[Bug target/23322] [4.1 regression] performance regression, possibly related to caching

["roger at eyesopen dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From roger at eyesopen dot com  2005-08-11 14:56 -------
I'll take a look, but on first inspection this looks more like a register
allocation issue than a reg-stack problem.  In the first (4.0) case, the
accumulator "result" is assigned a hard register in the loop, whilst in
the second (4.1) it is being placed in memory, at -16(%ebp).  This may also
explain why extracting that loop into a stand-alone function produces
optimal/original code, as the register allocator gets less confused by other
influences in the function.  The extracted code is also even better than 4.0's,
as it avoids writing "result" to memory on each iteration (store sinking).

The second failure does show an interesting reg-stack/reg-alloc interaction
though.  The "hot" accumulator value is live on the backedge and the exit
edge of the loop but not on the incoming edge.  Clearly, the best fix is to
make this value live on the incoming edge, but failing that it is actually
better to prevent it being live on the back and exit edges, and add compensation
code after the loop.  i.e. if the store to result in the loop used fstpl, you
wouldn't need to fstp %st(0) on each loop iteration, but would instead need a
compensating fldl after the loop.

I'm not sure how easy it would be to teach GCC's register allocation to take
these considerations into account, or failing that, whether reg-stack could be
tweaked/hacked to locally fix this up.  But the fundamental problem is that
reg-alloc should assign result to a hard resigster as it clearly knows there
are enough available in that block.

reg-stack.c is just doing what its told, and in this case its being told to
do something stupid.


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-08-11 14:56:31
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23322