RFR: JDK-8217723 Switch ld from bfd to gold on gcc toolchain

[Magnus Ihse Bursie]

I agree that the relationship between compiler, linker and OS is problematic. In the OpenJDK build system, we have used the concept of "toolchain", by which we mean (amongst other things) a compiler and a linker working together. For some platforms it's a perfect fit (eg Visual Studio), for others like gcc it's so-so, for yet others (solstudio) it's really bad. 

So, we already tacitly assume a specific linker with the gcc toolchain; we have just not made that check explicitly. 

As I said in a previous email, part of the reason for this switch is to allow --incremental. So for that reason, we must know that the linker supports that option. 

/Magnus

> 24 jan. 2019 kl. 15:48 skrev Martin Buchholz <martinrb at google.com>:
> 
> Getting into the business of choosing the linker is big trouble.
> 
> The system default toolchain may have already chosen a linker.
> 
> BFD might be configured to have either bfd ld or gold.
> # Handle --enable-gold, --enable-ld.
> # --disable-gold [--enable-ld]
> #     Build only ld.  Default option.
> # --enable-gold [--enable-ld]
> #     Build both gold and ld.  Install gold as "ld.gold", install ld
> #     as "ld.bfd" and "ld".
> # --enable-gold=default [--enable-ld]
> #     Build both gold and ld.  Install gold as "ld.gold" and "ld",
> #     install ld as "ld.bfd".
> # --enable-gold[=default] --disable-ld
> #     Build only gold, which is then installed as both "ld.gold" and "ld".
> # --enable-gold --enable-ld=default
> #     Build both gold (installed as "ld.gold") and ld (installed as "ld"
> #     and ld.bfd).
> #     In other words, ld is default
> 
> The compiler driver itself may have been configured to choose a linker.
> 
> The system administrator may have used update-alternatives to choose a linker.
> 
> A user might have configured openjdk to use -fuse-ld=... (we do this!)
> How do you resolve the conflict?
> 
> There's evidence that lld is even faster than gold.  The Internet says,
> """Liked linking 3x faster with gold? Link 10x faster with lld!"""
> so hardcoding gold might be a regression!
> 
> So ... -fuse-ld is a flag that is perfect for a local wrapper around
> configure that is __not__ part of openjdk (we do this!)