Thanks all for your review comments. To measure the actual performance difference between O2 and O3, I did the following tests:
Environments: * Hardware: AMD Athlon II X2 270 Processor, 3.4 GHz * Host OS: Fedora25 X86_64 * qemu: 2.8.0 * gcc: 6.2.1 * Guest OS: linux-0.2.img (downloaded from http://wiki.qemu.org/Testing/System_Images) * Benchmark: nbench within linux-0.2.img Results: Each test has been run at least 3 times to minimize the deviation. Higher score is better. | | O2 | O3 | ------------------------------------------------ | qemu-system-i386(stripped) | 8.1 MB | 8.9 MB | | nbench(NUMERIC SORT) | 254.72 | 260.48 | | | 259.2 | 259.52 | | | 261.04,| 259.68 | Conclusion: Compared with O2, O3 brings no obvious performance advantage. It's highly possible that the benchmark score is mainly determined by the quality of tcg, not the host compiler. Related work: (1) The same test is done on a Loongson 3A2000. It's a mips64el-compatible CPU, with 4 cores running at 1.5 GHz. The result and conclusion are almost the same with X86. (2) Another scene of OS booting is taken into consideration. I compare the booting time of linux-0.2.img under O2 and O3. Once again, I cannot see an observable difference. Based on these data I admit my patch has no benifit for performance. Jin Guojie ------------------ Original ------------------ From: "Peter Maydell";<peter.mayd...@linaro.org>; Send time: Monday, Jan 9, 2017 6:33 PM To: "Jin Guojie"<jinguo...@loongson.cn>; Cc: "QEMU Developers"<qemu-devel@nongnu.org>; "Paolo Bonzini"<pbonz...@redhat.com>; Subject: Re: [Qemu-devel] [PATCH 0/1] Change default CFLAGS to -O3 On 9 January 2017 at 10:01, Jin Guojie <jinguo...@loongson.cn> wrote: > * in configure, the default optimization level is changed to O3. > * Tested successfully on following machines: > > | Host machine | Host OS | qemu-system | Debian ISO | > |-----------------------------------------------------| > | x86 64 | Fedora25 | i386 | i386 | > | mips 64 le | Fedora25 | i386 | i386 | > > Signed-off-by: Jin Guojie <jinguo...@loongson.cn> > > Jin Guojie (1): > Change default CFLAGS to -O3 > > configure | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) ...so what are the measured performance improvements from this change? As I understand it -O3 is where the "trade off code size for speed" optimisations kick in, so we need to benchmark to be sure that the increased icache usage doesn't outweigh any speed gains. -O3 is not guaranteed to be strictly better than -O2 (which is why for instance Debian and Gentoo recommend -O2 as their default). thanks -- PMM
