Hi Gustavo, On Mon, 2018-09-10 at 10:15 -0300, Gustavo Romero wrote: > Hi Severin, > > On 09/10/2018 06:27 AM, Severin Gehwolf wrote: > > On Mon, 2018-09-10 at 10:05 +0100, Andrew Haley wrote: > > > On 09/05/2018 02:12 PM, Severin Gehwolf wrote: > > > > Is there a good > > > > reason to not use -O3 -ffp-contract=off everywhere? > > > > > > Is there a good reason to use -O3 rather than -O2? > > > > Not sure. I was following what JDK-8170153 did, which was using > > OPTIMIZATION := HIGH corresponding to -O3. cc'ing Gustavo. Gustavo, > > would you know why HIGH was chosen over, LOW? > > I don't remember exactly, but at least for ppc64 I discussed that a bit with > the toolchain folks (also regarding the precision issue, etc) and they never > said anything against using -O3. Unfortunately it was long time ago so I > don't remember exactly the numbers on ppc64 for -O2 to check if it was > worse and so I selected -O3 instead. > > > > > -O3 can bloat the > > > code which can increase cache pressure, which is not always noticeable > > > in benchmarks but hurts real-world programs. Unless benchmarks are > > > significantly better at -O3, -O2 is a good default choice. > > > > OK, thanks! I'll re-test and change to LOW (-O2) if it gives similar > > results. > > That's interesting. Andrew, do you mean bloat in the sense of final code size > (for instance, due to unrolling), right? > > BTW (I just remembered that), on RISC the lack of optimization hurts way more > than the lack of optimization on CISC, so I recall that it puzzled me the fact > that turning on the optimization on x86_64 did not change much the scenario, > contrary to the conspicuous gains on on ppc64 when turning on the > optimization. > I took me some time so to understand that the optimization flag was the > culprit > (a much simpler case lucky), because I tried first to profile and optimize the > fdlibm code (after extracting it from JVM for detailed analysis) and only > after > getting to a dead end I turned to look at simpler causes. > > Are you checking the difference between -O2 and -O3 only on x86_64?
So far yes. I'll see if I can get access to a ppc64 machine to check there as well. The plan is to run (some) TCK on a patched x86_64 build too. Thanks, Severin
