Crucial C++ inlining broken under -Os

[Taras Glek]

  On 07/01/2010 02:27 PM, Richard Guenther wrote:
> On Thu, Jul 1, 2010 at 10:29 PM, Taras Glek<tglek@mozilla.com>  wrote:
>> On 06/30/2010 03:06 PM, Jan Hubicka wrote:
>>> If you can find actual simple examples where -Os is losing size and speed
>>> we can try
>>> to do something about them.
>>>
>> According to our code size reports, inlining is completely screwed for C++
>> wrapper classes like ones often used for smart pointers, arrays, etc. See
>> http://people.mozilla.com/~tglek/codesize45.txt
>>
>> Would be really nice if this could be fixed in 4.5. It's tricky for us to
>> switch to 4.5 otherwise.
>>
>> The following code inlines as expected under -Os in 4.4. It also inlines
>> properly with -O1+ in 4.5. But it generates giant bloated code under -Os in
>> 4.5.
>>
>> class Container {
>> public:
>>   Container() {
>>     member = new int;
>>   }
>>
>>   void cleanup() {
>>     delete member;
>>     member = 0;
>>   }
>>
>>   int value() {
>>     return *member;
>>   }
>>
>>   ~Container() {
>>     cleanup();
>>   }
>> private:
>>   int *member;
>> };
>>
>>
>>
>> int gimme() {
>>   Container m;
>>   return m.value();
>> }
> Without looking I bet the issue here is call_cost at -Os (which is 1).
> In the above example we are only not inlining the constructor, which
> is estimated as size 2 (a function call with one (constant) parameter).
> Inlining that enlarges the caller as we'd replace a call without an
> argument with one with an argument.
>
> So the inlining decision isn't too bad for -Os here (which means
> your testcase isn't a good representative of what is the issue).
You are right. I tried a -finline-limit=50 flag that we used for gcc 4.1 
& 4.2 and that appears to bring performance to slightly above 4.3 levels 
with -Os.

However, this testcase that was most obvious regression from reading the 
above code size report. Seems like a pretty serious regression given that
size inline.o
   Returns 158 with -Os and 93 with -O1.
This doesn't get "fixed" by -finline-limit=50

Taras