diff options
Diffstat (limited to 'Kbuild-Add-an-option-to-enable-GCC-VTA.patch')
| -rw-r--r-- | Kbuild-Add-an-option-to-enable-GCC-VTA.patch | 89 |
1 files changed, 89 insertions, 0 deletions
diff --git a/Kbuild-Add-an-option-to-enable-GCC-VTA.patch b/Kbuild-Add-an-option-to-enable-GCC-VTA.patch new file mode 100644 index 000000000..4908a98ba --- /dev/null +++ b/Kbuild-Add-an-option-to-enable-GCC-VTA.patch @@ -0,0 +1,89 @@ +From: Josh Stone <jistone@redhat.com> +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[1], +although neither of those had any codegen impact. + [1] 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 <fche@redhat.com> +Cc: Jakub Jelinek <jakub@redhat.com> +Cc: Josh Boyer <jwboyer@fedoraproject.org> +Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> +Cc: Linus Torvalds <torvalds@linux-foundation.org> +Cc: Andrew Morton <akpm@linux-foundation.org> +Cc: Markus Trippelsdorf <markus@trippelsdorf.de> +Cc: Michel Dänzer <michel@daenzer.net> +Signed-off-by: Josh Stone <jistone@redhat.com> +--- + Makefile | 4 ++++ + lib/Kconfig.debug | 18 +++++++++++++++++- + 2 files changed, 21 insertions(+), 1 deletion(-) + +diff --git a/Makefile b/Makefile +index af6da040b952..2796a22bb0a4 100644 +--- a/Makefile ++++ b/Makefile +@@ -706,7 +706,11 @@ 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 c5cefb3c009c..b663e8e211b0 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" |