[PATCH] Makefile: Disable PLATFORM_LIBGCC for LLVM toolchain

[Tom Rini]

On Tue, Sep 27, 2022 at 09:29:51AM -0700, Alistair Delva 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.
> >
> > 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.

OK, that sounds familiar now that you say it, and part of why rpi_arm64
+ LLVM isn't part of my local testing loop. I believe I had been
thinking the best path forward was to port ashldi3.S and friends from
arch/arm64 in the Linux kernel over, so that USE_PRIVATE_LIBGCC (which
is a bad name!) could also be select'd on CONFIG_ARM64. That would just
leave x86, which just needs a div64 implementation for 64bit? I'm not
entirely sure.

All of that said, it might also be the case we don't need the fallback
to using libgcc at all, in most cases? If you're able to build and test
those platforms with it disabled, it might well be the case that we
just don't need the fallback anymore, and a world build with that just
removed would help figure out if that's true.

-- 
Tom
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