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Date:   Sat, 10 Oct 2020 22:58:36 +0200
From:   Mark Wielaard <mark@klomp.org>
To:     Andi Kleen <andi@firstfloor.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Stephane Eranian <eranian@google.com>,
        linux-toolchains@vger.kernel.org,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        "Phillips, Kim" <kim.phillips@amd.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: Additional debug info to aid cacheline analysis
Message-ID: <20201010205836.GA2666@wildebeest.org>
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On Thu, Oct 08, 2020 at 02:23:00PM -0700, Andi Kleen wrote:
> > Basically you simply want to remove this line in the top-level
> > Makefile:
> > 
> > DEBUG_CFLAGS    := $(call cc-option, -fno-var-tracking-assignments)
> 
> It looks like this was needed as a workaround for a gcc bug that was there
> from 4.5 to 4.9.
> 
> So I guess could disable it for 5.0+ only. 

Yes, that would work. I don't know what the lowest supported GCC
version is, but technically it was definitely fixed in 4.10.0, 4.8.4
and 4.9.2. And various distros would probably have backported the
fix. But checking for 5.0+ would certainly give you a good version.

How about the attached?

Cheers,

Mark
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Content-Disposition: attachment; filename="0001-Only-add-fno-var-tracking-assignments-workaround-for.patch"

From 48628d3cf2d829a90cd6622355eada1b30cb10c1 Mon Sep 17 00:00:00 2001
From: Mark Wielaard <mark@klomp.org>
Date: Sat, 10 Oct 2020 22:47:21 +0200
Subject: [PATCH] Only add -fno-var-tracking-assignments workaround for old GCC
 versions.

Some old GCC versions between 4.5.0 and 4.9.1 might miscompile code
with -fvar-tracking-assingments (which is enabled by default with -g -O2).
commit 2062afb4f added -fno-var-tracking-assignments unconditionally to
workaround this. But newer versions of GCC no longer have this bug, so
only add it for versions of GCC before 5.0.

Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 Makefile | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index f84d7e4ca0be..4f4a9416a87a 100644
--- a/Makefile
+++ b/Makefile
@@ -813,7 +813,9 @@ KBUILD_CFLAGS	+= -ftrivial-auto-var-init=zero
 KBUILD_CFLAGS	+= -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
 endif
 
-DEBUG_CFLAGS	:= $(call cc-option, -fno-var-tracking-assignments)
+# Workaround https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801
+# for old versions of GCC.
+DEBUG_CFLAGS	:= $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments))
 
 ifdef CONFIG_DEBUG_INFO
 ifdef CONFIG_DEBUG_INFO_SPLIT
-- 
2.18.4


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