[cfe-dev] [RFC] Recognize -fuse-ld={bfd, gold, lld} but don't prepend "ld." or "ld64." for other values

[Fangrui Song via cfe-dev]

On 2020-07-06, David Blaikie wrote:
>The design then is to support composing -fuse-ld and -fld-path? So
>things like -fuse-ld=gold
>-fld-path=some-thing-like-gold-but-maybe-not-quite (if I want the
>clang driver gold-specific behavior, but to use my custom linker
>binary?)?

Yes. -fuse-ld= specifies the flavor: bfd, gold, lld.
This can affect clang driver decisions on linker options.
(see https://github.com/llvm/llvm-project/blob/master/clang/lib/Driver/ToolChains/Fuchsia.cpp#L57 )

When -fuse-ld=lld is specified, you can still specify
-fld-path= to use a non-standard path, e.g.  -fuse-ld=lld -fld-path=/tmp/opt/bin/ld.lld

>On Wed, Jul 1, 2020 at 5:54 PM Fangrui Song via cfe-dev
><cfe-dev at lists.llvm.org> wrote:
>>
>> On 2020-06-16, Fangrui Song wrote:
>> >On 2020-06-03, Fangrui Song wrote:
>> >>On 2020-06-03, David Chisnall via cfe-dev wrote:
>> >>>On 02/06/2020 19:47, Fangrui Song via cfe-dev wrote:
>> >>>>If they use a relative path, they can't be benefiting from ld. prefix
>> >>>
>> >>>This depends on how they specify the path.  If they specify a path
>> >>>for the linker via -fuse-ld, they don't get the prefix.  Most
>> >>>cross-compile toolchains that I've seen don't do that.  Instead,
>> >>>they use -B to specify a path to find *all* of the tools and then
>> >>>use -fuse-ld={lld,gold,bfd,proprietary linker's name}.
>> >>>
>> >>>That is the use case I'm concerned about breaking.  The behaviour
>> >>>of -B is to prepend a search path, but still fall back to the
>> >>>default paths if a tool is not found there.  For example, if I
>> >>>have:
>> >>>
>> >>>/usr/bin/lld
>> >>>/my/custom/sdk/bin/ld.lld
>> >>>
>> >>>If I run:
>> >>>
>> >>>$ clang -B /my/custom/sdk/bin/ -fuse-ld=lld ...
>> >>>
>> >>>Today, I will run my cross-compile toolchain's linker.  With your
>> >>>proposed change, I will run my system LLD install.  As a user, I
>> >>>will see very confusing error messages from the system linker if
>> >>>it can't handle my cross-compile target.  Worse, I may see a
>> >>>binary linked with a linker that defaults to a different linker
>> >>>script, but still produces some output.
>> >>>
>> >>>David
>> >>
>> >>-fuse-ld={bfd,gold,lld} is compatible with the previous behavior.
>> >>
>> >>Do you propose that:
>> >>
>> >>* -fuse-ld={bfd,gold.lld} should find "ld.{bfd,gold,lld}" in COMPILER_PATH
>> >>* all -fuse-ld=word should find "word" in COMPILER_PATH
>> >>* -fuse-ld=relative/path should find "relative/path" in PATH?
>> >
>> >Thoughts?
>>
>> Created https://reviews.llvm.org/D83015 [Driver] Add -fld-path= and deprecate -fuse-ld=/abs/path and -fuse-ld=rel/path
>>
>> This should resolve jyknight's concern.
>>
>> I think GCC will be happy with whatever reasonable consensus we can reach:
>> https://gcc.gnu.org/pipermail/gcc-patches/2020-July/549236.html
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