This patch tries to fix the 2% regression in 510.parest_r on ampere1 in the tracker. (Previous discussion is here: https://gcc.gnu.org/pipermail/gcc-patches/2023-July/624893.html)
1. Add testcases for the problem. For an op list in the form of
"acc = a * b + c * d + acc", currently reassociation doesn't
Swap the operands so that more FMAs can be generated.
After widening_mul the result looks like:
_1 = .FMA(a, b, acc_0);
acc_1 = .FMA(c, d, _1);
While previously (before the "Handle FMA friendly..." patch),
widening_mul's result was like:
_1 = a * b;
_2 = .FMA (c, d, _1);
acc_1 = acc_0 + _2;
If the code fragment is in a loop, some architecture can execute
the latter in parallel, so the performance can be much faster than
the former. For the small testcase, the performance gap is over
10% on both ampere1 and neoverse-n1. So the point here is to avoid
turning the last statement into FMA, and keep it a PLUS_EXPR as
much as possible. (If we are rewriting the op list into parallel,
no special treatment is needed, since the last statement after
rewrite_expr_tree_parallel will be PLUS_EXPR anyway.)
2. Function result_feeds_back_from_phi_p is to check for cross
backedge dependency. Added new enum fma_state to describe the
state of FMA candidates.
With this patch, there's a 3% improvement in 510.parest_r 1-copy
run on ampere1. The compile options are:
"-Ofast -mcpu=ampere1 -flto --param avoid-fma-max-bits=512".
Best regards,
Di Zhao
----
PR tree-optimization/110279
gcc/ChangeLog:
* tree-ssa-reassoc.cc (enum fma_state): New enum to
describe the state of FMA candidates for an op list.
(rewrite_expr_tree_parallel): Changed boolean
parameter to enum type.
(result_feeds_back_from_phi_p): New function to check
for cross backedge dependency.
(rank_ops_for_fma): Return enum fma_state. Added new
parameter.
(reassociate_bb): If there's backedge dependency in an
op list, swap the operands before rewrite_expr_tree.
gcc/testsuite/ChangeLog:
* gcc.dg/pr110279.c: New test.
0001-swap-operands-in-reassoc-to-reduce-cross-backedge-FM.patch
Description: 0001-swap-operands-in-reassoc-to-reduce-cross-backedge-FM.patch
