kbuild: Add support for LLVM's Polly optimizer

This adds support for compiling the kernel with optimizations offered
by LLVM's polyhedral loop optimizer known as Polly, which can improve
performance by improving cache locality in loops. Note that LLVM is not
compiled with Polly by default -- it must be enabled explicitly.

[ghostrider-reborn]
- Removed polly DCE as it's no longer supported

Signed-off-by: Adithya R <gh0strider.2k18.reborn@gmail.com>

Makefile

@@ -808,6 +808,16 @@ else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS += -Os
 endif
 
+ifdef CONFIG_LLVM_POLLY
+KBUILD_CFLAGS	+= -mllvm -polly \
+		   -mllvm -polly-run-inliner \
+		   -mllvm -polly-opt-fusion=max \
+		   -mllvm -polly-ast-use-context \
+		   -mllvm -polly-detect-keep-going \
+		   -mllvm -polly-vectorizer=stripmine \
+		   -mllvm -polly-invariant-load-hoisting
+endif
+
 # Tell gcc to never replace conditional load with a non-conditional one
 KBUILD_CFLAGS	+= $(call cc-option,--param=allow-store-data-races=0)
 KBUILD_CFLAGS	+= $(call cc-option,-fno-allow-store-data-races)

arch/Kconfig

@@ -738,6 +738,14 @@ config CFI_PERMISSIVE
 	  warning instead of a kernel panic. This option is useful for finding
 	  CFI violations during development.
 
+config LLVM_POLLY
+	bool "Enable LLVM's polyhedral loop optimizer (Polly)"
+	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