
{
  return (u >> 0) & 0x1ff;
}

GCC generates:

        lsls    r0, r0, #23
        lsrs    r0, r0, #23
        bx      lr

The two shifts can be merged into 
        UBFX     r1,r0,#0,#9

If I change the source code into
  return (u >> 1) & 0x1ff;
Then gcc can generate ubfx instruction. So this case only occurs at the low end
of a register.

The same problem exists with option -Os and arm instructions.


-- 
           Summary: Use ubfx to extract unsigned bit fields at the low end
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44278