Re: subtle side effect of commit a1c48bb160f836

From: Vineet Gupta
Date: Thu Jun 18 2015 - 04:46:13 EST

On Thursday 18 June 2015 01:43 PM, Michal Marek wrote:
>> > Alternatively, as we already have CONFIG_CC_OPTIMIZE_FOR_SIZE,
>> > a(nother) Kconfig option may make sense.
> We can also introduce some ARCH_CFLAGS that is appended near the end of
> the list, and have arc/Makefile add its -O3 there. But I'd like to why
> the -O3 needs to be there in first place.

This is how historically ARC kernels have been built. We do track performance
results LMBench/hackbench... and going from -O3 to -O2 caused a sudden dip in some
of the numbers when we measured 3.18 (vs. 3.13)

> Obviously, the kernel works
> with -O2, otherwise the regression would have been identified earlier.

Its a performance thing - so yeah -O2 works, but -O3 works even better :-)

> So why can't users specify -O3 in KCFLAGS like on any other
> architecture.

Sweet, I didn't know about this. But I don't see any arch setting this - only tile
using it. So yeah - below does the trick !