Re: [PATCH] Add auto-resizing capability to irange's [PR109695]

[Aldy Hernandez <aldyh@redhat.com>]

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On 5/15/23 20:14, Aldy Hernandez wrote:
> On 5/15/23 17:07, Aldy Hernandez wrote:
>>
>>
>> On 5/15/23 12:42, Jakub Jelinek wrote:
>>> On Mon, May 15, 2023 at 12:35:23PM +0200, Aldy Hernandez wrote:
>>>> gcc/ChangeLog:
>>>>
>>>>     PR tree-optimization/109695
>>>>     * value-range.cc (irange::operator=): Resize range.
>>>>     (irange::union_): Same.
>>>>     (irange::intersect): Same.
>>>>     (irange::invert): Same.
>>>>     (int_range_max): Default to 3 sub-ranges and resize as needed.
>>>>     * value-range.h (irange::maybe_resize): New.
>>>>     (~int_range): New.
>>>>     (int_range::int_range): Adjust for resizing.
>>>>     (int_range::operator=): Same.
>>>
>>> LGTM.
>>>
>>> One question is if we shouldn't do it for GCC13/GCC12 as well, perhaps
>>> changing it to some larger number than 3 when the members aren't 
>>> wide_ints
>>> in there but just trees.  Sure, in 13/12 the problem is 10x less severe
>>> than in current trunk, but still we have some cases where we run out of
>>> stack because of it on some hosts.
>>
>> Sure, but that would require messing around with the gt_* GTY 
>> functions, and making sure we're allocating the trees from a sensible 
>> place, etc etc.  I'm less confident in my ability to mess with GTY 
>> stuff this late in the game.
> 
> Hmmm, maybe backporting this isn't too bad.  The only time we'd have a 
> chunk on the heap is for int_range_max, which will never live in GC 
> space.  So I don't think we need to worry about GC at all.
> 
> Although, legacy mode in GCC13 does get in a the way a bit.  Sigh.

I've adapted the patch to GCC13 and tested it on x86-64 Linux.  Please 
look over the new[] I do for trees to make sure I did things right.

int_range_max on GCC13 is currently 4112 bytes.  Here are the numbers 
for various defaults:

                 < 2> =  64 bytes, 3.02% for VRP.
                 < 3> =  80 bytes, 2.67% for VRP.
                 < 8> = 160 bytes, 2.46% for VRP.
                 <16> = 288 bytes, 2.40% for VRP.

Note that we don't have any runway on GCC13, so this would be a net loss 
in performance for VRP.  Threading shows about half as much of a drop 
than VRP.  Overall compilation is within 0.2%, so not noticeable.

I'm surprised 2 sub-ranges doesn't incur a  bigger penalty, but 3 seems 
to be the happy medium.  Anything more than that, and there's no difference.

The patch defaults to 3 sub-ranges.  I must say, 80 bytes looks mighty 
nice.  It's up to you what to do with the patch.  I'm chicken shit at 
heart and hate touching release compilers :).

Aldy

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From 777aa930b106fea2dd6ed9fe22b42a2717f1472d Mon Sep 17 00:00:00 2001
From: Aldy Hernandez <aldyh@redhat.com>
Date: Mon, 15 May 2023 12:25:58 +0200
Subject: [PATCH] [GCC13] Add auto-resizing capability to irange's [PR109695]

Backport the following from trunk.

	Note that the patch has been adapted to trees.

	The numbers for various sub-ranges on GCC13 are:
		< 2> =  64 bytes, -3.02% for VRP.
		< 3> =  80 bytes, -2.67% for VRP.
		< 8> = 160 bytes, -2.46% for VRP.
		<16> = 288 bytes, -2.40% for VRP.

<tldr>
We can now have int_range<N, RESIZABLE=false> for automatically
resizable ranges.  int_range_max is now int_range<3, true>
for a 69X reduction in size from current trunk, and 6.9X reduction from
GCC12.  This incurs a 5% performance penalty for VRP that is more than
covered by our > 13% improvements recently.
</tldr>

int_range_max is the temporary range object we use in the ranger for
integers.  With the conversion to wide_int, this structure bloated up
significantly because wide_ints are huge (80 bytes a piece) and are
about 10 times as big as a plain tree.  Since the temporary object
requires 255 sub-ranges, that's 255 * 80 * 2, plus the control word.
This means the structure grew from 4112 bytes to 40912 bytes.

This patch adds the ability to resize ranges as needed, defaulting to
no resizing, while int_range_max now defaults to 3 sub-ranges (instead
of 255) and grows to 255 when the range being calculated does not fit.

For example:

int_range<1> foo;	// 1 sub-range with no resizing.
int_range<5> foo;	// 5 sub-ranges with no resizing.
int_range<5, true> foo;	// 5 sub-ranges with resizing.

I ran some tests and found that 3 sub-ranges cover 99% of cases, so
I've set the int_range_max default to that:

	typedef int_range<3, /*RESIZABLE=*/true> int_range_max;

We don't bother growing incrementally, since the default covers most
cases and we have a 255 hard-limit.  This hard limit could be reduced
to 128, since my tests never saw a range needing more than 124, but we
could do that as a follow-up if needed.

With 3-subranges, int_range_max is now 592 bytes versus 40912 for
trunk, and versus 4112 bytes for GCC12!  The penalty is 5.04% for VRP
and 3.02% for threading, with no noticeable change in overall
compilation (0.27%).  This is more than covered by our 13.26%
improvements for the legacy removal + wide_int conversion.

I think this approach is a good alternative, while providing us with
flexibility going forward.  For example, we could try defaulting to a
8 sub-ranges for a noticeable improvement in VRP.  We could also use
large sub-ranges for switch analysis to avoid resizing.

Another approach I tried was always resizing.  With this, we could
drop the whole int_range<N> nonsense, and have irange just hold a
resizable range.  This simplified things, but incurred a 7% penalty on
ipa_cp.  This was hard to pinpoint, and I'm not entirely convinced
this wasn't some artifact of valgrind.  However, until we're sure,
let's avoid massive changes, especially since IPA changes are coming
up.

For the curious, a particular hot spot for IPA in this area was:

ipcp_vr_lattice::meet_with_1 (const value_range *other_vr)
{
...
...
  value_range save (m_vr);
  m_vr.union_ (*other_vr);
  return m_vr != save;
}

The problem isn't the resizing (since we do that at most once) but the
fact that for some functions with lots of callers we end up a huge
range that gets copied and compared for every meet operation.  Maybe
the IPA algorithm could be adjusted somehow??.

Anywhooo... for now there is nothing to worry about, since value_range
still has 2 subranges and is not resizable.  But we should probably
think what if anything we want to do here, as I envision IPA using
infinite ranges here (well, int_range_max) and handling frange's, etc.

gcc/ChangeLog:

	PR tree-optimization/109695
	* value-range.cc (irange::operator=): Resize range.
	(irange::union_): Same.
	(irange::intersect): Same.
	(irange::invert): Same.
	(int_range_max): Default to 3 sub-ranges and resize as needed.
	* value-range.h (irange::maybe_resize): New.
	(~int_range): New.
	(int_range::int_range): Adjust for resizing.
	(int_range::operator=): Same.
---
 gcc/value-range-storage.h |  2 +-
 gcc/value-range.cc        | 15 ++++++
 gcc/value-range.h         | 96 +++++++++++++++++++++++++++------------
 3 files changed, 83 insertions(+), 30 deletions(-)

diff --git a/gcc/value-range-storage.h b/gcc/value-range-storage.h
index 6da377ebd2e..1ed6f1ccd61 100644
--- a/gcc/value-range-storage.h
+++ b/gcc/value-range-storage.h
@@ -184,7 +184,7 @@ vrange_allocator::alloc_irange (unsigned num_pairs)
   // Allocate the irange and required memory for the vector.
   void *r = alloc (sizeof (irange));
   tree *mem = static_cast <tree *> (alloc (nbytes));
-  return new (r) irange (mem, num_pairs);
+  return new (r) irange (mem, num_pairs, /*resizable=*/false);
 }
 
 inline frange *
diff --git a/gcc/value-range.cc b/gcc/value-range.cc
index ec826c2fe1b..753f5e8cc76 100644
--- a/gcc/value-range.cc
+++ b/gcc/value-range.cc
@@ -831,6 +831,10 @@ irange::operator= (const irange &src)
       copy_to_legacy (src);
       return *this;
     }
+
+  int needed = src.num_pairs ();
+  maybe_resize (needed);
+
   if (src.legacy_mode_p ())
     {
       copy_legacy_to_multi_range (src);
@@ -2506,6 +2510,7 @@ irange::irange_union (const irange &r)
   // Now it simply needs to be copied, and if there are too many
   // ranges, merge some.  We wont do any analysis as to what the
   // "best" merges are, simply combine the final ranges into one.
+  maybe_resize (i / 2);
   if (i > m_max_ranges * 2)
     {
       res[m_max_ranges * 2 - 1] = res[i - 1];
@@ -2605,6 +2610,11 @@ irange::irange_intersect (const irange &r)
   if (r.irange_contains_p (*this))
     return intersect_nonzero_bits (r);
 
+  // ?? We could probably come up with something smarter than the
+  // worst case scenario here.
+  int needed = num_pairs () + r.num_pairs ();
+  maybe_resize (needed);
+
   signop sign = TYPE_SIGN (TREE_TYPE(m_base[0]));
   unsigned bld_pair = 0;
   unsigned bld_lim = m_max_ranges;
@@ -2831,6 +2841,11 @@ irange::invert ()
       m_num_ranges = 1;
       return;
     }
+
+  // At this point, we need one extra sub-range to represent the
+  // inverse.
+  maybe_resize (m_num_ranges + 1);
+
   // The algorithm is as follows.  To calculate INVERT ([a,b][c,d]), we
   // generate [-MIN, a-1][b+1, c-1][d+1, MAX].
   //
diff --git a/gcc/value-range.h b/gcc/value-range.h
index 969b2b68418..96e59ecfa72 100644
--- a/gcc/value-range.h
+++ b/gcc/value-range.h
@@ -172,7 +172,8 @@ public:
   bool legacy_verbose_intersect (const irange *);	// DEPRECATED
 
 protected:
-  irange (tree *, unsigned);
+  void maybe_resize (int needed);
+  irange (tree *, unsigned nranges, bool resizable);
   // potential promotion to public?
   tree tree_lower_bound (unsigned = 0) const;
   tree tree_upper_bound (unsigned) const;
@@ -200,6 +201,8 @@ protected:
   void copy_to_legacy (const irange &);
   void copy_legacy_to_multi_range (const irange &);
 
+  // Hard limit on max ranges allowed.
+  static const int HARD_MAX_RANGES = 255;
 private:
   friend void gt_ggc_mx (irange *);
   friend void gt_pch_nx (irange *);
@@ -214,15 +217,21 @@ private:
 
   bool intersect (const wide_int& lb, const wide_int& ub);
   unsigned char m_num_ranges;
+  bool m_resizable;
   unsigned char m_max_ranges;
   tree m_nonzero_mask;
+protected:
   tree *m_base;
 };
 
 // Here we describe an irange with N pairs of ranges.  The storage for
 // the pairs is embedded in the class as an array.
+//
+// If RESIZABLE is true, the storage will be resized on the heap when
+// the number of ranges needed goes past N up to a max of
+// HARD_MAX_RANGES.  This new storage is freed upon destruction.
 
-template<unsigned N>
+template<unsigned N, bool RESIZABLE = false>
 class GTY((user)) int_range : public irange
 {
 public:
@@ -233,7 +242,7 @@ public:
   int_range (tree type);
   int_range (const int_range &);
   int_range (const irange &);
-  virtual ~int_range () = default;
+  virtual ~int_range ();
   int_range& operator= (const int_range &);
 private:
   template <unsigned X> friend void gt_ggc_mx (int_range<X> *);
@@ -472,6 +481,38 @@ is_a <frange> (vrange &v)
   return v.m_discriminator == VR_FRANGE;
 }
 
+// For resizable ranges, resize the range up to HARD_MAX_RANGES if the
+// NEEDED pairs is greater than the current capacity of the range.
+
+inline void
+irange::maybe_resize (int needed)
+{
+  if (!m_resizable || m_max_ranges == HARD_MAX_RANGES)
+    return;
+
+  if (needed > m_max_ranges)
+    {
+      m_max_ranges = HARD_MAX_RANGES;
+      tree *newmem = new tree[m_max_ranges * 2];
+      memcpy (newmem, m_base, sizeof (tree) * num_pairs () * 2);
+      m_base = newmem;
+    }
+}
+
+template<unsigned N, bool RESIZABLE>
+inline
+int_range<N, RESIZABLE>::~int_range ()
+{
+  if (RESIZABLE && m_base != m_ranges)
+    delete m_base;
+}
+
+// This is an "infinite" precision irange for use in temporary
+// calculations.  It starts with a sensible default covering 99% of
+// uses, and goes up to HARD_MAX_RANGES when needed.  Any allocated
+// storage is freed upon destruction.
+typedef int_range<3, /*RESIZABLE=*/true> int_range_max;
+
 class vrange_visitor
 {
 public:
@@ -490,10 +531,6 @@ public:
 // There are copy operators to seamlessly copy to/fro multi-ranges.
 typedef int_range<1> value_range;
 
-// This is an "infinite" precision irange for use in temporary
-// calculations.
-typedef int_range<255> int_range_max;
-
 // This is an "infinite" precision range object for use in temporary
 // calculations for any of the handled types.  The object can be
 // transparently used as a vrange.
@@ -872,64 +909,65 @@ gt_pch_nx (int_range<N> *x, gt_pointer_operator op, void *cookie)
 // Constructors for irange
 
 inline
-irange::irange (tree *base, unsigned nranges)
+irange::irange (tree *base, unsigned nranges, bool resizable)
 {
   m_discriminator = VR_IRANGE;
   m_base = base;
   m_max_ranges = nranges;
+  m_resizable = resizable;
   set_undefined ();
 }
 
 // Constructors for int_range<>.
 
-template<unsigned N>
+template<unsigned N, bool RESIZABLE>
 inline
-int_range<N>::int_range ()
-  : irange (m_ranges, N)
+int_range<N, RESIZABLE>::int_range ()
+  : irange (m_ranges, N, RESIZABLE)
 {
 }
 
-template<unsigned N>
-int_range<N>::int_range (const int_range &other)
-  : irange (m_ranges, N)
+template<unsigned N, bool RESIZABLE>
+int_range<N, RESIZABLE>::int_range (const int_range &other)
+  : irange (m_ranges, N, RESIZABLE)
 {
   irange::operator= (other);
 }
 
-template<unsigned N>
-int_range<N>::int_range (tree min, tree max, value_range_kind kind)
-  : irange (m_ranges, N)
+template<unsigned N, bool RESIZABLE>
+int_range<N, RESIZABLE>::int_range (tree min, tree max, value_range_kind kind)
+  : irange (m_ranges, N, RESIZABLE)
 {
   irange::set (min, max, kind);
 }
 
-template<unsigned N>
-int_range<N>::int_range (tree type)
-  : irange (m_ranges, N)
+template<unsigned N, bool RESIZABLE>
+int_range<N, RESIZABLE>::int_range (tree type)
+  : irange (m_ranges, N, RESIZABLE)
 {
   set_varying (type);
 }
 
-template<unsigned N>
-int_range<N>::int_range (tree type, const wide_int &wmin, const wide_int &wmax,
+template<unsigned N, bool RESIZABLE>
+int_range<N, RESIZABLE>::int_range (tree type, const wide_int &wmin, const wide_int &wmax,
 			 value_range_kind kind)
-  : irange (m_ranges, N)
+  : irange (m_ranges, N, RESIZABLE)
 {
   tree min = wide_int_to_tree (type, wmin);
   tree max = wide_int_to_tree (type, wmax);
   set (min, max, kind);
 }
 
-template<unsigned N>
-int_range<N>::int_range (const irange &other)
-  : irange (m_ranges, N)
+template<unsigned N, bool RESIZABLE>
+int_range<N, RESIZABLE>::int_range (const irange &other)
+  : irange (m_ranges, N, RESIZABLE)
 {
   irange::operator= (other);
 }
 
-template<unsigned N>
-int_range<N>&
-int_range<N>::operator= (const int_range &src)
+template<unsigned N, bool RESIZABLE>
+int_range<N, RESIZABLE>&
+int_range<N, RESIZABLE>::operator= (const int_range &src)
 {
   irange::operator= (src);
   return *this;
-- 
2.40.0