On 10/14/22 09:56, Vineet Gupta wrote:
Hi,
When analyzing coremark build for RISC-V, noticed redundant constants
not being eliminated. While this is a recurrent issue with RV, this
specific instance is not unique to RV as I can trigger similar output
on aarch64 with -fno-if-conversion, hence something which could be
addressed in common passes.
-O3 -march=rv64gc_zba_zbb_zbc_zbs
crcu8:
xor a3,a0,a1
andi a3,a3,1
srli a4,a0,1
srli a5,a1,1
beq a3,zero,.L2
li a3,-24576 # 0xFFFF_A000
addi a3,a3,1 # 0xFFFF_A001
xor a5,a5,a3
zext.h a5,a5
.L2:
xor a4,a4,a5
andi a4,a4,1
srli a3,a0,2
srli a5,a5,1
beq a4,zero,.L3
li a4,-24576 # 0xFFFF_A000
addi a4,a4,1 # 0xFFFF_A001
xor a5,a5,a4
zext.h a5,a5
.L3:
xor a3,a3,a5
andi a3,a3,1
srli a4,a0,3
srli a5,a5,1
beq a3,zero,.L4
li a3,-24576 # 0xFFFF_A000
addi a3,a3,1 # 0xFFFF_A001
...
...
I see that with small tests cse1 is able to substitute redundant
constant reg with equivalent old reg.
I find it easier to reason about this stuff with a graphical CFG, so a
bit of ascii art...
2
/ \
3 ---> 4
/ \
5 ---> 6
Where BB4 corresponds to .L2 and BB6 corresponds to .L3. Evaluation of
the constants occurs in BB3 and BB5.
CSE isn't going to catch this. The best way to think about CSE's
capabilities is that it can work on extended basic blocks. An
extended basic block can have jumps out, but not jumps in. There are 3
EBBs in this code. (1,2), (4,5) and 6. So BB4 is in a different EBB
than BB3. So the evaluation in BB3 can't be used by CSE in the EBB
containing BB4, BB5.
PRE/GCSE is better suited for this scenario, but it has a critical
constraint. In particular our PRE formulation is never allowed to put
an evaluation of an expression on a path that didn't have one before.
So while there clearly a redundancy on the path 2->3->4->5 (BB3 and
BB5), there is nowhere we could put an evaluation that would reduce the
number of evaluation on that path without introducing an evaluation on
paths that didn't have one. So consider 2->4->6. On that path there
are zero evaluations. So we can't place an eval in BB2 because that
will cause evaluations on 2->4->6 which didn't have any evaluations.
There isn't a great place in GCC to handle this right now. If the
constraints were relaxed in PRE, then we'd have a chance, but getting
the cost model right is going to be tough.
Jeff
