[PATCH v2] riscv: don't specify -mno-save-restore when building with clang < 11

[Palmer Dabbelt]

On Tue, 28 Jul 2020 21:44:28 PDT (-0700), tklauser at distanz.ch wrote:
> Clang before version 11 doesn't support -msave-restore and
> -mno-save-restore [1].
>
> [1] https://github.com/ClangBuiltLinux/linux/issues/804
>
> This avoids the following message when building with clang 10 and older:
>
>   '-save-restore' is not a recognized feature for this target (ignoring feature)
>
> Signed-off-by: Tobias Klauser <tklauser at distanz.ch>
> ---
> v2: use cc-option and check for clang version
>
>  arch/riscv/Makefile | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index fb6e37db836d..30e34946af86 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -44,7 +44,14 @@ riscv-march-$(CONFIG_RISCV_ISA_C)	:= $(riscv-march-y)c
>  KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
>  KBUILD_AFLAGS += -march=$(riscv-march-y)
>
> -KBUILD_CFLAGS += -mno-save-restore
> +KBUILD_CFLAGS += $(call cc-option,-mno-save-restore)
> +# Clang versions less than 11 do not support save-restore. See
> +# https://github.com/ClangBuiltLinux/linux/issues/804
> +ifeq ($(CONFIG_CC_IS_CLANG), y)
> +  ifeq ($(shell test $(CONFIG_CLANG_VERSION) -lt 110000; echo $$?),0)
> +    KBUILD_CFLAGS := $(filter-out -mno-save-restore,$(KBUILD_CFLAGS))
> +  endif
> +endif
>  KBUILD_CFLAGS += -DCONFIG_PAGE_OFFSET=$(CONFIG_PAGE_OFFSET)
>
>  ifeq ($(CONFIG_CMODEL_MEDLOW),y)

Ya, that seems worse ;).

Does this work?  I still get a -mno-save-restore on GCC, but I don't have an
LLVM lying around to test it with.

>From 3e3ae8610db0abba9dd6a18e5d6b9bc8c495c2fc Mon Sep 17 00:00:00 2001
From: Palmer Dabbelt <palmerdabbelt at google.com>
Date: Thu, 30 Jul 2020 19:41:11 -0700
Subject: [PATCH 1/2] Kbuild: add cc-option-skipwarn

The documentation describes the behavior of this function.  In theory we could
just add -Werror to cc-option, but I don't want to chase around an infinate
tail of weird build bugs due to compilers that have this odd behavior.

Signed-off-by: Palmer Dabbelt <palmerdabbelt at google.com>
---
 Documentation/kbuild/makefiles.rst | 7 +++++++
 scripts/Kbuild.include             | 6 ++++++
 2 files changed, 13 insertions(+)

diff --git a/Documentation/kbuild/makefiles.rst b/Documentation/kbuild/makefiles.rst
index 6515ebc12b6f..dba9faf4af29 100644
--- a/Documentation/kbuild/makefiles.rst
+++ b/Documentation/kbuild/makefiles.rst
@@ -507,6 +507,13 @@ more details, with real examples.
 	cflags-y will be assigned no value if first option is not supported.
 	Note: cc-option uses KBUILD_CFLAGS for $(CC) options
 
+   cc-option-skipwarn
+	Some compilers emit a warning on unsupported options rather than an
+	error.  This trips up cc-option, as the test compiler run will succeed
+	only to spit out the warning for every file.  cc-option-skipwarn allows
+	the build infastructure to detect compiles that behave this way, but
+	otherwise acts exactly as cc-option does.
+
    cc-option-yn
 	cc-option-yn is used to check if gcc supports a given option
 	and return 'y' if supported, otherwise 'n'.
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 9a15fbf66aa1..2efd3a8890c7 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -129,6 +129,12 @@ CC_OPTION_CFLAGS = $(filter-out $(GCC_PLUGINS_CFLAGS),$(KBUILD_CFLAGS))
 cc-option = $(call __cc-option, $(CC),\
 	$(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS),$(1),$(2))
 
+# cc-option-skipwarn
+# Usage: cflags-y += $(call cc-option-skipwarn,-mno-save-restore)
+
+cc-option-skipwarn = $(call __cc-option, $(CC),\
+	$(KBUILD_CPPFLAGS) -Werror $(CC_OPTION_CFLAGS),$(1),$(2))
+
 # cc-option-yn
 # Usage: flag := $(call cc-option-yn,-march=winchip-c6)
 cc-option-yn = $(call try-run,\
-- 
2.28.0.163.g6104cc2f0b6-goog


>From 8509566a1a3480ea309fd81317d8cd6125b5e294 Mon Sep 17 00:00:00 2001
From: Tobias Klauser <tklauser at distanz.ch>
Date: Wed, 29 Jul 2020 06:44:28 +0200
Subject: [PATCH 2/2] riscv: don't specify -mno-save-restore unless it's
 supported

Clang before version 11 doesn't support -msave-restore and
-mno-save-restore [1].

[1] https://github.com/ClangBuiltLinux/linux/issues/804

This avoids the following message when building with clang 10 and older:

  '-save-restore' is not a recognized feature for this target (ignoring feature)

Signed-off-by: Tobias Klauser <tklauser at distanz.ch>
[Palmer: Use cc-option-nowarn]
Signed-off-by: Palmer Dabbelt <palmerdabbelt at google.com>
---
 arch/riscv/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index fb6e37db836d..532029c887f1 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -44,7 +44,7 @@ riscv-march-$(CONFIG_RISCV_ISA_C)	:= $(riscv-march-y)c
 KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
 KBUILD_AFLAGS += -march=$(riscv-march-y)
 
-KBUILD_CFLAGS += -mno-save-restore
+KBUILD_CFLAGS += $(call cc-option-skipwarn,-mno-save-restore)
 KBUILD_CFLAGS += -DCONFIG_PAGE_OFFSET=$(CONFIG_PAGE_OFFSET)
 
 ifeq ($(CONFIG_CMODEL_MEDLOW),y)
-- 
2.28.0.163.g6104cc2f0b6-goog