Makefile: remove stale cc-option checks

cc-option, cc-option-yn, and cc-disable-warning all invoke the compiler
during build time, and can slow down the build when these checks become
stale for our supported compilers, whose minimally supported versions
increases over time. See Documentation/process/changes.rst for the
current supported minimal versions (GCC 4.9+, clang 10.0.1+). Compiler
version support for these flags may be verified on godbolt.org.

The following flags are GCC only and supported since at least GCC 4.9.
Remove cc-option and cc-disable-warning tests.
* -fno-tree-loop-im
* -Wno-maybe-uninitialized
* -fno-reorder-blocks
* -fno-ipa-cp-clone
* -fno-partial-inlining
* -femit-struct-debug-baseonly
* -fno-inline-functions-called-once
* -fconserve-stack

The following flags are supported by all supported versions of GCC and
Clang. Remove their cc-option, cc-option-yn, and cc-disable-warning tests.
* -fno-delete-null-pointer-checks
* -fno-var-tracking
* -Wno-array-bounds

The following configs are made dependent on GCC, since they use GCC
specific flags.
* READABLE_ASM
* DEBUG_SECTION_MISMATCH

-mfentry was not supported by s390-linux-gnu-gcc until gcc-9+, add a
comment.

--param=allow-store-data-races=0 was renamed to -fno-allow-store-data-races
in the GCC 10 release; add a comment.

-Wmaybe-uninitialized (GCC specific) was being added for CONFIG_GCOV,
then again unconditionally; add it only once.

Also, base RETPOLINE_CFLAGS and RETPOLINE_VDSO_CFLAGS on CONFIC_CC_IS_*
then remove cc-option tests for Clang.

Link: https://github.com/ClangBuiltLinux/linux/issues/1436
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

diff --git a/Makefile b/Makefile
index 891866af078720610e125ee90486e9387564ae47..ce5a297ecd7c8da9fbd12f50df4d55195a388e35 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -669,9 +669,10 @@ endif # KBUILD_EXTMOD
 # Defaults to vmlinux, but the arch makefile usually adds further targets
 all: vmlinux
 
-CFLAGS_GCOV    := -fprofile-arcs -ftest-coverage \
-       $(call cc-option,-fno-tree-loop-im) \
-       $(call cc-disable-warning,maybe-uninitialized,)
+CFLAGS_GCOV    := -fprofile-arcs -ftest-coverage
+ifdef CONFIG_CC_IS_GCC
+CFLAGS_GCOV    += -fno-tree-loop-im
+endif
 export CFLAGS_GCOV
 
 # The arch Makefiles can override CC_FLAGS_FTRACE. We may also append it later.
@@ -679,12 +680,14 @@ ifdef CONFIG_FUNCTION_TRACER
   CC_FLAGS_FTRACE := -pg
 endif
 
-RETPOLINE_CFLAGS_GCC := -mindirect-branch=thunk-extern -mindirect-branch-register
-RETPOLINE_VDSO_CFLAGS_GCC := -mindirect-branch=thunk-inline -mindirect-branch-register
-RETPOLINE_CFLAGS_CLANG := -mretpoline-external-thunk
-RETPOLINE_VDSO_CFLAGS_CLANG := -mretpoline
-RETPOLINE_CFLAGS := $(call cc-option,$(RETPOLINE_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_CFLAGS_CLANG)))
-RETPOLINE_VDSO_CFLAGS := $(call cc-option,$(RETPOLINE_VDSO_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_VDSO_CFLAGS_CLANG)))
+ifdef CONFIG_CC_IS_GCC
+RETPOLINE_CFLAGS       := $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
+RETPOLINE_VDSO_CFLAGS  := $(call cc-option,-mindirect-branch=thunk-inline -mindirect-branch-register)
+endif
+ifdef CONFIG_CC_IS_CLANG
+RETPOLINE_CFLAGS       := -mretpoline-external-thunk
+RETPOLINE_VDSO_CFLAGS  := -mretpoline
+endif
 export RETPOLINE_CFLAGS
 export RETPOLINE_VDSO_CFLAGS
 
@@ -737,7 +740,7 @@ include/config/auto.conf:
 endif # may-sync-config
 endif # need-config
 
-KBUILD_CFLAGS  += $(call cc-option,-fno-delete-null-pointer-checks,)
+KBUILD_CFLAGS  += -fno-delete-null-pointer-checks
 KBUILD_CFLAGS  += $(call cc-disable-warning,frame-address,)
 KBUILD_CFLAGS  += $(call cc-disable-warning, format-truncation)
 KBUILD_CFLAGS  += $(call cc-disable-warning, format-overflow)
@@ -752,17 +755,19 @@ KBUILD_CFLAGS += -Os
 endif
 
 # Tell gcc to never replace conditional load with a non-conditional one
+ifdef CONFIG_CC_IS_GCC
+# gcc-10 renamed --param=allow-store-data-races=0 to
+# -fno-allow-store-data-races.
 KBUILD_CFLAGS  += $(call cc-option,--param=allow-store-data-races=0)
 KBUILD_CFLAGS  += $(call cc-option,-fno-allow-store-data-races)
+endif
 
 ifdef CONFIG_READABLE_ASM
 # Disable optimizations that make assembler listings hard to read.
 # reorder blocks reorders the control in the function
 # ipa clone creates specialized cloned functions
 # partial inlining inlines only parts of functions
-KBUILD_CFLAGS += $(call cc-option,-fno-reorder-blocks,) \
-                 $(call cc-option,-fno-ipa-cp-clone,) \
-                 $(call cc-option,-fno-partial-inlining)
+KBUILD_CFLAGS += -fno-reorder-blocks -fno-ipa-cp-clone -fno-partial-inlining
 endif
 
 ifneq ($(CONFIG_FRAME_WARN),0)
@@ -854,8 +859,10 @@ DEBUG_CFLAGS       += -gdwarf-$(dwarf-version-y)
 endif
 
 ifdef CONFIG_DEBUG_INFO_REDUCED
-DEBUG_CFLAGS   += $(call cc-option, -femit-struct-debug-baseonly) \
-                  $(call cc-option,-fno-var-tracking)
+DEBUG_CFLAGS   += -fno-var-tracking
+ifdef CONFIG_CC_IS_GCC
+DEBUG_CFLAGS   += -femit-struct-debug-baseonly
+endif
 endif
 
 ifdef CONFIG_DEBUG_INFO_COMPRESSED
@@ -889,6 +896,7 @@ ifdef CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
   endif
 endif
 ifdef CONFIG_HAVE_FENTRY
+  # s390-linux-gnu-gcc did not support -mfentry until gcc-9.
   ifeq ($(call cc-option-yn, -mfentry),y)
     CC_FLAGS_FTRACE    += -mfentry
     CC_FLAGS_USING     += -DCC_USING_FENTRY
@@ -901,7 +909,7 @@ endif
 
 # We trigger additional mismatches with less inlining
 ifdef CONFIG_DEBUG_SECTION_MISMATCH
-KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
+KBUILD_CFLAGS += -fno-inline-functions-called-once
 endif
 
 ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
@@ -980,14 +988,16 @@ KBUILD_CFLAGS += $(call cc-disable-warning, stringop-truncation)
 
 # We'll want to enable this eventually, but it's not going away for 5.7 at least
 KBUILD_CFLAGS += $(call cc-disable-warning, zero-length-bounds)
-KBUILD_CFLAGS += $(call cc-disable-warning, array-bounds)
+KBUILD_CFLAGS += -Wno-array-bounds
 KBUILD_CFLAGS += $(call cc-disable-warning, stringop-overflow)
 
 # Another good warning that we'll want to enable eventually
 KBUILD_CFLAGS += $(call cc-disable-warning, restrict)
 
 # Enabled with W=2, disabled by default as noisy
-KBUILD_CFLAGS += $(call cc-disable-warning, maybe-uninitialized)
+ifdef CONFIG_CC_IS_GCC
+KBUILD_CFLAGS += -Wno-maybe-uninitialized
+endif
 
 # disable invalid "can't wrap" optimizations for signed / pointers
 KBUILD_CFLAGS  += -fno-strict-overflow
@@ -996,7 +1006,9 @@ KBUILD_CFLAGS      += -fno-strict-overflow
 KBUILD_CFLAGS  += -fno-stack-check
 
 # conserve stack if available
-KBUILD_CFLAGS   += $(call cc-option,-fconserve-stack)
+ifdef CONFIG_CC_IS_GCC
+KBUILD_CFLAGS   += -fconserve-stack
+endif
 
 # Prohibit date/time macros, which would make the build non-deterministic
 KBUILD_CFLAGS   += -Werror=date-time

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 5ddd575159fb82f2f9d0bc95b7b162f2b47d8f5f..7d3d29203a72f36d49e5caee34bbc166c859de7b 100644 (file)
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -365,6 +365,7 @@ config STRIP_ASM_SYMS
 config READABLE_ASM
        bool "Generate readable assembler code"
        depends on DEBUG_KERNEL
+       depends on CC_IS_GCC
        help
          Disable some compiler optimizations that tend to generate human unreadable
          assembler output. This may make the kernel slightly slower, but it helps
@@ -383,6 +384,7 @@ config HEADERS_INSTALL
 
 config DEBUG_SECTION_MISMATCH
        bool "Enable full Section mismatch analysis"
+       depends on CC_IS_GCC
        help
          The section mismatch analysis checks if there are illegal
          references from one section to another section.