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Re: operator new returns nonzero

On Sat, 7 Sep 2013, Gabriel Dos Reis wrote:

On Sat, Sep 7, 2013 at 11:42 AM, Marc Glisse <> wrote:
On Sat, 7 Sep 2013, Marc Glisse wrote:

this patch teaches the compiler that operator new, when it can throw,
isn't allowed to return a null pointer. Note that it doesn't work for
placement new (that one may have to be handled in the front-end or the

Placement new is a completely different thing. For one, it is nothrow (so
the test doesn't apply). But mostly, it is a condition on the argument more
than the result. Using the nonnull attribute would make sense, but the
compiler doesn't seem clever enough to use that information. The easiest
seems to be in the library:

--- o/new       2013-09-07 11:11:17.388756320 +0200
+++ i/new       2013-09-07 18:00:32.204797291 +0200
@@ -144,9 +144,9 @@

 // Default placement versions of operator new.
 inline void* operator new(std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
-{ return __p; }
+{ if (!__p) __builtin_unreachable(); return __p; }
 inline void* operator new[](std::size_t, void* __p) _GLIBCXX_USE_NOEXCEPT
-{ return __p; }
+{ if (!__p) __builtin_unreachable(); return __p; }

 // Default placement versions of operator delete.
 inline void operator delete  (void*, void*) _GLIBCXX_USE_NOEXCEPT { }

Though I am not sure what the policy is for this kind of thing. And that's
assuming it is indeed undefined to pass a null pointer to it, I don't have a
good reference for that.

and it will not work on user-defined operator new if they are inlined.

But then if that operator new is inlined, it may already contain a line like
if(p==0)throw(); which lets the compiler know all it needs.

I am not sure I like this version of the patch.

The 2 patches are really independent, one (in the compiler) for regular operator new, and one (in the library) for the placement version.

I mostly like this second patch because it is so easy ;-)
But I will be happy if someone posts a better solution.

I think the appropriate patch should be in the compiler, not
here.  C++ has several places where certain parameters cannot
have non-null values. For example, the implicit parameter 'this'
in non-static member functions. This is another instance.

Indeed, I didn't check how gcc handles "this" being non-0.

Furthermore, I do think that the compiler should have special nodes
for both standard placement new and the global operator new functions,

That's one way to do it. Since this is the first time I look at those, I don't really see the advantage compared to the current status, but I trust you. What would you do with this special-node placement new? Keep it as is until after vrp so we can use the !=0 property and then expand it to its first argument? Or expand it early to the equivalent of the library code I wrote above?

as I explained in previous messages.

I couldn't find them, sorry if they contained information that makes this post irrelevant.

Marc Glisse

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