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

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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 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"