http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57301
Steven Bosscher <steven at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2013-05-16
CC| |jakub at gcc dot gnu.org
Component|c++ |testsuite
Ever confirmed|0 |1
--- Comment #3 from Steven Bosscher <steven at gcc dot gnu.org> ---
(In reply to Andy Lutomirski from comment #0)
> It's well known [citation needed] that modern compilers are excellent at
> optimizing simple bit twiddling.
:-)
> 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.)
The gcc code is slightly better in most cases but it depends on
the target. ix86 tuning is as irregular as the architecture itself.
> 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...
That is not so easy to do, but apparently it's necessary. gcc and
g++ are not supposed to generate different code for such a trivial
example.
We'd have to figure out a subset of the gcc test suite that actually
tests middle-end features (such as your instruction combining/folding
example).
For GCC 4.9 the middle-end rotation optimizations have been improved
by Jakub's patches for bug 55278. For GCC 4.7 and GCC 4.8 I would
expect the -fdump-tree-optimized dumps for gcc and g++ to be the same.
Can you try that and have a look at those dumps?
