[Bug middle-end/17886] variable rotate and long long rotate should be better optimized

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

------- Additional Comments From mmitchel at gcc dot gnu dot org  2005-09-29 03:52 -------
Here is the current status for the four functions in Andi's testcase, with "f2"
changed to use "32 - y" so that it is a proper rotation:

* f still generates a complex code sequence, but I'm not sure how much better we
can do.  Our code sequence doesn't look a lot worse than the sequence generated
by icc 9.0, at first glance.  We could try something like:  

  if %ecx > 31:
    mov %eax, %ebx
    shldl $31, %edx, %eax
    shldl $31, %ebx, %edx
    %ecx -= 31
  if %ecx > 31:
    mov %eax, %ebx
    shldl $31, %edx, %eax
    shldl $31, %ebx, %edx
    %ecx -= 31
  if %ecx != 0:
    mov %eax, %ebx
    shldl %cl, %edx, %eax
    shldl %cl, %ebx, %edx

but, that doesn't seem clearly better than what we presently generate.

* f2 uses the roll instruction, which appears optimal.

* f3 uses two shdl instructions, which appears optimal.

* f4 uses the rorl instruction, which appears optimal.

For all of f2 and f3, it looks like we generate code better than you get with
icc 9.0.

I have no plans to work on this further, for the time being, but I'll not close
out the PRt; someone else might want to try to attack the code generated for the
variable rotation case.   Or, if people are satisfied, we can close the PR.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|mark at codesourcery dot com|unassigned at gcc dot gnu
                   |                            |dot org
             Status|ASSIGNED                    |NEW


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17886