[Bug c++/57301] New: bit rotation is not optimized in c but not c++

[luto at mit dot edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57301

            Bug ID: 57301
           Summary: bit rotation is not optimized in c but not c++
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: luto at mit dot edu

It's well known [citation needed] that modern compilers are excellent at
optimizing simple bit twiddling.  Unfortunately, while gcc is, g++ is not. 
(See PR29749 for where gcc gained this ability.)

Here's a trivial example borrowed from gcc/testsuite/gcc.dg/fold-rotate-1.c:

unsigned short
f1 (unsigned short a)
{
  return a >> 8 | a << 8;
}

gcc -O2 (4.7 and some random 4.8 build) generates:

f1:
.LFB0:
        .cfi_startproc
        movl    %edi, %eax
        rolw    $8, %ax
        ret
        .cfi_endproc

clang -O2 generates:

f1:                                     # @f1
        .cfi_startproc
# BB#0:
        rolw    $8, %di
        movzwl  %di, %eax
        ret

(I suspect that these are equally good.)

clang++ -O2 generates:

_Z2f1t:                                 # @_Z2f1t
        .cfi_startproc
# BB#0:
        rolw    $8, %di
        movzwl  %di, %eax
        ret

All unsurprising.  But g++ -O2 generates:

_Z2f1t:
.LFB0:
        .cfi_startproc
        movzwl  %di, %edi
        movl    %edi, %eax
        sarl    $8, %edi
        sall    $8, %eax
        orl     %edi, %eax
        ret
        .cfi_endproc

Perhaps some decent subset of the gcc testsuite should also be built with the
c++ frontend to make sure it still passes...