[v3] Slightly improve operator new

Marc Glisse marc.glisse@inria.fr
Sat May 17 16:30:00 GMT 2014


Ping.

On Tue, 15 Apr 2014, Marc Glisse wrote:

> Ping
> http://gcc.gnu.org/ml/gcc-patches/2014-03/msg00051.html
>
>
> On Sun, 2 Mar 2014, Marc Glisse wrote:
>
>> Hello,
>> 
>> inlining operator new (with LTO or otherwise), I noticed that it has a 
>> complicated implementation, which makes it hard to use this inlined code 
>> for optimizations. This patch does two things:
>> 
>> 1) there are 2 calls to malloc, I am turning them into just one. At -Os, it 
>> does not change the generated code (RTL optimizers manage to merge the 
>> calls to malloc). At other levels (-O2, -O3, and especially with -g) it 
>> gives a smaller object file. And with just one malloc, some optimizations 
>> become much easier (see my recent calloc patch for instance).
>> 
>> 2) malloc is predicted to return null 19 times out of 20 because of the 
>> loop (that didn't change with the patch), so I am adding __builtin_expect 
>> to let gcc optimize the fast path.
>> 
>> Further discussion:
>> 
>> a) I didn't add __builtin_expect for the test (sz == 0), it didn't change 
>> the generated code in my limited test. I was wondering if this test is 
>> necessary (new doesn't seem to ever call operator new(0)) or could be moved 
>> to operator new[] (new type[0] does call operator new[](0)), but since one 
>> can call operator new directly, it has to be protected indeed, so let's 
>> forget this point ;-)
>> (too bad malloc is replacable, so we can't use the fact that glibc already 
>> does the right thing)
>> 
>> b) I have a bit of trouble parsing the standard. Is the nothrow operator 
>> new supposed to call the regular operator new? In particular, if a user 
>> replaces only the throwing operator new, should the nothrow operator new 
>> automatically call that function? That's not what we are currently doing 
>> (and it would be a perf regression).
>> 
>> "Required behavior: Return a non-null pointer to suitably aligned storage 
>> (3.7.4), or else return a null pointer. This nothrow version of operator 
>> new returns a pointer obtained as if acquired from the (possibly replaced) 
>> ordinary version. This requirement is binding on a replacement version of 
>> this function.
>> 
>> Default behavior: Calls operator new(size). If the call returns normally, 
>> returns the result of that call. Otherwise, returns a null pointer."
>> 
>> 
>> 
>> Passes bootstrap+testsuite on x86_64-linux-gnu. Stage 1?
>> 
>> 2014-03-03  Marc Glisse  <marc.glisse@inria.fr>
>>
>> 	* libsupc++/new_op.cc: Factor the calls to malloc, use 
>> __builtin_expect.
>> 	* libsupc++/new_opnt.cc: Likewise.

-- 
Marc Glisse



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