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Re: operator new returns nonzero


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 inliner),

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 guess it would be possible to teach the inliner to insert an assert_expr or something when inlining such a function, but that's not for now. The code I removed from vrp_visit_stmt was duplicated in stmt_interesting_for_vrp and thus useless.

I ran the c,c++ testsuite on a non-bootstrap compiler. I'll do more proper testing when trunk bootstraps again.

2013-09-07  Marc Glisse  <marc.glisse@inria.fr>

	PR c++/19476
gcc/
	* fold-const.c (tree_expr_nonzero_warnv_p): Handle operator new.
	* tree-vrp.c (gimple_stmt_nonzero_warnv_p, stmt_interesting_for_vrp):
	Likewise.
	(vrp_visit_stmt): Remove duplicated code.

gcc/testsuite/
	* g++.dg/tree-ssa/pr19476-1.C: New file.
	* g++.dg/tree-ssa/pr19476-2.C: Likewise.

--
Marc Glisse


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