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Re: Overwhelmed by GCC frustration


On Wed, Aug 2, 2017 at 3:54 PM, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
> On Tue, Aug 01, 2017 at 01:50:14PM +0200, David Brown wrote:
>> I would not expect that to be good at all.  With no optimisation (-O0),
>> gcc produces quite poor code - local variables are not put in registers
>> or "optimised away", there is no strength reduction, etc.  For an
>> architecture like the AVR with a fair number of registers (32, albeit
>> 8-bit registers) and relatively inefficient stack access, -O0 produces
>> /terrible/ code.
>
> -Og is better though (better than any other -O for this test at least).
>
> The regression happened before 4.7, it seems the big jump was with 4.6?
> So what happened there?  This seems to happen on x86 as well, maybe
> on everything.

And one function (of the two) shrinks compared to 3.4 and the other increases
so the jumps are probably mis-bisected anyway.

Richard.

>
> Segher


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