RFR: 8301869: Regression ~14% in J2dBench-bimg_misc-* in 21-b5 only on linux-aarch64 [v2]

[Erik Joelsson]

On Tue, 28 Feb 2023 06:47:37 GMT, Jayathirth D V <jdv at openjdk.org> wrote:

>> make/modules/java.desktop/lib/Awt2dLibraries.gmk line 130:
>> 
>>> 128:   BUILD_LIBAWT_debug_mem.c_CFLAGS := -w
>>> 129:   # This option improves performance of FillRect/FillOval in Java2D by 15% for some gcc
>>> 130:   LIBAWT_CFLAGS += -fpic
>> 
>> So .. acc. to the eval in JBS we can get rid of -fgcse-after-reload because it is implied by -O3
>> And Erik says -fPIC is implied by -fpic 
>> 
>> So how is -fpic different than -fPIC in a way that helps here ?
>> So far as I can tell the -fpic are using just limits the size of the global offset table that -fPIC will generate.
>> Since we had an unlimited offset table before, then -fpic won't change anything. 
>> So what's the explanation ?
>
> Thanks @prrace for your inputs.
> I dont think -fPIC is implied by -fpic. @erikj79  please clarify.
> 
> Difference between -fpic and -fPIC:
> 1) With -fPIC we have unlimited storage capacity for global offset table and with -fpic on some platforms we have limitation on this size. For aarch64 it is 28k, but in case of x86 size of GOT is unlimited irrespective of -fPIC or -fpic. This is already captured.
> 2) Also it looks like if we use -fpic option instead of -fPIC we may generate smaller/faster code as mentioned at http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html. Looks like in our case, 28k GOT size suffices and we are able to create smaller/faster code for libawt in case of aarch64. This inturn looks to be related to usage of -msmall-data or -mlarge-data flags. -fpic implies -msmall-data and -fPIC implies -mlarge-data while generating code for shared libraries. -msmall-data also talks about data being nearer and easily accessible compared to  -mlarge-data(https://man7.org/linux/man-pages/man1/gcc.1.html)
> 
> Also i can make this change specific to aarch64 since we are seeing its effect only on aarch64.

I think you have read more about the differences between -fpic and -fPIC than I have, so nothing I can add here.

Making this option only on aarch64 seems like a reasonable idea if that's where we see the effect.

-------------

PR: https://git.openjdk.org/jdk/pull/12761