master: Build failure in malloc with GCC 7

[Florian Weimer]

On 07/12/2017 02:29 PM, Andreas Schwab wrote:
> On Jul 12 2017, Florian Weimer <fweimer@redhat.com> wrote:
> 
>> The attached patch adds an assert which reveals to the GCC optimizers
>> that the global_max_fast can never MAX_FAST_SIZE, so this particular
>> issue goes away.  However, there is a cost in terms of code size because
>> it affects many places in malloc.
> 
> Can __builtin_unreachable avoid the runtime overhead?

Interesting idea.  I have never used __builtin_unreachable before, I think.

Current master:

   text    data     bss     dec     hex filename
  64592    2392     104   67088   10610 /root/build/malloc/malloc.o

My second patch:

   text    data     bss     dec     hex filename
  64424    2392     104   66920   10568 /root/build/malloc/malloc.o

Attached patch:

   text    data     bss     dec     hex filename
  64504    2392     104   67000   105b8 /root/build/malloc/malloc.o

Looking at the GIMPLE dumps, the code is somewhat improved over my
second patch.

Thanks,
Florian
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