[PATCH] powerpc/xmon: Relax frame size for clang

[PATCH] powerpc/xmon: Relax frame size for clang

[Joel Stanley]

When building with clang (8 trunk, 7.0 release) the frame size limit is
hit:

 arch/powerpc/xmon/xmon.c:452:12: warning: stack frame size of 2576
 bytes in function 'xmon_core' [-Wframe-larger-than=]

Some investigation by Naveen indicates this is due to clang saving the
addresses to printf format strings on the stack.

While this issue is investigated, bump up the frame size limit for xmon
when building with clang.

Link: https://github.com/ClangBuiltLinux/linux/issues/252
Signed-off-by: Joel Stanley <joel@jms.id.au>
---
 arch/powerpc/xmon/Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/powerpc/xmon/Makefile b/arch/powerpc/xmon/Makefile
index 69e7fb47bcaa..878f9c1d3615 100644
--- a/arch/powerpc/xmon/Makefile
+++ b/arch/powerpc/xmon/Makefile
@@ -11,6 +11,12 @@ UBSAN_SANITIZE := n
 ORIG_CFLAGS := $(KBUILD_CFLAGS)
 KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))
 
+ifdef CONFIG_CC_IS_CLANG
+# clang stores addresses on the stack causing the frame size to blow
+# out. See https://github.com/ClangBuiltLinux/linux/issues/252
+KBUILD_CFLAGS += -Wframe-larger-than=4096
+endif
+
 ccflags-$(CONFIG_PPC64) := $(NO_MINIMAL_TOC)
 
 obj-y			+= xmon.o nonstdio.o spr_access.o
-- 
2.19.1

Re: powerpc/xmon: Relax frame size for clang

[Michael Ellerman]

On Wed, 2018-10-31 at 01:09:34 UTC, Joel Stanley wrote:
> When building with clang (8 trunk, 7.0 release) the frame size limit is
> hit:
> 
>  arch/powerpc/xmon/xmon.c:452:12: warning: stack frame size of 2576
>  bytes in function 'xmon_core' [-Wframe-larger-than=]
> 
> Some investigation by Naveen indicates this is due to clang saving the
> addresses to printf format strings on the stack.
> 
> While this issue is investigated, bump up the frame size limit for xmon
> when building with clang.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/252
> Signed-off-by: Joel Stanley <joel@jms.id.au>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/9c87156cce5a63735d1218f0096a65

cheers

RE: powerpc/xmon: Relax frame size for clang

[David Laight]

From: Linuxppc-dev [mailto:linuxppc-dev-bounces+david.laight=aculab.com@lists.ozlabs.org] On Behalf Of
> Michael Ellerman
> Subject: Re: powerpc/xmon: Relax frame size for clang
> 
> On Wed, 2018-10-31 at 01:09:34 UTC, Joel Stanley wrote:
> > When building with clang (8 trunk, 7.0 release) the frame size limit is
> > hit:
> >
> >  arch/powerpc/xmon/xmon.c:452:12: warning: stack frame size of 2576
> >  bytes in function 'xmon_core' [-Wframe-larger-than=]
> >
> > Some investigation by Naveen indicates this is due to clang saving the
> > addresses to printf format strings on the stack.
> >
> > While this issue is investigated, bump up the frame size limit for xmon
> > when building with clang.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/252
> > Signed-off-by: Joel Stanley <joel@jms.id.au>

Would it be better to stop some of the functions being inlined?

Clearly clang shouldn't be using separate on-stack temporaries
for every printf() call.
That is indicative of a bigger problem.

	David

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