[LLD][ELF][ARM] Accept and ignore -p and -no-pipeline-knowledge

[peter.smith]

Summary

The linux kernel uses an old flag -p/-no-pipeline-knowledge that is accepted by bfd and gold but ignored by modern versions of them. The original option is very old and is pre-ABI, it sometimes comes up in code-bases that had support for pre ABI toolchains. Linux uses the option in its shorter -p form in 3 places arch/arm/Makefile arch/arm/boot/bootp/Makefile and arch/arm/boot/compressed/Makefile and a defconfig link will fail with LLD unless I manually remove it. Given that it is simple to accept and ignore I thought it would be worth doing that for increased GNU compatibility.

Gold documentation of options https://manpages.debian.org/stretch/binutils-arm-linux-gnueabi/arm-linux-gnueabi-ld.gold.1.en.html search for no-pipeline-knowledge, the -p option is a short form alias. It is marked as ignored for compatibility.

[grimar]

I have no objections.

[ruiu]

LGTM

Perhaps we should remove the usage of these flags from the Linux kernel even after this patch has landed because they are just dead flags, but this patch per se looks good to me.

[MaskRay]

The three places were removed https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=091bb549f7722723b284f63ac665e2aedcf9dec9

This has been a no-op in binutils since 2004 (see commit dea514f51da1 in that tree). Given that the lowest officially supported of binutils for the kernel is 2.20, which was released in 2009, nobody needs this flag around so just remove it. Commit 1a381d4a0a9a ("arm64: remove no-op -p linker flag") did the same for arm64.

Maybe we can delete the the compatibility code in a future release, lld 10, or 11?

[peter.smith]

In D58540#1646340, @MaskRay wrote:

The three places were removed https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=091bb549f7722723b284f63ac665e2aedcf9dec9

This has been a no-op in binutils since 2004 (see commit dea514f51da1 in that tree). Given that the lowest officially supported of binutils for the kernel is 2.20, which was released in 2009, nobody needs this flag around so just remove it. Commit 1a381d4a0a9a ("arm64: remove no-op -p linker flag") did the same for arm64.

Maybe we can delete the the compatibility code in a future release, lld 10, or 11?

Sounds reasonable, I don't think that there is an existing community that have standardized a linux version and are varying their LLVM release on ARM, and I've not seen this option anywhere else. Having said that this does raise the issue of deprecation, in an ideal world when we want to take an option out in release N then we should have the option print a "This option is deprecated and will be removed from the next release." in release N - 1. That way we should flush out anyone depending on the option.

lld/trunk/ELF/Options.td

 def: F<"detect-odr-violations">;
 def: Flag<["-"], "g">;
 def: F<"long-plt">;
 def: F<"no-add-needed">;
 def: F<"no-copy-dt-needed-entries">;
 def: F<"no-ctors-in-init-array">;
 def: F<"no-keep-memory">;
 def: F<"no-mmap-output-file">;
+def: F<"no-pipeline-knowledge">;
 def: F<"no-warn-mismatch">;
+def: Flag<["-"], "p">;
 def: Separate<["--", "-"], "rpath-link">;
 def: J<"rpath-link=">;
 def: F<"sort-common">;
 def: F<"stats">;
 def: F<"warn-execstack">;
 def: F<"warn-once">;
 def: F<"warn-shared-textrel">;
 def: F<"EB">;
 def: F<"EL">;
 def: JoinedOrSeparate<["-"], "G">;
 def: F<"Qy">;
 
 // Hidden option used for testing MIPS multi-GOT implementation.
 defm mips_got_size:
   Eq<"mips-got-size", "Max size of a single MIPS GOT. 0x10000 by default.">,
   Flags<[HelpHidden]>;

lld/trunk/test/ELF/silent-ignore.test

 RUN: ld.lld --version \
 RUN:   -allow-shlib-undefined \
 RUN:   -g \
 RUN:   -no-add-needed \
 RUN:   -no-allow-shlib-undefined \
 RUN:   -no-copy-dt-needed-entries \
 RUN:   -no-ctors-in-init-array \
 RUN:   -no-keep-memory \
+RUN:   -no-pipeline-knowledge \
 RUN:   -no-warn-common \
 RUN:   -no-warn-mismatch \
+RUN:   -p \
 RUN:   -sort-common \
 RUN:   -stats \
 RUN:   -warn-execstack \
 RUN:   -warn-once \
 RUN:   -warn-shared-textrel \
 RUN:   -EB \
 RUN:   -EL \
 RUN:   -Qy
 RUN: not ld.lld --version --not-an-ignored-argument