On 31.05.2017 11:00, Jakub Jelinek wrote:
On Wed, May 31, 2017 at 10:48:07AM +0200, Georg-Johann Lay wrote:
because divmod in not a single_set:
(gdb) p seq
$10 = (const rtx_insn *) 0x7ffff730d500
(gdb) pr
warning: Expression is not an assignment (and might have no effect)
(insn 14 13 0 (parallel [
(set (reg:HI 52)
(div:HI (reg:HI 47)
(reg:HI 54)))
(set (reg:HI 53)
(mod:HI (reg:HI 47)
(reg:HI 54)))
(clobber (reg:QI 21 r21))
(clobber (reg:HI 22 r22))
(clobber (reg:HI 24 r24))
(clobber (reg:HI 26 r26))
]) "scale.c":7 -1
(nil))
(gdb)
Hence the divmod appears to be much less expensive than the unsigned
variant that computed the costs for mult_highpart.
Then you should fix the cost computation - be able to use a target hook
on insns that are not a single set or something similar.
Are you saying that cost computation in GCC is fundamentally flawed
for anything that it not a single_set?
The division/modulo optimization I've added as well as many other spots
in GCC rely on reasonable cost, just grep e.g. all places that call
seq_cost. So, if it returns something that is a very wrong estimate,
it won't affect just that single optimization, but all others. Therefore,
you should fix the cost computation, rather than disabling all the places
that use the costs. Many targets have instructions with multiple sets,
I didn't intend to disable anything...
Would the following addition be in order?
gcc/
PR middle-end/80929
* rtlanal.c (seq_cost) [PARALLEL]: Get cost from insn_rtx_cost
instead of assuming cost of 1.
Index: rtlanal.c
===================================================================
--- rtlanal.c (revision 248737)
+++ rtlanal.c (working copy)
@@ -5300,6 +5300,8 @@ seq_cost (const rtx_insn *seq, bool spee
set = single_set (seq);
if (set)
cost += set_rtx_cost (set, speed);
+ else if (PARALLEL == GET_CODE (PATTERN (seq)))
+ cost += insn_rtx_cost (PATTERN (seq), speed);
else
cost++;
}
so I'm surprised assuming cost of 1 for them doesn't break many more things.
Maybe because PARALLEL is not common, and when expand tests for costs of
DIV or MOD, it passes respective RTXes to the RTL cost functions, and
*not* what the target expands in divmod insns.
I think either we should have a separate target hook for multiple sets
instructions, or just call the targetm.rtx_costs on the PARALLEL in that
case and see if the targets compute something reasonable for it, otherwise
either use the cost of the first set, or maximum of all sets (that might be
best) or something similar.
Jakub
The patch uses whatever insn_rtx_costs comes up with. For PARALLEL,
it's the cost of the 1st SET which is reasonable imo (at least for the
divmod case).
Johann
