RFR: JDK-8238380: java.base/unix/native/libjava/childproc.c "multiple definition" link errors with GCC10

[Patrick Zhang OS]

Hi

I split the original patch into parts for corresponding function review, here is the very simple change to java.base/unix/native/libjava/childproc.c and .h.
Could core-libs-dev group help review and sponsor this? thanks in advance.

JBS: https://bugs.openjdk.java.net/browse/JDK-8238380 (a sub-task of JDK-8235903)
Webrev: http://cr.openjdk.java.net/~qpzhang/8238380/webrev.01/
Test: ran jtreg tier1, jdk built with GCC4.8.5, GCC9, and GCC10, no regression found. I don’t have any specific function tests, so this is just a smoke test.

Regards
Patrick

From: Martin Buchholz <martinrb at google.com<mailto:martinrb at google.com>>
Sent: Monday, December 16, 2019 10:44 AM
To: Patrick Zhang OS <patrick at os.amperecomputing.com<mailto:patrick at os.amperecomputing.com>>; net-dev <net-dev at openjdk.java.net<mailto:net-dev at openjdk.java.net>>; OpenJDK <security-dev at openjdk.java.net<mailto:security-dev at openjdk.java.net>>
Cc: core-libs-dev <core-libs-dev at openjdk.java.net<mailto:core-libs-dev at openjdk.java.net>>
Subject: Re: RFR: JDK-8235903: GCC default -fno-common exposes "multiple definition" link errors

forwarded to other teams for review.

On Fri, Dec 13, 2019 at 3:14 AM Patrick Zhang OS <patrick at os.amperecomputing.com<mailto:patrick at os.amperecomputing.com>> wrote:
Hi

Please review this patch, if it should be reviewed by any group other than core-libs, please help forward it. Thanks.

JBS: https://bugs.openjdk.java.net/browse/JDK-8235903
Webrev: http://cr.openjdk.java.net/~qpzhang/8235903/webrev.01/

A recent GCC patch (supposed to be in GCC 10) exposes a couple of "multiple definition" link errors when building the jdk tip.

[PATCH] PR85678: Change default to -fno-common
https://gcc.gnu.org/ml/gcc-patches/2019-10/msg01847.html

For example, the error message looks like:
* For target support_native_java.base_libjava_BUILD_LIBJAVA_link:
build/support/native/java.base/libjava/childproc.o:(.bss+0x0): multiple definition of `parentPathv'
build/support/native/java.base/libjava/ProcessImpl_md.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status

This was not an issue because the original default -fcommon allowed "global variables defined without an initializer" be handled as COMMON symbols, so it would not warn the problem like "same variable is accidentally defined in more than one compilation unit".

About -fcommon vs -fno-cmmon:
https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fno-common

Moving forward, building jdk with latest versions of GCC will trigger this error. Specifying "--with-extra-cflags='-fcommon'"  can make it work, but it just got things hidden again.

In addition, -fcommon's behavior "is inconsistent with C++, and on many targets implies a speed and code size penalty on global variable references. It is mainly useful to enable legacy code to link without errors."

Last, in case that other jdk developers would revisit this problem once again, I suggest fixing the error explicitly instead of using "-fcommon"

Regards
Patrick