[PATCH v3 2/7] Revert "kbuild: disable clang's default use of -fmerge-all-constants"

[Nick Desaulniers]

This reverts commit 87e0d4f0f37fb0c8c4aeeac46fff5e957738df79.

-fno-merge-all-constants has been the default since clang-6; the minimum
supported version of clang in the kernel is clang-10 (10.0.1).

Suggested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Reviewed-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://reviews.llvm.org/rL329300.
Link: https://github.com/ClangBuiltLinux/linux/issues/9
---
 Makefile | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/Makefile b/Makefile
index 1cfa5029fd2f..e475650c9599 100644
--- a/Makefile
+++ b/Makefile
@@ -932,15 +932,6 @@ KBUILD_CFLAGS += $(call cc-disable-warning, maybe-uninitialized)
 # disable invalid "can't wrap" optimizations for signed / pointers
 KBUILD_CFLAGS	+= $(call cc-option,-fno-strict-overflow)
 
-# clang sets -fmerge-all-constants by default as optimization, but this
-# is non-conforming behavior for C and in fact breaks the kernel, so we
-# need to disable it here generally.
-KBUILD_CFLAGS	+= $(call cc-option,-fno-merge-all-constants)
-
-# for gcc -fno-merge-all-constants disables everything, but it is fine
-# to have actual conforming behavior enabled.
-KBUILD_CFLAGS	+= $(call cc-option,-fmerge-constants)
-
 # Make sure -fstack-check isn't enabled (like gentoo apparently did)
 KBUILD_CFLAGS  += $(call cc-option,-fno-stack-check,)
 
-- 
2.28.0.402.g5ffc5be6b7-goog