Patchwork [C++] Omit overflow check for new char[n]

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Submitter Florian Weimer
Date Oct. 31, 2012, 5:13 p.m.
Message ID <50915C48.1050305@redhat.com>
Download mbox | patch
Permalink /patch/195944/
State New
Headers show

Comments

Florian Weimer - Oct. 31, 2012, 5:13 p.m.
On 10/31/2012 10:05 AM, Jakub Jelinek wrote:

> I don't see any == double_int_one (or zero) comparisons in grep,
> so I'd say inner_size.is_one () should be used instead (which is used
> pretty frequently).  Ditto in the second spot.
> Otherwise the patch looks good to me, but I'd like Jason to chime in too.

Thanks, I'm attaching the updated patch.

Bootstrapped and tested on x86_64-redhat-linux-gnu, with no apparent 
regressions.
Florian Weimer - Oct. 31, 2012, 5:22 p.m.
On 10/31/2012 06:13 PM, Florian Weimer wrote:

> +	      if (outer_nelts_check != NULL && inner_size.is_one())

Uhm, I will add the missing space before commit.  Sorry.
Jason Merrill - Nov. 6, 2012, 7:12 p.m.
OK.

Jason

Patch

gcc/:

2012-10-31  Florian Weimer  <fweimer@redhat.com>

	* init.c (build_new_1): Do not check for arithmetic overflow if
	inner array size is 1.

gcc/testsuite/:

2012-10-31  Florian Weimer  <fweimer@redhat.com>

	* g++.dg/init/new40.C: New.

Index: gcc/cp/init.c
===================================================================
--- gcc/cp/init.c	(revision 193033)
+++ gcc/cp/init.c	(working copy)
@@ -2185,6 +2185,8 @@ 
   bool outer_nelts_from_type = false;
   double_int inner_nelts_count = double_int_one;
   tree alloc_call, alloc_expr;
+  /* Size of the inner array elements. */
+  double_int inner_size;
   /* The address returned by the call to "operator new".  This node is
      a VAR_DECL and is therefore reusable.  */
   tree alloc_node;
@@ -2346,8 +2348,6 @@ 
       double_int max_size
 	= double_int_one.llshift (TYPE_PRECISION (sizetype) - 1,
 				  HOST_BITS_PER_DOUBLE_INT);
-      /* Size of the inner array elements. */
-      double_int inner_size;
       /* Maximum number of outer elements which can be allocated. */
       double_int max_outer_nelts;
       tree max_outer_nelts_tree;
@@ -2451,7 +2451,13 @@ 
 	  if (array_p && TYPE_VEC_NEW_USES_COOKIE (elt_type))
 	    size = size_binop (PLUS_EXPR, size, cookie_size);
 	  else
-	    cookie_size = NULL_TREE;
+	    {
+	      cookie_size = NULL_TREE;
+	      /* No size arithmetic necessary, so the size check is
+		 not needed. */
+	      if (outer_nelts_check != NULL && inner_size.is_one())
+		outer_nelts_check = NULL_TREE;
+	    }
 	  /* Perform the overflow check.  */
 	  if (outer_nelts_check != NULL_TREE)
             size = fold_build3 (COND_EXPR, sizetype, outer_nelts_check,
@@ -2487,7 +2493,13 @@ 
 	  /* Use a global operator new.  */
 	  /* See if a cookie might be required.  */
 	  if (!(array_p && TYPE_VEC_NEW_USES_COOKIE (elt_type)))
-	    cookie_size = NULL_TREE;
+	    {
+	      cookie_size = NULL_TREE;
+	      /* No size arithmetic necessary, so the size check is
+		 not needed. */
+	      if (outer_nelts_check != NULL && inner_size.is_one())
+		outer_nelts_check = NULL_TREE;
+	    }
 
 	  alloc_call = build_operator_new_call (fnname, placement,
 						&size, &cookie_size,
Index: gcc/testsuite/g++.dg/init/new40.C
===================================================================
--- gcc/testsuite/g++.dg/init/new40.C	(revision 0)
+++ gcc/testsuite/g++.dg/init/new40.C	(working copy)
@@ -0,0 +1,112 @@ 
+// Testcase for overflow handling in operator new[].
+// Optimization of unnecessary overflow checks.
+// { dg-do run }
+
+#include <assert.h>
+#include <stdlib.h>
+#include <stdexcept>
+
+static size_t magic_allocation_size
+  = 1 + (size_t (1) << (sizeof (size_t) * 8 - 1));
+
+struct exc : std::bad_alloc {
+};
+
+static size_t expected_size;
+
+struct pod_with_new {
+  char ch;
+  void *operator new[] (size_t sz)
+  {
+    if (sz != expected_size)
+      abort ();
+    throw exc ();
+  }
+};
+
+struct with_new {
+  char ch;
+  with_new () { }
+  ~with_new () { }
+  void *operator new[] (size_t sz)
+  {
+    if (sz != size_t (-1))
+      abort ();
+    throw exc ();
+  }
+};
+
+struct non_pod {
+  char ch;
+  non_pod () { }
+  ~non_pod () { }
+};
+
+void *
+operator new (size_t sz) _GLIBCXX_THROW (std::bad_alloc)
+{
+  if (sz != expected_size)
+    abort ();
+  throw exc ();
+}
+
+int
+main ()
+{
+  if (sizeof (pod_with_new) == 1)
+    expected_size = magic_allocation_size;
+  else
+    expected_size = -1;
+
+  try {
+    new pod_with_new[magic_allocation_size];
+    abort ();
+  } catch (exc &) {
+  }
+
+  if (sizeof (with_new) == 1)
+    expected_size = magic_allocation_size;
+  else
+    expected_size = -1;
+
+  try {
+    new with_new[magic_allocation_size];
+    abort ();
+  } catch (exc &) {
+  }
+
+  expected_size = magic_allocation_size;
+  try {
+    new char[magic_allocation_size];
+    abort ();
+  } catch (exc &) {
+  }
+
+  expected_size = -1;
+
+  try {
+    new pod_with_new[magic_allocation_size][2];
+    abort ();
+  } catch (exc &) {
+  }
+
+  try {
+    new with_new[magic_allocation_size][2];
+    abort ();
+  } catch (exc &) {
+  }
+
+  try {
+    new char[magic_allocation_size][2];
+    abort ();
+  } catch (exc &) {
+  }
+
+  try {
+    new non_pod[magic_allocation_size];
+    abort ();
+  } catch (exc &) {
+  }
+
+  return 0;
+}