Re: [wwwdocs] Improve example at https://gcc.gnu.org/gcc-6/porting_to.html#flifetime-dse

[Jonathan Wakely]

On 16/08/16 05:16 +0000, Bernd Edlinger wrote:

Hi Jonathan,

I think this would be an improvement, although I still can't get the
assertion to fail:

Probably because the memory is still initialized to zero,
when it is used for the first time.

Try this:

#include <stdlib.h>
#include <string.h>
#include <assert.h>

struct A
{
A() {}

void* operator new(size_t s)
{
  void* ptr = malloc(s);
  memset(ptr, 0xFF, s);
  return ptr;
}

void operator delete(void* ptr) { free(ptr); }

int value;
};

int main()
{
A* a =  new A;
assert(a->value == -1); /* Use of uninitialized value */
delete a;
}

I've committed Bernd's improved example that demonstrates the new
optimization.

Thanks for everyone's input.


Index: gcc-6/porting_to.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-6/porting_to.html,v
retrieving revision 1.24
diff -u -r1.24 porting_to.html
--- gcc-6/porting_to.html	15 Aug 2016 17:32:29 -0000	1.24
+++ gcc-6/porting_to.html	18 Aug 2016 08:58:33 -0000
@@ -357,29 +357,25 @@
 
 struct A
 {
-  A () {}
-  void *operator new (size_t s)
-  {
-    void *ptr = malloc (s);
-    memset (ptr, 0, s);
-    return ptr;
-  }
+ A() {}
 
-  int value;
-};
+ void* operator new(size_t s)
+ {
+   void* ptr = malloc(s);
+   memset(ptr, 0xFF, s);
+   return ptr;
+ }
 
-A *
-__attribute__ ((noinline))
-build (void)
-{
-  return new A ();
-}
+ void operator delete(void* ptr) { free(ptr); }
+
+ int value;
+};
 
 int main()
 {
-  A *a =  build ();
-  assert (a->value == 0); /* Use of uninitialized value */
-  free (a);
+ A* a =  new A;
+ assert(a->value == -1); // Use of uninitialized value
+ delete a;
 }
 </code></pre>