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