|
 |
f2c60e |
From: Josh Stone <jistone@redhat.com>
|
|
|
f2c60e |
Date: Fri, 21 Nov 2014 10:40:00 -0800
|
|
|
f2c60e |
Subject: [PATCH] Kbuild: Add an option to enable GCC VTA
|
|
|
f2c60e |
MIME-Version: 1.0
|
|
|
f2c60e |
Content-Type: text/plain; charset=UTF-8
|
|
|
f2c60e |
Content-Transfer-Encoding: 8bit
|
|
|
f2c60e |
|
|
|
f2c60e |
Due to recent codegen issues, gcc -fvar-tracking-assignments was
|
|
|
f2c60e |
unconditionally disabled in commit 2062afb4f804a ("Fix gcc-4.9.0
|
|
|
f2c60e |
miscompilation of load_balance() in scheduler"). However, this reduces
|
|
|
f2c60e |
the debuginfo coverage for variable locations, especially in inline
|
|
|
f2c60e |
functions. VTA is certainly not perfect either in those cases, but it
|
|
|
f2c60e |
is much better than without. With compiler versions that have fixed the
|
|
|
f2c60e |
codegen bugs, we would prefer to have the better details for SystemTap,
|
|
|
f2c60e |
and surely other debuginfo consumers like perf will benefit as well.
|
|
|
f2c60e |
|
|
|
f2c60e |
This patch simply makes CONFIG_DEBUG_INFO_VTA an option. I considered
|
|
|
f2c60e |
Frank and Linus's discussion of a cc-option-like -fcompare-debug test,
|
|
|
f2c60e |
but I'm convinced that a narrow test of an arch-specific codegen issue
|
|
|
f2c60e |
is not really useful. GCC has their own regression tests for this, so
|
|
|
f2c60e |
I'd suggest GCC_COMPARE_DEBUG=-fvar-tracking-assignments-toggle is more
|
|
|
f2c60e |
useful for kernel developers to test confidence.
|
|
|
f2c60e |
|
|
|
f2c60e |
In fact, I ran into a couple more issues when testing for this patch[1],
|
|
|
f2c60e |
although neither of those had any codegen impact.
|
|
|
f2c60e |
[1] https://bugzilla.redhat.com/show_bug.cgi?id=1140872
|
|
|
f2c60e |
|
|
|
f2c60e |
With gcc-4.9.2-1.fc22, I can now build v3.18-rc5 with Fedora's i686 and
|
|
|
f2c60e |
x86_64 configs, and this is completely clean with GCC_COMPARE_DEBUG.
|
|
|
f2c60e |
|
|
|
f2c60e |
Cc: Frank Ch. Eigler <fche@redhat.com>
|
|
|
f2c60e |
Cc: Jakub Jelinek <jakub@redhat.com>
|
|
|
f2c60e |
Cc: Josh Boyer <jwboyer@fedoraproject.org>
|
|
|
f2c60e |
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
|
|
|
f2c60e |
Cc: Linus Torvalds <torvalds@linux-foundation.org>
|
|
|
f2c60e |
Cc: Andrew Morton <akpm@linux-foundation.org>
|
|
|
f2c60e |
Cc: Markus Trippelsdorf <markus@trippelsdorf.de>
|
|
|
f2c60e |
Cc: Michel Dänzer <michel@daenzer.net>
|
|
|
f2c60e |
Signed-off-by: Josh Stone <jistone@redhat.com>
|
|
|
f2c60e |
---
|
|
|
f2c60e |
Makefile | 4 ++++
|
|
|
f2c60e |
lib/Kconfig.debug | 18 +++++++++++++++++-
|
|
|
f2c60e |
2 files changed, 21 insertions(+), 1 deletion(-)
|
|
|
f2c60e |
|
|
|
f2c60e |
diff --git a/Makefile b/Makefile
|
|
|
f2c60e |
index 257ef5892ab7..3cc6f4477e78 100644
|
|
|
f2c60e |
--- a/Makefile
|
|
|
f2c60e |
+++ b/Makefile
|
|
|
f2c60e |
@@ -701,7 +701,11 @@ KBUILD_CFLAGS += -fomit-frame-pointer
|
|
|
f2c60e |
endif
|
|
|
f2c60e |
endif
|
|
|
f2c60e |
|
|
|
f2c60e |
+ifdef CONFIG_DEBUG_INFO_VTA
|
|
|
f2c60e |
+KBUILD_CFLAGS += $(call cc-option, -fvar-tracking-assignments)
|
|
|
f2c60e |
+else
|
|
|
f2c60e |
KBUILD_CFLAGS += $(call cc-option, -fno-var-tracking-assignments)
|
|
|
f2c60e |
+endif
|
|
|
f2c60e |
|
|
|
f2c60e |
ifdef CONFIG_DEBUG_INFO
|
|
|
f2c60e |
ifdef CONFIG_DEBUG_INFO_SPLIT
|
|
|
f2c60e |
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
|
|
|
f2c60e |
index e2894b23efb6..d98afe18f704 100644
|
|
|
f2c60e |
--- a/lib/Kconfig.debug
|
|
|
f2c60e |
+++ b/lib/Kconfig.debug
|
|
|
f2c60e |
@@ -165,7 +165,23 @@ config DEBUG_INFO_DWARF4
|
|
|
f2c60e |
Generate dwarf4 debug info. This requires recent versions
|
|
|
f2c60e |
of gcc and gdb. It makes the debug information larger.
|
|
|
f2c60e |
But it significantly improves the success of resolving
|
|
|
f2c60e |
- variables in gdb on optimized code.
|
|
|
f2c60e |
+ variables in gdb on optimized code. The gcc docs also
|
|
|
f2c60e |
+ recommend enabling -fvar-tracking-assignments for maximum
|
|
|
f2c60e |
+ benefit. (see DEBUG_INFO_VTA)
|
|
|
f2c60e |
+
|
|
|
f2c60e |
+config DEBUG_INFO_VTA
|
|
|
f2c60e |
+ bool "Enable var-tracking-assignments for debuginfo"
|
|
|
f2c60e |
+ depends on DEBUG_INFO
|
|
|
f2c60e |
+ help
|
|
|
f2c60e |
+ Enable gcc -fvar-tracking-assignments for improved debug
|
|
|
f2c60e |
+ information on variable locations in optimized code. Per
|
|
|
f2c60e |
+ gcc, DEBUG_INFO_DWARF4 is recommended for best use of VTA.
|
|
|
f2c60e |
+
|
|
|
f2c60e |
+ VTA has been implicated in codegen bugs (gcc PR61801,
|
|
|
f2c60e |
+ PR61904), so this may deserve some caution. One can set
|
|
|
f2c60e |
+ GCC_COMPARE_DEBUG=-fvar-tracking-assignments-toggle in the
|
|
|
f2c60e |
+ environment to automatically compile everything both ways,
|
|
|
f2c60e |
+ generating an error if anything differs.
|
|
|
f2c60e |
|
|
|
f2c60e |
config GDB_SCRIPTS
|
|
|
f2c60e |
bool "Provide GDB scripts for kernel debugging"
|