[Bug middle-end/111743] New: shifts in bit field accesses don't combine with other shifts

["andi-gcc at firstfloor dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111743

            Bug ID: 111743
           Summary: shifts in bit field accesses don't combine with other
                    shifts
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andi-gcc at firstfloor dot org
  Target Milestone: ---

(not sure it's the middle-end, picked arbitrarily)

The following code

struct bf { 
        unsigned a : 10, b : 20, c : 10;
};
unsigned fbc(struct bf bf) { return bf.b | (bf.c << 20); }


generates:

        movq    %rdi, %rax
        shrq    $10, %rdi
        shrq    $32, %rax               
        andl    $1048575, %edi
        andl    $1023, %eax
        sall    $20, %eax
        orl     %edi, %eax
        ret

It doesn't understand that the shift right can be combined with the shift left.
Also not sure why the shift left is arithmetic (this should be all unsigned) 

clang does the simplification which ends up one instruction shorter:
        movl    %edi, %eax
        shrl    $10, %eax
        andl    $1048575, %eax                  # imm = 0xFFFFF
        shrq    $12, %rdi
        andl    $1072693248, %edi               # imm = 0x3FF00000
        orl     %edi, %eax
        retq