Add patch from Josh Stone to restore var-tracking via Kconfig (rhbz 1126580)
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Josh Boyer
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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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MIME-Version: 1.0
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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 fd80c6e9bc23..4ba81e0dce46 100644
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--- a/Makefile
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+++ b/Makefile
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@@ -704,7 +704,11 @@ 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 4e35a5d767ed..27410417de1d 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 ENABLE_WARN_DEPRECATED
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bool "Enable __deprecated logic"
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--
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2.1.0
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@ -4372,6 +4372,7 @@ CONFIG_FRAME_WARN=1024
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CONFIG_MAGIC_SYSRQ=y
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CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x0
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CONFIG_DEBUG_INFO=y
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CONFIG_DEBUG_INFO_VTA=y
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# Revisit both of these options
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# CONFIG_DEBUG_INFO_SPLIT is not set
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# CONFIG_DEBUG_INFO_DWARF4 is not set
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@ -519,6 +519,9 @@ Patch530: silence-fbcon-logo.patch
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Patch600: lib-cpumask-Make-CPUMASK_OFFSTACK-usable-without-deb.patch
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#rhbz 1126580
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Patch601: Kbuild-Add-an-option-to-enable-GCC-VTA.patch
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Patch800: crash-driver.patch
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# crypto/
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@ -1278,6 +1281,8 @@ ApplyPatch input-silence-i8042-noise.patch
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ApplyPatch silence-fbcon-logo.patch
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# Changes to upstream defaults.
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#rhbz 1126580
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ApplyPatch Kbuild-Add-an-option-to-enable-GCC-VTA.patch
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# /dev/crash driver.
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ApplyPatch crash-driver.patch
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@ -2243,6 +2248,9 @@ fi
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# ||----w |
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# || ||
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%changelog
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* Tue Dec 16 2014 Josh Boyer <jwboyer@fedoraproject.org>
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- Add patch from Josh Stone to restore var-tracking via Kconfig (rhbz 1126580)
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* Mon Dec 15 2014 Josh Boyer <jwboyer@fedoraproject.org>
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- Fix ppc64 boot with smt-enabled=off (rhbz 1173806)
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- CVE-2014-8133 x86: espfix(64) bypass via set_thread_area and CLONE_SETTLS (rhbz 1172797 1174374)
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