From: Josh Stone <firstname.lastname@example.org> Date: Fri, 21 Nov 2014 10:40:00 -0800 Subject: [PATCH] Kbuild: Add an option to enable GCC VTA MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Due to recent codegen issues, gcc -fvar-tracking-assignments was unconditionally disabled in commit 2062afb4f804a ("Fix gcc-4.9.0 miscompilation of load_balance() in scheduler"). However, this reduces the debuginfo coverage for variable locations, especially in inline functions. VTA is certainly not perfect either in those cases, but it is much better than without. With compiler versions that have fixed the codegen bugs, we would prefer to have the better details for SystemTap, and surely other debuginfo consumers like perf will benefit as well. This patch simply makes CONFIG_DEBUG_INFO_VTA an option. I considered Frank and Linus's discussion of a cc-option-like -fcompare-debug test, but I'm convinced that a narrow test of an arch-specific codegen issue is not really useful. GCC has their own regression tests for this, so I'd suggest GCC_COMPARE_DEBUG=-fvar-tracking-assignments-toggle is more useful for kernel developers to test confidence. In fact, I ran into a couple more issues when testing for this patch, although neither of those had any codegen impact.  https://bugzilla.redhat.com/show_bug.cgi?id=1140872 With gcc-4.9.2-1.fc22, I can now build v3.18-rc5 with Fedora's i686 and x86_64 configs, and this is completely clean with GCC_COMPARE_DEBUG. Cc: Frank Ch. Eigler <email@example.com> Cc: Jakub Jelinek <firstname.lastname@example.org> Cc: Josh Boyer <email@example.com> Cc: Greg Kroah-Hartman <firstname.lastname@example.org> Cc: Linus Torvalds <email@example.com> Cc: Andrew Morton <firstname.lastname@example.org> Cc: Markus Trippelsdorf <email@example.com> Cc: Michel Dänzer <firstname.lastname@example.org> Signed-off-by: Josh Stone <email@example.com> --- Makefile | 4 ++++ lib/Kconfig.debug | 18 +++++++++++++++++- 2 files changed, 21 insertions(+), 1 deletion(-) diff --git a/Makefile b/Makefile index 257ef5892ab7..3cc6f4477e78 100644 --- a/Makefile +++ b/Makefile @@ -701,7 +701,11 @@ KBUILD_CFLAGS += -fomit-frame-pointer endif endif +ifdef CONFIG_DEBUG_INFO_VTA +KBUILD_CFLAGS += $(call cc-option, -fvar-tracking-assignments) +else KBUILD_CFLAGS += $(call cc-option, -fno-var-tracking-assignments) +endif ifdef CONFIG_DEBUG_INFO ifdef CONFIG_DEBUG_INFO_SPLIT diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index e2894b23efb6..d98afe18f704 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -165,7 +165,23 @@ config DEBUG_INFO_DWARF4 Generate dwarf4 debug info. This requires recent versions of gcc and gdb. It makes the debug information larger. But it significantly improves the success of resolving - variables in gdb on optimized code. + variables in gdb on optimized code. The gcc docs also + recommend enabling -fvar-tracking-assignments for maximum + benefit. (see DEBUG_INFO_VTA) + +config DEBUG_INFO_VTA + bool "Enable var-tracking-assignments for debuginfo" + depends on DEBUG_INFO + help + Enable gcc -fvar-tracking-assignments for improved debug + information on variable locations in optimized code. Per + gcc, DEBUG_INFO_DWARF4 is recommended for best use of VTA. + + VTA has been implicated in codegen bugs (gcc PR61801, + PR61904), so this may deserve some caution. One can set + GCC_COMPARE_DEBUG=-fvar-tracking-assignments-toggle in the + environment to automatically compile everything both ways, + generating an error if anything differs. config GDB_SCRIPTS bool "Provide GDB scripts for kernel debugging"