RFR: 8210416: [linux] Poor StrictMath performance due to non-optimized compilation

Gustavo Romero gromero at linux.vnet.ibm.com
Mon Sep 10 13:15:52 UTC 2018

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?

Best regards,

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