Re: [PATCH] [ranger] Force buffer alignment in Value_Range [PR114912]

[Aldy Hernandez <aldyh@redhat.com>]

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Ahh, that is indeed cleaner, and there's no longer a need to assert
the sizeof of individual ranges.

It looks like a default constructor is needed for the buffer now, but
only for the default constructor of Value_Range.

I have verified that the individual range constructors are not called
on initialization to Value_Range, which was the original point of the
patch.  I have also run our performance suite, and there are no
changes to VRP or overall.

I would appreciate a review from someone more C++ savvy than me :).

OK for trunk?

On Fri, May 3, 2024 at 11:32 AM Andrew Pinski <pinskia@gmail.com> wrote:
>
> On Fri, May 3, 2024 at 2:24 AM Aldy Hernandez <aldyh@redhat.com> wrote:
> >
> > Sparc requires strict alignment and is choking on the byte vector in
> > Value_Range.  Is this the right approach, or is there a more canonical
> > way of forcing alignment?
>
> I think the suggestion was to change over to use an union and use the
> types directly in the union (anonymous unions and unions containing
> non-PODs are part of C++11).
> That is:
> union {
>   int_range_max int_range;
>   frange fload_range;
>   unsupported_range un_range;
> };
> ...
> m_vrange = new (&int_range) int_range_max ();
> ...
>
> Also the canonical way of forcing alignment in C++ is to use aliagnas
> as my patch in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114912
> did.
> Also I suspect the alignment is not word alignment but rather the
> alignment of HOST_WIDE_INT which is not always the same as the
> alignment of the pointer but bigger and that is why it is failing on
> sparc (32bit rather than 64bit).
>
> Thanks,
> Andrew Pinski
>
> >
> > If this is correct, OK for trunk?
> >
> > gcc/ChangeLog:
> >
> >         * value-range.h (class Value_Range): Use a union.
> > ---
> >  gcc/value-range.h | 24 +++++++++++++++---------
> >  1 file changed, 15 insertions(+), 9 deletions(-)
> >
> > diff --git a/gcc/value-range.h b/gcc/value-range.h
> > index 934eec9e386..31af7888018 100644
> > --- a/gcc/value-range.h
> > +++ b/gcc/value-range.h
> > @@ -740,9 +740,14 @@ private:
> >    void init (const vrange &);
> >
> >    vrange *m_vrange;
> > -  // The buffer must be at least the size of the largest range.
> > -  static_assert (sizeof (int_range_max) > sizeof (frange), "");
> > -  char m_buffer[sizeof (int_range_max)];
> > +  union {
> > +    // The buffer must be at least the size of the largest range, and
> > +    // be aligned on a word boundary for strict alignment targets
> > +    // such as sparc.
> > +    static_assert (sizeof (int_range_max) > sizeof (frange), "");
> > +    char m_buffer[sizeof (int_range_max)];
> > +    void *align;
> > +  } u;
> >  };
> >
> >  // The default constructor is uninitialized and must be initialized
> > @@ -816,11 +821,11 @@ Value_Range::init (tree type)
> >    gcc_checking_assert (TYPE_P (type));
> >
> >    if (irange::supports_p (type))
> > -    m_vrange = new (&m_buffer) int_range_max ();
> > +    m_vrange = new (&u.m_buffer) int_range_max ();
> >    else if (frange::supports_p (type))
> > -    m_vrange = new (&m_buffer) frange ();
> > +    m_vrange = new (&u.m_buffer) frange ();
> >    else
> > -    m_vrange = new (&m_buffer) unsupported_range ();
> > +    m_vrange = new (&u.m_buffer) unsupported_range ();
> >  }
> >
> >  // Initialize object with a copy of R.
> > @@ -829,11 +834,12 @@ inline void
> >  Value_Range::init (const vrange &r)
> >  {
> >    if (is_a <irange> (r))
> > -    m_vrange = new (&m_buffer) int_range_max (as_a <irange> (r));
> > +    m_vrange = new (&u.m_buffer) int_range_max (as_a <irange> (r));
> >    else if (is_a <frange> (r))
> > -    m_vrange = new (&m_buffer) frange (as_a <frange> (r));
> > +    m_vrange = new (&u.m_buffer) frange (as_a <frange> (r));
> >    else
> > -    m_vrange = new (&m_buffer) unsupported_range (as_a <unsupported_range> (r));
> > +    m_vrange
> > +      = new (&u.m_buffer) unsupported_range (as_a <unsupported_range> (r));
> >  }
> >
> >  // Assignment operator.  Copying incompatible types is allowed.  That
> > --
> > 2.44.0
> >
>

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From a022147e7a9a93d4fc8919aca77ed7fabc99eff7 Mon Sep 17 00:00:00 2001
From: Aldy Hernandez <aldyh@redhat.com>
Date: Fri, 3 May 2024 11:17:32 +0200
Subject: [PATCH] [ranger] Force buffer alignment in Value_Range [PR114912]

gcc/ChangeLog:

	* value-range.h (class Value_Range): Use a union.
---
 gcc/value-range.h | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/gcc/value-range.h b/gcc/value-range.h
index 934eec9e386..f124dca34be 100644
--- a/gcc/value-range.h
+++ b/gcc/value-range.h
@@ -740,9 +740,13 @@ private:
   void init (const vrange &);
 
   vrange *m_vrange;
-  // The buffer must be at least the size of the largest range.
-  static_assert (sizeof (int_range_max) > sizeof (frange), "");
-  char m_buffer[sizeof (int_range_max)];
+  union buffer_type {
+    int_range_max ints;
+    frange floats;
+    unsupported_range unsupported;
+    buffer_type () { }
+    ~buffer_type () { }
+  } m_buffer;
 };
 
 // The default constructor is uninitialized and must be initialized
@@ -750,6 +754,7 @@ private:
 
 inline
 Value_Range::Value_Range ()
+  : m_buffer ()
 {
   m_vrange = NULL;
 }
@@ -816,11 +821,11 @@ Value_Range::init (tree type)
   gcc_checking_assert (TYPE_P (type));
 
   if (irange::supports_p (type))
-    m_vrange = new (&m_buffer) int_range_max ();
+    m_vrange = new (&m_buffer.ints) int_range_max ();
   else if (frange::supports_p (type))
-    m_vrange = new (&m_buffer) frange ();
+    m_vrange = new (&m_buffer.floats) frange ();
   else
-    m_vrange = new (&m_buffer) unsupported_range ();
+    m_vrange = new (&m_buffer.unsupported) unsupported_range ();
 }
 
 // Initialize object with a copy of R.
@@ -829,11 +834,12 @@ inline void
 Value_Range::init (const vrange &r)
 {
   if (is_a <irange> (r))
-    m_vrange = new (&m_buffer) int_range_max (as_a <irange> (r));
+    m_vrange = new (&m_buffer.ints) int_range_max (as_a <irange> (r));
   else if (is_a <frange> (r))
-    m_vrange = new (&m_buffer) frange (as_a <frange> (r));
+    m_vrange = new (&m_buffer.floats) frange (as_a <frange> (r));
   else
-    m_vrange = new (&m_buffer) unsupported_range (as_a <unsupported_range> (r));
+    m_vrange = new (&m_buffer.unsupported)
+      unsupported_range (as_a <unsupported_range> (r));
 }
 
 // Assignment operator.  Copying incompatible types is allowed.  That
-- 
2.44.0