[8u] RFR: 8210416: [linux] Poor StrictMath performance due to non-optimized compilation

Baesken, Matthias matthias.baesken at sap.com
Tue Apr 30 12:52:02 UTC 2019

Hi  Severin,   looks okay to me (not a reviewer however).
In our  proprietary  JVM8  we had    set  (on Linux)    BUILD_LIBFDLIBM_OPTIMIZATION := HIGH   and     FDLIBM_CFLAGS += -ffp-contract=off    for a long time and did not observe any issues .
(but we always build with gcc 4.8.x  so cannot tell much about lower gcc versions ).

But I really wonder - wouldn’t it  be better in the long run to go for a min.  gcc versions  >= 4.6   (or even 4.8)   in OpenJDK8  ?

Maybe some people  from the distros  could  comment on  this .

Best regards, Matthias

> -----Original Message-----
> From: ppc-aix-port-dev <ppc-aix-port-dev-bounces at openjdk.java.net> On
> Behalf Of Severin Gehwolf
> Sent: Dienstag, 30. April 2019 13:38
> To: jdk8u-dev <jdk8u-dev at openjdk.java.net>; build-dev <build-
> dev at openjdk.java.net>
> Cc: ppc-aix-port-dev <ppc-aix-port-dev at openjdk.java.net>
> Subject: [8u] RFR: 8210416: [linux] Poor StrictMath performance due to non-
> optimized compilation
> Hi,
> Could I please get a review for this 8u backport related to fdlibm
> optimization on Linux? The JDK 12 patch doesn't apply as-is as the JDK
> 8 build system is drastically different from JDK 11+.
> Bug: https://bugs.openjdk.java.net/browse/JDK-8210416
> webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-
> 8210416/jdk8/02/
> The main differences to the original fix are: a) optimization level and
> b) additional hook for older GCC. This backport keeps the optimization
> level at -O3 (HIGH) over -O2 (LOW) for JDK 8u as this would otherwise
> regress Linux ppc64{,le} which currently use -O3. As the current code
> has the implicit assumption of ppc64 being compiled on older GCCs too
> (JDK-8172053), this backport maintains compatibility in this regard. If
> -ffp-contract=off is not available, a machine specific set of flags is
> being used if the compiler supports them (-mno-fused-madd -fno-strict-
> aliasing).
> For older GCCs (< 4.6) specific machine flags are being used. That is,
> for ppc64{,le} and x86{,_64}. ppc64{,le} machine specific flags have
> already been determined (See JDK-8172053). x86_64 and x86 have the same
> machine specific flags available, so I've used them there too[1].
> Testing: build/test on gcc 8.x Linux x86_64. build/test on gcc 4.4.7
> x86_64/ppc64. Manual inspection of build logs for fdlibm files (e.g.
> w_asin.c).
> Thoughts?
> Thanks,
> Severin
> [1] https://gcc.gnu.org/onlinedocs/gcc-4.4.7/gcc/i386-and-x86_002d64-
> Options.html#i386-and-x86_002d64-Options

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