kbuild: Add support for Clang's polyhedral loop optimizer

* Default N

Makefile

@@ -721,6 +721,21 @@ KBUILD_CFLAGS += $(call cc-option,-fno-reorder-blocks,) \
                  $(call cc-option,-fno-partial-inlining)
 endif
 
+ifdef CONFIG_LLVM_POLLY
+KBUILD_CFLAGS	+= -mllvm -polly \
+		   -mllvm -polly-ast-use-context \
+		   -mllvm -polly-invariant-load-hoisting \
+		   -mllvm -polly-run-inliner \
+		   -mllvm -polly-vectorizer=stripmine
+# Polly may optimise loops with dead paths beyound what the linker
+# # can understand. This may negate the effect of the linker's DCE
+# # so we tell Polly to perfom proven DCE on the loops it optimises
+# # in order to preserve the overall effect of the linker's DCE.
+ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
+KBUILD_CFLAGS	+= -mllvm -polly-run-dce
+endif
+endif
+
 ifneq ($(CONFIG_FRAME_WARN),0)
 KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
 endif

arch/Kconfig

@@ -594,6 +594,15 @@ config SHADOW_CALL_STACK_VMAP
 	  provides better stack exhaustion protection, but increases per-thread
 	  memory consumption as a full page is allocated for each shadow stack.
 
+config LLVM_POLLY
+	bool "Enable LLVM's polyhedral loop optimizer (Polly)"
+	default n
+	help
+	  This option enables LLVM's polyhedral loop optimizer known as Polly.
+	  Polly is able to optimize various loops throughout the kernel for
+	  maximum cache locality. This requires an LLVM toolchain explicitly
+	  compiled with Polly support.
+
 config HAVE_ARCH_WITHIN_STACK_FRAMES
 	bool
 	help