Subject: Re: [PATCH 02/22] kbuild: add support for Clang LTO

From: Sami Tolvanen <[email-masked]>
Date: Thu, 25 Jun 2020 09:13:39 -0700

On Wed, Jun 24, 2020 at 07:26:47PM -0700, Nathan Chancellor wrote:
> Hi Sami,
> 
> On Wed, Jun 24, 2020 at 01:31:40PM -0700, 'Sami Tolvanen' via Clang Built Linux wrote:
> > This change adds build system support for Clang's Link Time
> > Optimization (LTO). With -flto, instead of ELF object files, Clang
> > produces LLVM bitcode, which is compiled into native code at link
> > time, allowing the final binary to be optimized globally. For more
> > details, see:
> > 
> >   https://llvm.org/docs/LinkTimeOptimization.html
> > 
> > The Kconfig option CONFIG_LTO_CLANG is implemented as a choice,
> > which defaults to LTO being disabled. To use LTO, the architecture
> > must select ARCH_SUPPORTS_LTO_CLANG and support:
> > 
> >   - compiling with Clang,
> >   - compiling inline assembly with Clang's integrated assembler,
> >   - and linking with LLD.
> > 
> > While using full LTO results in the best runtime performance, the
> > compilation is not scalable in time or memory. CONFIG_THINLTO
> > enables ThinLTO, which allows parallel optimization and faster
> > incremental builds. ThinLTO is used by default if the architecture
> > also selects ARCH_SUPPORTS_THINLTO:
> > 
> >   https://clang.llvm.org/docs/ThinLTO.html
> > 
> > To enable LTO, LLVM tools must be used to handle bitcode files. The
> > easiest way is to pass the LLVM=1 option to make:
> > 
> >   $ make LLVM=1 defconfig
> >   $ scripts/config -e LTO_CLANG
> >   $ make LLVM=1
> > 
> > Alternatively, at least the following LLVM tools must be used:
> > 
> >   CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm
> > 
> > To prepare for LTO support with other compilers, common parts are
> > gated behind the CONFIG_LTO option, and LTO can be disabled for
> > specific files by filtering out CC_FLAGS_LTO.
> > 
> > Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in
> > follow-up patches.
> > 
> > Signed-off-by: Sami Tolvanen <[email-masked]>
> > ---
> >  Makefile                          | 16 ++++++++
> >  arch/Kconfig                      | 66 +++++++++++++++++++++++++++++++
> >  include/asm-generic/vmlinux.lds.h | 11 ++++--
> >  scripts/Makefile.build            |  9 ++++-
> >  scripts/Makefile.modfinal         |  9 ++++-
> >  scripts/Makefile.modpost          | 24 ++++++++++-
> >  scripts/link-vmlinux.sh           | 32 +++++++++++----
> >  7 files changed, 151 insertions(+), 16 deletions(-)
> > 
> > diff --git a/Makefile b/Makefile
> > index ac2c61c37a73..0c7fe6fb2143 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -886,6 +886,22 @@ KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
> >  export CC_FLAGS_SCS
> >  endif
> >  
> > +ifdef CONFIG_LTO_CLANG
> > +ifdef CONFIG_THINLTO
> > +CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit)
> > +KBUILD_LDFLAGS	+= --thinlto-cache-dir=.thinlto-cache
> > +else
> > +CC_FLAGS_LTO_CLANG := -flto
> > +endif
> > +CC_FLAGS_LTO_CLANG += -fvisibility=default
> > +endif
> > +
> > +ifdef CONFIG_LTO
> > +CC_FLAGS_LTO	:= $(CC_FLAGS_LTO_CLANG)
> > +KBUILD_CFLAGS	+= $(CC_FLAGS_LTO)
> > +export CC_FLAGS_LTO
> > +endif
> > +
> >  # arch Makefile may override CC so keep this after arch Makefile is included
> >  NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
> >  
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 8cc35dc556c7..e00b122293f8 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -552,6 +552,72 @@ config SHADOW_CALL_STACK
> >  	  reading and writing arbitrary memory may be able to locate them
> >  	  and hijack control flow by modifying the stacks.
> >  
> > +config LTO
> > +	bool
> > +
> > +config ARCH_SUPPORTS_LTO_CLANG
> > +	bool
> > +	help
> > +	  An architecture should select this option if it supports:
> > +	  - compiling with Clang,
> > +	  - compiling inline assembly with Clang's integrated assembler,
> > +	  - and linking with LLD.
> > +
> > +config ARCH_SUPPORTS_THINLTO
> > +	bool
> > +	help
> > +	  An architecture should select this option if it supports Clang's
> > +	  ThinLTO.
> > +
> > +config THINLTO
> > +	bool "Clang ThinLTO"
> > +	depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO
> > +	default y
> > +	help
> > +	  This option enables Clang's ThinLTO, which allows for parallel
> > +	  optimization and faster incremental compiles. More information
> > +	  can be found from Clang's documentation:
> > +
> > +	    https://clang.llvm.org/docs/ThinLTO.html
> > +
> > +choice
> > +	prompt "Link Time Optimization (LTO)"
> > +	default LTO_NONE
> > +	help
> > +	  This option enables Link Time Optimization (LTO), which allows the
> > +	  compiler to optimize binaries globally.
> > +
> > +	  If unsure, select LTO_NONE.
> > +
> > +config LTO_NONE
> > +	bool "None"
> > +
> > +config LTO_CLANG
> > +	bool "Clang's Link Time Optimization (EXPERIMENTAL)"
> > +	depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD
> 
> I am curious, what is the reason for gating this at clang 11.0.0?
> 
> Presumably this? https://github.com/ClangBuiltLinux/linux/issues/510
> 
> It might be nice to notate this so that we do not have to wonder :)

Yes, that's the reason. I'll add a note about it. Thanks!

Sami

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