Re: [PATCH v7 02/17] kbuild: add support for Clang LTO
From: Sami Tolvanen <samitolvanen@google.com>
Date: 2020-11-21 01:47:10
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linux-arch, linux-kbuild, linux-pci, lkml
On Fri, Nov 20, 2020 at 3:59 PM Kees Cook [off-list ref] wrote:
On Fri, Nov 20, 2020 at 12:58:41PM -0800, Sami Tolvanen wrote:quoted
On Fri, Nov 20, 2020 at 12:43 PM Kees Cook [off-list ref] wrote:quoted
On Fri, Nov 20, 2020 at 01:29:35PM -0700, Nathan Chancellor wrote:quoted
On Fri, Nov 20, 2020 at 11:47:21AM -0800, Kees Cook wrote:quoted
On Fri, Nov 20, 2020 at 08:23:11AM -0800, Sami Tolvanen wrote:quoted
Changing the ThinLTO config to a choice and moving it after the main LTO config sounds like a good idea to me. I'll see if I can change this in v8. Thanks!Originally, I thought this might be a bit ugly once GCC LTO is added, but this could be just a choice like we're done for the stack initialization. Something like an "LTO" choice of NONE, CLANG_FULL, CLANG_THIN, and in the future GCC, etc.Having two separate choices might be a little bit cleaner though? One for the compiler (LTO_CLANG versus LTO_GCC) and one for the type (THINLTO versus FULLLTO). The type one could just have a "depends on CC_IS_CLANG" to ensure it only showed up when needed.Right, that's how the stack init choice works. Kconfigs that aren't supported by the compiler won't be shown. I.e. after Sami's future patch, the only choice for GCC will be CONFIG_LTO_NONE. But building under Clang, it would offer CONFIG_LTO_NONE, CONFIG_LTO_CLANG_FULL, CONFIG_LTO_CLANG_THIN, or something. (and I assume CONFIG_LTO would be def_bool y, depends on !LTO_NONE)I'm fine with adding ThinLTO as another option to the LTO choice, but it would duplicate the dependencies and a lot of the help text. I suppose we could add another config for the dependencies and have both LTO options depend on that instead.How about something like this? This separates the arch support, compiler support, and user choice into three separate Kconfig areas, which I think should work.
Sure, this looks good to me, I'll use this in v8. The only minor concern I have is that ThinLTO cannot be set as the default LTO mode, but I assume anyone who selects LTO is also capable of deciding which mode is better for them.
quoted hunk ↗ jump to hunk
diff --git a/Makefile b/Makefile index e397c4caec1b..af902718e882 100644 --- a/Makefile +++ b/Makefile@@ -897,7 +897,7 @@ export CC_FLAGS_SCS endif ifdef CONFIG_LTO_CLANG -ifdef CONFIG_THINLTO +ifdef CONFIG_LTO_CLANG_THIN CC_FLAGS_LTO += -flto=thin -fsplit-lto-unit KBUILD_LDFLAGS += --thinlto-cache-dir=$(extmod-prefix).thinlto-cache elsediff --git a/arch/Kconfig b/arch/Kconfig index cdd29b5fdb56..5c22e10e4c12 100644 --- a/arch/Kconfig +++ b/arch/Kconfig@@ -600,6 +600,14 @@ config SHADOW_CALL_STACK config LTO bool + help + Selected if the kernel will be built using the compiler's LTO feature. + +config LTO_CLANG + bool + select LTO + help + Selected if the kernel will be built using Clang's LTO feature. config ARCH_SUPPORTS_LTO_CLANG bool@@ -609,28 +617,25 @@ config ARCH_SUPPORTS_LTO_CLANG - compiling inline assembly with Clang's integrated assembler, - and linking with LLD. -config ARCH_SUPPORTS_THINLTO +config ARCH_SUPPORTS_LTO_CLANG_THIN bool help - An architecture should select this option if it supports Clang's - ThinLTO. + An architecture should select this option if it can supports Clang's + ThinLTO mode. -config THINLTO - bool "Clang ThinLTO" - depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO - default y +config HAS_LTO_CLANG + def_bool y + # Clang >= 11: https://github.com/ClangBuiltLinux/linux/issues/510 + depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD + depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm) + depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm) + depends on ARCH_SUPPORTS_LTO_CLANG + depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT + depends on !KASAN + depends on !GCOV_KERNEL 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 - - If you say N here, the compiler will use full LTO, which may - produce faster code, but building the kernel will be significantly - slower as the linker won't efficiently utilize multiple threads. - - If unsure, say Y. + The compiler and Kconfig options support building with Clang's + LTO. choice prompt "Link Time Optimization (LTO)"@@ -644,20 +649,14 @@ choice config LTO_NONE bool "None" + help + Build the kernel normally, without Link Time Optimization (LTO). -config LTO_CLANG - bool "Clang's Link Time Optimization (EXPERIMENTAL)" - # Clang >= 11: https://github.com/ClangBuiltLinux/linux/issues/510 - depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD - depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm) - depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm) - depends on ARCH_SUPPORTS_LTO_CLANG - depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT - depends on !KASAN - depends on !GCOV_KERNEL - select LTO +config LTO_CLANG_FULL + bool "Clang Full LTO (EXPERIMENTAL)" + select LTO_CLANG help - This option enables Clang's Link Time Optimization (LTO), which + This option enables Clang's full Link Time Optimization (LTO), which allows the compiler to optimize the kernel globally. If you enable this option, the compiler generates LLVM bitcode instead of ELF object files, and the actual compilation from bitcode happens at@@ -667,9 +666,22 @@ config LTO_CLANG https://llvm.org/docs/LinkTimeOptimization.html - To select this option, you also need to use LLVM tools to handle - the bitcode by passing LLVM=1 to make. + During link time, this option can use a large amount of RAM, and + may take much longer than the ThinLTO option. +config LTO_CLANG_THIN + bool "Clang ThinLTO (EXPERIMENTAL)" + depends on ARCH_SUPPORTS_LTO_CLANG_THIN + select LTO_CLANG + help + This option enables Clang's ThinLTO, which allows for parallel + optimization and faster incremental compiles compared to the + CONFIG_LTO_CLANG_FULL option. More information can be found + from Clang's documentation: + + https://clang.llvm.org/docs/ThinLTO.html + + If unsure, say Y. endchoice
The two LTO_CLANG_* options need to depend on HAS_LTO_CLANG, of course. Sami _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel