Re: PATCH: Fix linker plugin support for gnu linker

[Richard Sandiford <rdsandiford at googlemail dot com>]

"H.J. Lu" <hongjiu.lu@intel.com> writes:
> This patch fixes linker plugin support for gnu linker with 2 stage
> linking and supports mixed LTO objects:
>
> http://www.kernel.org/pub/linux/devel/gcc/lto/mixed-IR/mixed-IR.pdf
>
> Any objections?

One problem with the patch as written is that it doesn't cope with
the kind of embedded setup in which the startup file is specified
in a linker script.  E.g. on mipsisa64-elf, GCC doesn't know what
system you're targetting, and therefore doesn't what startup files or
board-specific libraries are needed.  You instead link with something
like -Tidt64.ld.  idt64.ld then contains:

STARTUP(crt0.o)

-T options are parsed immediately in-place, and don't show up in the
cmdline_options.  Things like STARTUP don't either.  This means that
crt0.o is dropped by the second stage link.

The handling of -T also means that the system libraries:

GROUP(-lc -lidt -lgcc)

don't show up in the second link.  However, that brings up a broader
question (which might already have been answered, sorry).  The current
process -- that is, the process before the 2-stage patch -- seems to
rely on the GCC driver passing the equivalent of:

  -pass-through=-lgcc -pass-through=-lc -pass-through=-lgcc

to the plugin.  But on targets like mipsisa64-elf, GCC doesn't know
the correct C library sequence (-lc -lidt in the case of idt64.ld),
so LIB_SPEC is "".  The only options that GCC passes are therefore:

  -pass-through=-lgcc -pass-through=-lgcc

It seems to be necessary to pass:

  -Wl,-plugin-opt=-pass-through=-lc
  -Wl,-plugin-opt=-pass-through=-lidt
  -Wl,-plugin-opt=-pass-through=-lgcc

in addition to -Tidt64.ld.  Is that right, or am I missing something?
If the former, and if the -T problem with the 2-stage patch could be
fixed, perhaps 2-stage linking might solve this problem for free.

Richard