[PATCH] Makefile: Disable PLATFORM_LIBGCC for LLVM toolchain

[Nick Desaulniers]

On Tue, Sep 27, 2022 at 9:30 AM Alistair Delva <adelva at google.com> wrote:
>
> Hi Tom,
>
> On Tue, Sep 27, 2022 at 8:57 AM Tom Rini <trini at konsulko.com> wrote:
> >
> > On Mon, Sep 26, 2022 at 08:47:47PM +0000, Alistair Delva wrote:
> >
> > > The LLVM toolchain does not have or need libgcc, so do not require
> > > it to exist on the library path. Even if "-print-libgcc-file-name"
> > > returned the empty string, -lgcc would be specified.
> > >
> > > This leaves CONFIG_USE_PRIVATE_LIBGCC alone because I did not have
> > > a target/toolchain combination available for testing.
> > >
> > > Signed-off-by: Alistair Delva <adelva at google.com>
> > > Cc: Simon Glass <sjg at chromium.org>
> > > Cc: Tom Rini <trini at konsulko.com>
> > > Cc: Nick Desaulniers <ndesaulniers at google.com>
> > > ---
> > >  Makefile | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/Makefile b/Makefile
> > > index 8af67ebd63..af06b7aa19 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -874,8 +874,10 @@ u-boot-main := $(libs-y)
> > >  ifeq ($(CONFIG_USE_PRIVATE_LIBGCC),y)
> > >  PLATFORM_LIBGCC = arch/$(ARCH)/lib/lib.a
> > >  else
> > > +ifneq ($(cc-name),clang)
> > >  PLATFORM_LIBGCC := -L $(shell dirname `$(CC) $(c_flags) -print-libgcc-file-name`) -lgcc
> > >  endif
> > > +endif
> > >  PLATFORM_LIBS += $(PLATFORM_LIBGCC)
> > >
> > >  ifdef CONFIG_CC_COVERAGE
> >
> > So this one isn't quite right, and will result in some platforms /
> > architectures just failing to build as the handful of functions we get
> > provided by CONFIG_USE_PRIVATE_LIBGCC end up being missing.

So sounds like based on the configs we build for Android, we don't
need the compiler runtime for ARCH=arm64 and ARCH=x86_64 (possibly
ARCH=arm).

LLVM's equivalent is called compiler-rt; `$ clang
--rtlib=compiler-rt`, which might need to be passed explicitly
depending on whether clang was configured at build time of clang
itself to use libgcc or compiler-rt as the implicit default --rtlib.

FWIW, the Linux kernel is a bit of a mess when it comes to compiler
runtime.  Some architecture ports require a compiler runtime, some
provide their own impl, but only for some symbols which makes
targeting them with any given compiler...interesting.

If these flags are going to the compiler as the driver, I wonder if
`--rtlib=libgcc` vs `--rtlib=none` might be more descriptive.

> >
> > It's also the case that this is a badly named option, and something we
> > really should figure out how to remove and then always provide the
> > required functions for. What configuration did you end up having this
> > problem on exactly?
>
> We have CI set up for qemu_arm64_defconfig, qemu-x86_64_defconfig,
> am57xx_evm_defconfig and rock-pi-4-rk3399_defconfig. I think all were
> affected, since they don't use CONFIG_USE_PRIVATE_LIBGCC, so they all
> hit this fallback.
>
> AFAIK, for a long time, Android used an LLVM toolchain built with some
> workarounds such as knowledge of a corresponding GCC-based toolchain,
> so -print-libgcc-file-name could provide a path to libgcc.a. However,
> our newer toolchains no longer do this and the symbols are either
> automatically provided or provided by compiler-rt. In U-Boot's case, I
> don't think our targets ever required libgcc but because the
> fall-through expands to "-L . -lgcc" when -print-libgcc-file-name
> returns the empty string, the link will fail because it will look for
> libgcc but not find it.
>
> Alistair.



-- 
Thanks,
~Nick Desaulniers