[PATCH v6 2/3] xen: use explicit function alignment if supported by compiler

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Introduce a new Kconfig check for whether the compiler supports
-falign-functions and if supported use it to align functions to the per-arch
selected value, just like it's done for assembly ENTRY() and FUNC() symbols.

Note that it's possible for the compiler to end up using a higher function
alignment regardless of the passed value.  Different compilers handle the
option differently, as clang will ignore -falign-functions value if it's
smaller than the one that would be set by the optimization level, while gcc
seems to always honor the function alignment passed in -falign-functions.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v5:
 - New in this version.
---
 xen/Kconfig  | 5 +++++
 xen/Makefile | 1 +
 2 files changed, 6 insertions(+)

diff --git a/xen/Kconfig b/xen/Kconfig
index 1e1b041fd52f..040cba1b4b73 100644
--- a/xen/Kconfig
+++ b/xen/Kconfig
@@ -41,6 +41,11 @@ config CC_SPLIT_SECTIONS
 #
 # Allow setting on a boolean basis, and then convert such selection to an
 # integer for the build system and code to consume more easily.
+#
+# Requires clang >= 7.0.0
+config CC_HAS_FUNCTION_ALIGNMENT
+       def_bool $(cc-option,-falign-functions)
+
 config FUNCTION_ALIGNMENT_4B
        bool
 config FUNCTION_ALIGNMENT_8B
diff --git a/xen/Makefile b/xen/Makefile
index 21832d640225..7c8249ab3a33 100644
--- a/xen/Makefile
+++ b/xen/Makefile
@@ -390,6 +390,7 @@ CFLAGS += -fomit-frame-pointer
 endif
 
 CFLAGS-$(CONFIG_CC_SPLIT_SECTIONS) += -ffunction-sections -fdata-sections
+CFLAGS-$(CONFIG_CC_HAS_FUNCTION_ALIGNMENT) += 
-falign-functions=$(CONFIG_FUNCTION_ALIGNMENT)
 
 CFLAGS += -nostdinc -fno-builtin -fno-common
 CFLAGS += -Werror -Wredundant-decls -Wwrite-strings -Wno-pointer-arith
-- 
2.43.0