Re: [Patch match.pd] Fold (A / (1 << B)) to (A >> B)

[James Greenhalgh <james.greenhalgh@arm.com>]

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On Fri, Jun 16, 2017 at 11:41:57AM +0200, Richard Biener wrote:
> On Fri, 16 Jun 2017, James Greenhalgh wrote:
> > On Mon, Jun 12, 2017 at 03:56:25PM +0200, Richard Biener wrote:
> > > +   We can't do the same for signed A, as it might be negative, which
> > > would
> > > +   introduce undefined behaviour.  */
> > >
> > > huh, AFAIR it is _left_ shift of negative values that invokes
> > > undefined behavior.
> >
> > You're right this is not a clear comment. The problem is not undefined
> > behaviour, so that text needs to go, but rounding towards/away from zero
> > for signed negative values. Division will round towards zero, arithmetic
> > right shift away from zero. For example in:
> >
> >     -1 / (1 << 1)   !=    -1 >> 1
> >   = -1 / 2
> >   = 0                     = -1
> >
> > I've rewritten the comment to make it clear this is why we can only make
> > this optimisation for unsigned values.
>
> Ah, of course.  You could use
>
>  if ((TYPE_UNSIGNED (type)
>       || tree_expr_nonnegative_p (@0))
>
> here as improvement.

Thanks, I've made that change.

> > See, for example, gcc.c-torture/execute/pr34070-2.c
> >
> > > Note that as you are accepting vectors you need to make sure the
> > > target actually supports arithmetic right shift of vectors
> > > (you only know it supports left shift and division -- so it might
> > > be sort-of-superfluous to check in case there is no arch that supports
> > > those but not the other).
> >
> > I've added a check for that using optabs, is that the right way to do this?
>
> +      && (!VECTOR_TYPE_P (type)
> +          || optab_for_tree_code (RSHIFT_EXPR, type, optab_vector)
> +          || optab_for_tree_code (RSHIFT_EXPR, type, optab_scalar)))
>
> is not enough -- you need sth like
>
>  optab ot = optab_for_tree_code (RSHIFT_EXPR, type, optab_vector);
>  if (ot != unknown_optab
>      && optab_handler (ot, TYPE_MODE (type)) != CODE_FOR_nothing)
>    .. ok! ...
>
> ideally we'd have a helper for this in optab-tree.[ch],
> tree-vect-patterns.c could also make use of that.

OK. I've added "target_has_vector_rshift_p" for this purpose.

Bootstrapped and tested on aarch64-none-linux-gnu with no issues.

OK?

Thanks,
James

---
gcc/

2017-06-19  James Greenhalgh  <james.greenhalgh@arm.com>

	* match.pd (A / (1 << B) -> A >> B): New.
	* generic-match-head.c: Include optabs-tree.h.
	* gimple-match-head.c: Likewise.
	* optabs-tree.h (target_has_vector_rshift_p): New.
	* optabs-tree.c (target_has_vector_rshift_p): New.

gcc/testsuite/

2017-06-19  James Greenhalgh  <james.greenhalgh@arm.com>

	* gcc.dg/tree-ssa/forwprop-37.c: New.


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diff --git a/gcc/generic-match-head.c b/gcc/generic-match-head.c
index 0c0d182..4504401 100644
--- a/gcc/generic-match-head.c
+++ b/gcc/generic-match-head.c
@@ -33,6 +33,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "builtins.h"
 #include "case-cfn-macros.h"
 #include "gimplify.h"
+#include "optabs-tree.h"
 
 
 /* Routine to determine if the types T1 and T2 are effectively
diff --git a/gcc/gimple-match-head.c b/gcc/gimple-match-head.c
index e7e9839..5f6aa27 100644
--- a/gcc/gimple-match-head.c
+++ b/gcc/gimple-match-head.c
@@ -39,6 +39,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "internal-fn.h"
 #include "case-cfn-macros.h"
 #include "gimplify.h"
+#include "optabs-tree.h"
 
 
 /* Forward declarations of the private auto-generated matchers.
diff --git a/gcc/match.pd b/gcc/match.pd
index 244e9eb..eb6bd59 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -147,6 +147,17 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
     (op @0 integer_onep)
     (non_lvalue @0)))
 
+/* (A / (1 << B)) -> (A >> B).
+   Only for unsigned A.  For signed A, this would not preserve rounding
+   toward zero.
+   For example: (-1 / ( 1 << B)) !=  -1 >> B.  */
+(simplify
+ (trunc_div @0 (lshift integer_onep@1 @2))
+ (if ((TYPE_UNSIGNED (type) || tree_expr_nonnegative_p (@0))
+      && (!VECTOR_TYPE_P (type)
+	  || target_has_vector_rshift_p (type, optab_default)))
+  (rshift @0 @2)))
+
 /* Preserve explicit divisions by 0: the C++ front-end wants to detect
    undefined behavior in constexpr evaluation, and assuming that the division
    traps enables better optimizations than these anyway.  */
diff --git a/gcc/optabs-tree.c b/gcc/optabs-tree.c
index 4bb54ba..4a513d2 100644
--- a/gcc/optabs-tree.c
+++ b/gcc/optabs-tree.c
@@ -376,3 +376,24 @@ init_tree_optimization_optabs (tree optnode)
       ggc_free (tmp_optabs);
     }
 }
+
+/* Return TRUE if the target has support for vector right shift of an
+   operand of type TYPE.  If OT_TYPE is OPTAB_DEFAULT, check for existence
+   of a shift by either a scalar or a vector.  Otherwise, check only
+   for a shift that matches OT_TYPE.  */
+
+bool
+target_has_vector_rshift_p (tree type, enum optab_subtype ot_type)
+{
+  gcc_assert (VECTOR_TYPE_P (type));
+  if (ot_type != optab_default)
+    {
+      optab ot = optab_for_tree_code (RSHIFT_EXPR, type, optab_vector);
+      return ot != unknown_optab
+	&& optab_handler (ot, TYPE_MODE (type)) != CODE_FOR_nothing;
+    }
+  else
+    return target_has_vector_rshift_p (type, optab_scalar)
+	   || target_has_vector_rshift_p (type, optab_vector);
+}
+
diff --git a/gcc/optabs-tree.h b/gcc/optabs-tree.h
index d0b27e0..958a701 100644
--- a/gcc/optabs-tree.h
+++ b/gcc/optabs-tree.h
@@ -41,5 +41,6 @@ bool supportable_convert_operation (enum tree_code, tree, tree, tree *,
 bool expand_vec_cmp_expr_p (tree, tree, enum tree_code);
 bool expand_vec_cond_expr_p (tree, tree, enum tree_code);
 void init_tree_optimization_optabs (tree);
+bool target_has_vector_rshift_p (tree type, enum optab_subtype);
 
 #endif
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/forwprop-37.c b/gcc/testsuite/gcc.dg/tree-ssa/forwprop-37.c
new file mode 100644
index 0000000..dec826c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/forwprop-37.c
@@ -0,0 +1,25 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-forwprop1-raw" } */
+
+unsigned int
+f1 (unsigned int a, unsigned int b)
+{
+  unsigned int x = 1U << b;
+  return a / x;
+}
+
+unsigned long
+f2 (unsigned long a, int b)
+{
+  unsigned long x = 1UL << b;
+  return a / x;
+}
+
+unsigned long long
+f3 (unsigned long long a, int b)
+{
+  unsigned long long x = 1ULL << b;
+  return a / x;
+}
+
+/* { dg-final { scan-tree-dump-not "trunc_div_expr" "forwprop1" } } */