Messages in this thread | | | Date | Wed, 21 Mar 2018 10:03:22 +0100 | From | Stefan Agner <> | Subject | Re: [PATCH 5/5] ARM: add support for building ARM kernel with clang |
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On 21.03.2018 01:20, Matthias Kaehlcke wrote: > El Tue, Mar 20, 2018 at 11:18:33PM +0000 Russell King - ARM Linux ha dit: > >> On Wed, Mar 21, 2018 at 12:02:06AM +0100, Stefan Agner wrote: >> > Use cc-options call for compiler options which are not available >> > in clang. With this patch an ARMv7 multi platform kernel can be >> > successfully build using clang (tested with version 5.0.1). >> > >> > Based-on-patches-by: Behan Webster <behanw@converseincode.com> >> > Signed-off-by: Stefan Agner <stefan@agner.ch> > > Great to see your work on bringing clang support for 32-bit ARM > upstream! > >> > --- >> > arch/arm/Makefile | 2 +- >> > arch/arm/boot/compressed/Makefile | 2 +- >> > 2 files changed, 2 insertions(+), 2 deletions(-) >> > >> > diff --git a/arch/arm/Makefile b/arch/arm/Makefile >> > index e9e3fde3c657..20e9fee1ccc5 100644 >> > --- a/arch/arm/Makefile >> > +++ b/arch/arm/Makefile >> > @@ -39,7 +39,7 @@ KBUILD_CFLAGS += $(call cc-option,-mno-unaligned-access) >> > endif >> > >> > ifeq ($(CONFIG_FRAME_POINTER),y) >> > -KBUILD_CFLAGS +=-fno-omit-frame-pointer -mapcs -mno-sched-prolog >> > +KBUILD_CFLAGS +=-fno-omit-frame-pointer $(call cc-option,-mapcs,) $(call cc-option,-mno-sched-prolog,) >> >> Some of these options here are to ensure that we generate the following >> code, so we can backtrace: >> >> mov ip, sp >> stmfd sp!, {fp, ip, lr, pc} >> sub fp, ip, #4 >> >> If clang isn't producing that code at the start of functions with >> CONFIG_FRAME_POINTER=y, then backtracing will not work, and arguably >> CONFIG_FRAME_POINTER=y is useless there. In that circumstance, it's >> probably better to fail so the user can configure something more >> debuggable, rather than having the kernel potentially producing >> undebuggable oopses.
With clang and -fno-omit-frame-pointer function prologue looks something like this:
0: e92d4ff0 push {r4, r5, r6, r7, r8, r9, sl, fp, lr} 4: e28db01c add fp, sp, #28 ...
This bug seems to be related: https://bugs.llvm.org/show_bug.cgi?id=18505
It seems that LLVM/clang does not plan to add APCS format/gcc interoperability for the same frame layout.
I guess it would be possible to support the LLVM/clang frame layout?
But until then, I am with Russel here: Better just let clang fail on CONFIG_FRAME_POINTER=y.
Most configs probably anyway use CONFIG_ARM_UNWIND. At least multi_v7_defconfig does.
> > Which option in particular is important to generate the above code for > backstracing?
For gcc, I guess really all of them.
-- Stefan
> > According to the gcc doc -mapcs(-frame) is deprecated. > > For -mno-sched-prolog the doc says: > > "Prevent the reordering of instructions in the function prologue, or > the merging of those instruction with the instructions in the > function’s body. This means that all functions start with a > recognizable set of instructions (or in fact one of a choice from a > small set of different function prologues), and this information can > be used to locate the start of functions inside an executable piece of > code. The default is -msched-prolog." > > Thanks > > Matthias
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