On Mon, Jun 12, 2017 at 03:56:25PM +0200, Richard Biener wrote:
> On Mon, 12 Jun 2017, James Greenhalgh wrote:
>
> >
> > Hi,
> >
> > As subject, for the testcase in the patch:
> >
> > unsigned long
> > f2 (unsigned long a, int b)
> > {
> > unsigned long x = 1UL << b;
> > return a / x;
> > }
> >
> > We currently generate:
> >
> > f2:
> > mov x2, 1
> > lsl x1, x2, x1
> > udiv x0, x0, x1
> > ret
> >
> > Which could instead be transformed to:
> >
> > f2:
> > lsr x0, x0, x1
> > ret
> >
> > OK?
>
> + 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.
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?
Bootstrapped and tested on aarch64-none-linux-gnu with no issues.
OK?
Thanks,
James
---
gcc/
2017-06-13 James Greenhalgh <james.greenha...@arm.com>
* match.pd (A / (1 << B) -> A >> B): New.
* generic-match-head.c: Include optabs-tree.h.
* gimple-match-head.c: Likewise.
gcc/testsuite/
2017-06-13 James Greenhalgh <james.greenha...@arm.com>
* gcc.dg/tree-ssa/forwprop-37.c: New.
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..2bea268 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -147,6 +147,18 @@ 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)
+ && (!VECTOR_TYPE_P (type)
+ || optab_for_tree_code (RSHIFT_EXPR, type, optab_vector)
+ || optab_for_tree_code (RSHIFT_EXPR, type, optab_scalar)))
+ (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/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" } } */
