copnix/modules/nixos/boot/patches/0001-clang-polly.patch

From 836c16fbbd981fcf4f6b39faad2e7e7be13d5434 Mon Sep 17 00:00:00 2001
From: Peter Jung <[email protected]>
Date: Thu, 12 Dec 2024 17:28:45 +0100
Subject: [PATCH 1/3] kbuild: Add support for Clang's polyhedral loop
 optimizer.

Polly is able to optimize various loops throughout the kernel for cache
locality. A mathematical representation of the program, based on
polyhedra, is analysed to find opportunistic optimisations in memory
access patterns which then leads to loop transformations.

Polly is not built with LLVM by default, and requires LLVM to be compiled
with the Polly "project". This can be done by adding Polly to
-DLLVM_ENABLE_PROJECTS, for example:

-DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi;polly"

Preliminary benchmarking seems to show an improvement of around two
percent across perf benchmarks:

Benchmark                         | Control    | Polly
--------------------------------------------------------
bonnie++ -x 2 -s 4096 -r 0        | 12.610s    | 12.547s
perf bench futex requeue          | 33.553s    | 33.094s
perf bench futex wake             |  1.032s    |  1.021s
perf bench futex wake-parallel    |  1.049s    |  1.025s
perf bench futex requeue          |  1.037s    |  1.020s

Furthermore, Polly does not produce a much larger image size netting it
to be a "free" optimisation. A comparison of a bzImage for a kernel with
and without Polly is shown below:

bzImage        | stat --printf="%s\n"
-------------------------------------
Control        | 9333728
Polly          | 9345792

Compile times were one percent different at best, which is well within
the range of noise. Therefore, I can say with certainty that Polly has
a minimal effect on compile times, if none.

Signed-off-by: Peter Jung <[email protected]>
---
 Makefile     | 16 ++++++++++++++++
 init/Kconfig | 13 +++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/Makefile b/Makefile
index 64c594bd7..5f0cb6238 100644
--- a/Makefile
+++ b/Makefile
@@ -870,6 +870,22 @@ endif
 KBUILD_RUSTFLAGS += -Cdebug-assertions=$(if $(CONFIG_RUST_DEBUG_ASSERTIONS),y,n)
 KBUILD_RUSTFLAGS += -Coverflow-checks=$(if $(CONFIG_RUST_OVERFLOW_CHECKS),y,n)
 
+ifdef CONFIG_POLLY_CLANG
+KBUILD_CFLAGS	+= -mllvm -polly \
+		   -mllvm -polly-ast-use-context \
+		   -mllvm -polly-invariant-load-hoisting \
+		   -mllvm -polly-opt-fusion=max \
+		   -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
+
 # 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
diff --git a/init/Kconfig b/init/Kconfig
index a20e6efd3..66adb2fc9 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -251,6 +251,19 @@ config BUILD_SALT
 	  This is mostly useful for distributions which want to ensure the
 	  build is unique between builds. It's safe to leave the default.
 
+config POLLY_CLANG
+	bool "Use Clang Polly optimizations"
+	depends on CC_IS_CLANG && $(cc-option,-mllvm -polly -fplugin=LLVMPolly.so)
+	depends on !COMPILE_TEST
+	help
+	  This option enables Clang's polyhedral loop optimizer known as
+	  Polly. Polly is able to optimize various loops throughout the
+	  kernel for cache locality. This requires a Clang toolchain
+	  compiled with support for Polly. More information can be found
+	  from Polly's website:
+
+	    https://polly.llvm.org
+
 config HAVE_KERNEL_GZIP
 	bool
 
-- 
2.47.1


From 8b5347cd5e745e8d0a68b28e55b48c62e6c0c513 Mon Sep 17 00:00:00 2001
From: Username404-59 <[email protected]>
Date: Thu, 12 Dec 2024 21:25:40 +0100
Subject: [PATCH 2/3] kbuild: Update old polly option

Signed-off-by: Username404-59 <[email protected]>
---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 5f0cb6238..fa433f1d9 100644
--- a/Makefile
+++ b/Makefile
@@ -874,7 +874,7 @@ ifdef CONFIG_POLLY_CLANG
 KBUILD_CFLAGS	+= -mllvm -polly \
 		   -mllvm -polly-ast-use-context \
 		   -mllvm -polly-invariant-load-hoisting \
-		   -mllvm -polly-opt-fusion=max \
+		   -mllvm -polly-loopfusion-greedy \
 		   -mllvm -polly-run-inliner \
 		   -mllvm -polly-vectorizer=stripmine
 # Polly may optimise loops with dead paths beyound what the linker
-- 
2.47.1


From 562335afe34f24f9940d198d536e941e326a9716 Mon Sep 17 00:00:00 2001
From: Username404-59 <[email protected]>
Date: Thu, 12 Dec 2024 21:41:01 +0100
Subject: [PATCH 3/3] kbuild: load polly llvm plugin

Signed-off-by: Username404-59 <[email protected]>
---
 Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index fa433f1d9..e8737f5ea 100644
--- a/Makefile
+++ b/Makefile
@@ -871,7 +871,8 @@ KBUILD_RUSTFLAGS += -Cdebug-assertions=$(if $(CONFIG_RUST_DEBUG_ASSERTIONS),y,n)
 KBUILD_RUSTFLAGS += -Coverflow-checks=$(if $(CONFIG_RUST_OVERFLOW_CHECKS),y,n)
 
 ifdef CONFIG_POLLY_CLANG
-KBUILD_CFLAGS	+= -mllvm -polly \
+KBUILD_CFLAGS	+= -fplugin=LLVMPolly.so \
+		   -mllvm -polly \
 		   -mllvm -polly-ast-use-context \
 		   -mllvm -polly-invariant-load-hoisting \
 		   -mllvm -polly-loopfusion-greedy \
-- 
2.47.1