[COMMITTED,1/2] Remove simple ranges from trailing zero bitmasks.
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Commit Message
WHen we set bitmasks indicating known zero or one bits, we see some
"obvious" things once in a while that are easy to prevent. ie
unsigned int [2, +INF] MASK 0xfffffffe VALUE 0x1
the range [2, 2] is obviously impossible since the final bit must be a
one. This doesn't usually cause us too much trouble, but the
subsequent patch triggers some more interesting situations in which it
helps to remove the obvious ranges when we have mask that is trailing
zeros.
Its too much of a performance impact to constantly be checking the range
every time we set the bitmask, but it turns out that if we simply try to
take care of it during intersection operations (which happen at most key
times, like changing an existing value), the impact is pretty minimal..
like 0.6% of VRP.
This patch looks for trailing zeros in the mask, and replaces the low
end range covered by those bits with those bits from the value field.
Bootstraps on build-x86_64-pc-linux-gnu with no regressions. Pushed.
Andrew
From b20f1dce46fb8bb1b142e9087530e546a40edec8 Mon Sep 17 00:00:00 2001
From: Andrew MacLeod <amacleod@redhat.com>
Date: Tue, 31 Oct 2023 11:51:34 -0400
Subject: [PATCH 1/2] Remove simple ranges from trailing zero bitmasks.
During the intersection operation, it can be helpful to remove any
low-end ranges when the bitmask has trailing zeros. This prevents
obviously incorrect ranges from appearing without requiring a bitmask
check.
* value-range.cc (irange_bitmask::adjust_range): New.
(irange::intersect_bitmask): Call adjust_range.
* value-range.h (irange_bitmask::adjust_range): New prototype.
---
gcc/value-range.cc | 30 ++++++++++++++++++++++++++++++
gcc/value-range.h | 2 ++
2 files changed, 32 insertions(+)
@@ -1857,6 +1857,35 @@ irange::get_bitmask_from_range () const
return irange_bitmask (wi::zero (prec), min | xorv);
}
+// Remove trailing ranges that this bitmask indicates can't exist.
+
+void
+irange_bitmask::adjust_range (irange &r) const
+{
+ if (unknown_p () || r.undefined_p ())
+ return;
+
+ int_range_max range;
+ tree type = r.type ();
+ int prec = TYPE_PRECISION (type);
+ // If there are trailing zeros, create a range representing those bits.
+ gcc_checking_assert (m_mask != 0);
+ int z = wi::ctz (m_mask);
+ if (z)
+ {
+ wide_int ub = (wi::one (prec) << z) - 1;
+ range = int_range<5> (type, wi::zero (prec), ub);
+ // Then remove the specific value these bits contain from the range.
+ wide_int value = m_value & ub;
+ range.intersect (int_range<2> (type, value, value, VR_ANTI_RANGE));
+ // Inverting produces a list of ranges which can be valid.
+ range.invert ();
+ // And finally select R from only those valid values.
+ r.intersect (range);
+ return;
+ }
+}
+
// If the the mask can be trivially converted to a range, do so and
// return TRUE.
@@ -2002,6 +2031,7 @@ irange::intersect_bitmask (const irange &r)
if (!set_range_from_bitmask ())
normalize_kind ();
+ m_bitmask.adjust_range (*this);
if (flag_checking)
verify_range ();
return true;
@@ -139,6 +139,8 @@ public:
void verify_mask () const;
void dump (FILE *) const;
+ void adjust_range (irange &r) const;
+
// Convenience functions for nonzero bitmask compatibility.
wide_int get_nonzero_bits () const;
void set_nonzero_bits (const wide_int &bits);
--
2.41.0