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

[Fāng-ruì Sòng via cfe-dev]

On Mon, Jul 6, 2020 at 2:32 PM Dimitry Andric <dimitry at andric.com> wrote:
>
> As far as I know, -fuse-ld has been copied from gcc, which documents it here: https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html#index-fuse-ld_003dbfd. Obviously we can make any alias we want, but the gcc spelling should still be supported.
>
> -Dimitry

> On 6 Jul 2020, at 21:52, Petr Hosek via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>
> -f<flag> options are typically used for flags that control code generation, that's what https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html says as well. Neither of these flags are related to code generation. Have you considered using just -use-ld= and -ld-path= and deprecating -fuse-ld= altogether? I think that would also be more consistent with other --*-path flags we have such as --cuda-path= or ptxas-path=.

Renamed -fld-path= to --ld-path= in https://reviews.llvm.org/D83015
The -f difference may be too small to warrant an option name change
for -fuse-ld= . I'll keep it for now.

There are some discussions about -B/COMPILER_PATH vs PATH/CWD. I
currently think PATH/CWD makes more sense

> On Mon, Jul 6, 2020 at 12:36 PM Fangrui Song via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>>
>> 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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