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

[Severin Gehwolf]

On Wed, 2018-09-12 at 17:58 +1000, David Holmes wrote:
> But I don't understand why the optimization setting is being tied to the 
> availability of the -ffp-contract flag?

In configure we perform a check for gcc or clang whether that flag is
supported. If it is, it would be non-empty exactly having -ffp-contract 
as value. It could be another set of flags for other arches if somebody
wanted to do the same, fwiw. In JDK 8, for example, it's "-mno-fused-
madd -fno-strict-aliasing" for ppc64:
http://hg.openjdk.java.net/jdk8u/jdk8u-dev/jdk/file/2660b127b407/make/lib/CoreLibraries.gmk#l63

We need support for that flag (or a set of flags) when we optimize
fdlibm since otherwise we would lose precision. If the flag is empty
we'd not optimize as we can't guarantee precision. That's why we tie
optimization to the availability of that flag. The expectation is for
this flag to be available on gcc/clang arches only at this point. Does
that make sense?

Thanks,
Severin