This test case demonstrates an unnoticed exponential situation in range-ops.
We end up unrolling the loop, and the pattern of code creates a set of
cascading multiplies for which we can precisely evaluate them with
sub-ranges.
For instance, we calculated :
_38 = int [8192, 8192][24576, 24576][40960, 40960][57344, 57344]
so _38 has 4 sub-ranges, and then we calculate:
_39 = _38 * _38;
we do 16 sub-range multiplications and end up with: int [67108864,
67108864][201326592, 201326592][335544320, 335544320][469762048,
469762048][603979776, 603979776][1006632960, 1006632960][1409286144,
1409286144][1677721600, 1677721600][+INF, +INF]
This feeds other multiplies (_39 * _39) and progresses rapidly to blow
up the number of sub-ranges in subsequent operations.
Folding of sub-ranges is an O(n*m) process. We perform the operation on
each pair of sub-ranges and union them. Values like _38 * _38 that
continue feeding each other quickly become exponential.
Then combining that with union (an inherently linear operation over the
number of sub-ranges) at each step of the way adds an additional
quadratic operation on top of the exponential factor.
This patch adjusts the wi_fold routine to recognize when the calculation
is moving in an exponential direction, simply produce a summary result
instead of a precise one. The attached patch does this if (#LH
sub-ranges * #RH sub-ranges > 12)... then it just performs the operation
with the lower and upper bound instead. We could choose a different
number, but that one seems to keep things under control, and allows us
to process up to a 3x4 operation for precision (there is a testcase in
the testsuite for this combination gcc.dg/tree-ssa/pr61839_2.c).
Longer term, we might want adjust this routine to be slightly smarter
than that, but this is a virtually zero-risk solution this late in the
release cycle.
This also a generalize ~1% speedup in the VRP2 pass across 380 gcc
source files, but I'm sure has much more dramatic results at -O3 that
this testcase exposes.
Bootstraps on x86_64-pc-linux-gnu with no regressions. OK for trunk?
Andrew
commit d8c5c37d5362bd876118949de76086daba756ace
Author: Andrew MacLeod <amacl...@redhat.com>
Date: Mon Jan 10 13:33:44 2022 -0500
Prevent exponential range calculations.
Produce a summary result for any operation involving too many subranges.
PR tree-optimization/103821
* range-op.cc (range_operator::fold_range): Only do precise ranges
when there are not too many subranges.
range_operator::fold_range
diff --git a/gcc/range-op.cc b/gcc/range-op.cc
index 1af42ebc376..a4f6e9eba29 100644
--- a/gcc/range-op.cc
+++ b/gcc/range-op.cc
@@ -209,10 +209,12 @@ range_operator::fold_range (irange &r, tree type,
unsigned num_rh = rh.num_pairs ();
// If both ranges are single pairs, fold directly into the result range.
- if (num_lh == 1 && num_rh == 1)
+ // If the number of subranges grows too high, produce a summary result as the
+ // loop becomes exponential with little benefit. See PR 103821.
+ if ((num_lh == 1 && num_rh == 1) || num_lh * num_rh > 12)
{
- wi_fold_in_parts (r, type, lh.lower_bound (0), lh.upper_bound (0),
- rh.lower_bound (0), rh.upper_bound (0));
+ wi_fold_in_parts (r, type, lh.lower_bound (), lh.upper_bound (),
+ rh.lower_bound (), rh.upper_bound ());
op1_op2_relation_effect (r, type, lh, rh, rel);
return true;
}
