| Message ID | 20201017120135.4004-1-mark@klomp.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [V2] Only add -fno-var-tracking-assignments workaround for old GCC versions. | expand |
On Sat, Oct 17, 2020 at 5:02 AM Mark Wielaard <mark@klomp.org> wrote: > > 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 > work around this. But newer versions of GCC no longer have this bug, so > only add it for versions of GCC before 5.0. This allows various tools > such as a perf probe or gdb debuggers or systemtap to resolve variable > locations using dwarf locations in more code. > > Changes in V2: > - Update commit message explaining purpose. > - Explicitly mention GCC version in comment. > - Wrap workaround in ifdef CONFIG_CC_IS_GCC > > Signed-off-by: Mark Wielaard <mark@klomp.org> > Acked-by: Ian Rogers <irogers@google.com> > Reviewed-by: Andi Kleen <andi@firstfloor.org> > Cc: linux-toolchains@vger.kernel.org > Cc: Nick Desaulniers <ndesaulniers@google.com> > Cc: Segher Boessenkool <segher@kernel.crashing.org> > Cc: Florian Weimer <fw@deneb.enyo.de> > Cc: Sedat Dilek <sedat.dilek@gmail.com> > --- > Makefile | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/Makefile b/Makefile > index 51540b291738..964754b4cedf 100644 > --- a/Makefile > +++ b/Makefile > @@ -813,7 +813,11 @@ 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 for GCC versions < 5.0 > +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801 > +ifdef CONFIG_CC_IS_GCC > +DEBUG_CFLAGS := $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments)) Thanks for adding the comment. That will help us find+remove this when the kernel's minimum supported version of GCC advances to gcc-5. The current minimum supported version of GCC according to Documentation/process/changes.rst is gcc-4.9 (so anything older is irrelevant, and we drop support for it). If gcc 4.9 supports `-fno-var-tracking-assignments` (it looks like it does: https://godbolt.org/z/oa53f5), then we should drop the `cc-option` call, which will save us a compiler invocation for each invocation of `make`. > +endif > > ifdef CONFIG_DEBUG_INFO > ifdef CONFIG_DEBUG_INFO_SPLIT > -- > 2.18.4 >
On Sat, Oct 17, 2020 at 9:02 PM Mark Wielaard <mark@klomp.org> wrote: > > 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 > work around this. But newer versions of GCC no longer have this bug, so > only add it for versions of GCC before 5.0. This allows various tools > such as a perf probe or gdb debuggers or systemtap to resolve variable > locations using dwarf locations in more code. > > Changes in V2: > - Update commit message explaining purpose. > - Explicitly mention GCC version in comment. > - Wrap workaround in ifdef CONFIG_CC_IS_GCC > > Signed-off-by: Mark Wielaard <mark@klomp.org> > Acked-by: Ian Rogers <irogers@google.com> > Reviewed-by: Andi Kleen <andi@firstfloor.org> > Cc: linux-toolchains@vger.kernel.org > Cc: Nick Desaulniers <ndesaulniers@google.com> > Cc: Segher Boessenkool <segher@kernel.crashing.org> > Cc: Florian Weimer <fw@deneb.enyo.de> > Cc: Sedat Dilek <sedat.dilek@gmail.com> > --- Applied to linux-kbuild. Thanks. > Makefile | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/Makefile b/Makefile > index 51540b291738..964754b4cedf 100644 > --- a/Makefile > +++ b/Makefile > @@ -813,7 +813,11 @@ 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 for GCC versions < 5.0 > +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801 > +ifdef CONFIG_CC_IS_GCC > +DEBUG_CFLAGS := $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments)) > +endif > > ifdef CONFIG_DEBUG_INFO > ifdef CONFIG_DEBUG_INFO_SPLIT > -- > 2.18.4 >
diff --git a/Makefile b/Makefile index 51540b291738..964754b4cedf 100644 --- a/Makefile +++ b/Makefile @@ -813,7 +813,11 @@ 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 for GCC versions < 5.0 +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801 +ifdef CONFIG_CC_IS_GCC +DEBUG_CFLAGS := $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments)) +endif ifdef CONFIG_DEBUG_INFO ifdef CONFIG_DEBUG_INFO_SPLIT