[Bug middle-end/81161] New: poor code concatenating bitfields

["nathan at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81161

            Bug ID: 81161
           Summary: poor code concatenating bitfields
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nathan at gcc dot gnu.org
  Target Milestone: ---

Created attachment 41604
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41604&action=edit
example

The example code is trying to use a set of adjacent single bit fields as a
wider value.  (Think TREE_LANG_FLAG_{N,N+2} to explain why I can't just make a
3-bit bitfield).  We don't spot this is just extrating a wider bitfield[*] 
here's the x86_64 code I get:
        movzbl  (%rdi), %edx
        movl    %edx, %eax
        movl    %edx, %ecx
        shrb    $2, %dl
        shrb    $4, %al
        shrb    $3, %cl
        andl    $1, %edx
        andl    $1, %eax
        andl    $1, %ecx
        sall    $2, %eax
        addl    %ecx, %ecx
        orl     %ecx, %eax
        orl     %edx, %eax
        ret

which is roughly doing:
   t1 = (val >> 2) & 1
   t2 = (val >> 3) & 1
   t3 = (val >> 4) & 1
   r = (t3 << 2) | (t2 + t1) | t1

optimal code would be something like:
   movzbl       (%rdi), %eax
   shrb $2,%al
   andl $7,%eax

this is similar to 68360, but looks sufficiently different to warrant a new
report.

[*] where bitfield packing was from the other end of the object, one might want
to swap the bitfields around to get a nice ordering.  Let's just assume
little-endian packing for the sake of argument.