blob: 528be1d4d1daa7621f0472648e71211431c0bdcd [file] [log] [blame]
From ec1f1957bbf4cc4a58877214061a52fd5f38e25f Mon Sep 17 00:00:00 2001
From: Masahiro Yamada <>
Date: Tue, 10 Mar 2020 19:12:49 +0900
Subject: [PATCH] kconfig: introduce m32-flag and m64-flag
commit 8cc4fd73501d9f1370c3eebb70cfe8cc9e24062b upstream.
When a compiler supports multiple architectures, some compiler features
can be dependent on the target architecture.
This is typical for Clang, which supports multiple LLVM backends.
Even for GCC, we need to take care of biarch compiler cases.
It is not a problem when we evaluate cc-option in Makefiles because
cc-option is tested against the flag in question + $(KBUILD_CFLAGS).
The cc-option in Kconfig, on the other hand, does not accumulate
tested flags. Due to this simplification, it could potentially test
cc-option against a different target.
At first, Kconfig always evaluated cc-option against the host
Since commit e8de12fb7cde ("kbuild: Check for unknown options with
cc-option usage in Kconfig and clang"), in case of cross-compiling
with Clang, the target triple is correctly passed to Kconfig.
The case with biarch GCC (and native build with Clang) is still not
handled properly. We need to pass some flags to specify the target
machine bit.
Due to the design, all the macros in Kconfig are expanded in the
parse stage, where we do not know the target bit size yet.
For example, arch/x86/Kconfig allows a user to toggle CONFIG_64BIT.
If a compiler flag -foo depends on the machine bit, it must be tested
twice, one with -m32 and the other with -m64.
However, -m32/-m64 are not always recognized. So, this commits adds
m64-flag and m32-flag macros. They expand to -m32, -m64, respectively
if supported. Or, they expand to an empty string if unsupported.
The typical usage is like this:
config FOO
default $(cc-option,$(m64-flag) -foo) if 64BIT
default $(cc-option,$(m32-flag) -foo)
This is clumsy, but there is no elegant way to handle this in the
current static macro expansion.
There was discussion for static functions vs dynamic functions.
The consensus was to go as far as possible with the static functions.
Signed-off-by: Masahiro Yamada <>
Tested-by: George Spelvin <>
Reviewed-by: Nathan Chancellor <>
Signed-off-by: Paul Gortmaker <>
diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
index 35d6c4f8da46..e9551628ca6c 100644
--- a/scripts/Kconfig.include
+++ b/scripts/Kconfig.include
@@ -37,3 +37,10 @@ $(error-if,$(failure,command -v $(LD)),linker '$(LD)' not found)
# gcc version including patch level
gcc-version := $(shell,$(srctree)/scripts/ $(CC))
+# machine bit flags
+# $(m32-flag): -m32 if the compiler supports it, or an empty string otherwise.
+# $(m64-flag): -m64 if the compiler supports it, or an empty string otherwise.
+cc-option-bit = $(if-success,$(CC) -Werror $(1) -E -x c /dev/null -o /dev/null,$(1))
+m32-flag := $(cc-option-bit,-m32)
+m64-flag := $(cc-option-bit,-m64)