Tweak vector::_M_realloc_insert for code size

[Marc Glisse]

Hello,

operator new can clobber memory, it is hard to teach the compiler 
otherwise since it is replaceable. Here I cache a couple values before the 
call to the allocator. I checked the result on this simple example:

#include <vector>
void f(std::vector<int>&v){ v.push_back(0); }

The patch does not affect the fast path where no reallocation is needed. 
Compiling with -O3 (everything gets inlined into f), I see a nice decrease 
in code size on x86_64

$ size old.o new.o
    text	   data	    bss	    dec	    hex	filename
     462	      0	      0	    462	    1ce	old.o
     376	      0	      0	    376	    178	new.o

Even at -O2 where _M_realloc_insert is not inlined, I get a slight 
decrease in code size (490 -> 470). On x86, that's 531 -> 519 (465 -> 
387 at -O3).

I'm not going to modify every function like that, I just happened to be 
looking at this example for other reasons, and the size gain is larger 
than I expected, so I am posting the patch.

Bootstrap+regtest on powerpc64le-unknown-linux-gnu.

2017-11-13  Marc Glisse  <marc.glisse@inria.fr>

 	* include/bits/vector.tcc (vector::_M_realloc_insert): Cache old
 	values before the allocation.

-- 
Marc Glisse
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