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[Solved] How to set CFLAGS to kernels compiled by genkernel?
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Cruslan
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Joined: 06 Jan 2024
Posts: 26

PostPosted: Sun Jun 30, 2024 7:33 pm    Post subject: [Solved] How to set CFLAGS to kernels compiled by genkernel? Reply with quote

I want to compile kernel with CFLAGS added by genkernel. But i can't find any information in Gentoo documentation. I found a topic that the guy succeed by adding "KERNEL_CFLAGS" variable to /etc/genkernel.conf; https://forums.gentoo.org/viewtopic-p-8785123.html?sid=ffc4f016ae4af7f94b4288619e030a1b It will work or is there any method to kernel built by requested CFLAGS by genkernel? Is it correct usage for CFLAGS in kernel building;

/etc/genkernel.conf
Code:
KERNEL_CFLAGS="-march=native -mtune=native -O3 -pipe -flto=12 -fuse-linker-plugin -ftree-loop-optimize -fgraphite-identity -floop-nest-optimize -floop-parallelize-all -fdevirtualize-at-ltrans -fipa-pta -fno-semantic-interposition"


Last edited by Cruslan on Mon Jul 01, 2024 4:30 pm; edited 1 time in total
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Cruslan
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Joined: 06 Jan 2024
Posts: 26

PostPosted: Mon Jul 01, 2024 4:29 pm    Post subject: Reply with quote

I succeed my goal by cherry-picking TKG-Kernel's "0013-optimize_harder_O3.patch" file in https://github.com/Frogging-Family/linux-tkg/blob/master/linux-tkg-patches/6.9/0013-optimize_harder_O3.patch. I changed default options to my desired CFLAGS. After that i applied patch by emerge to sys-kernel/gentoo-kernel. Lastly i compiled kernel as usual.

Original patch;
Code:
diff --git a/Makefile b/Makefile
--- a/Makefile
+++ b/Makefile
@@ -442,7 +442,7 @@ endif
 HOSTPKG_CONFIG   = pkg-config

 KBUILD_USERHOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
-          -O2 -fomit-frame-pointer -std=gnu11
+          -O3 -fomit-frame-pointer -std=gnu11
 KBUILD_USERCFLAGS  := $(KBUILD_USERHOSTCFLAGS) $(USERCFLAGS)
 KBUILD_USERLDFLAGS := $(USERLDFLAGS)

@@ -474,7 +474,7 @@ endif
              -Wclippy::dbg_macro

 KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
-KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
+KBUILD_HOSTCXXFLAGS := -Wall -O3 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
 KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
          -Zallow-features= $(HOSTRUSTFLAGS)
 KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
@@ -757,7 +757,7 @@ KBUILD_CFLAGS       += $(call cc-disable-warning, format-overflow)
 KBUILD_CFLAGS  += $(call cc-disable-warning, address-of-packed-member)

 ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
-KBUILD_CFLAGS += -O2
+KBUILD_CFLAGS += -O3
 KBUILD_RUSTFLAGS += -Copt-level=2
 else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS += -Os
diff --git a/init/Kconfig b/init/Kconfig
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1401,10 +1401,10 @@ choice
    default CC_OPTIMIZE_FOR_PERFORMANCE

 config CC_OPTIMIZE_FOR_PERFORMANCE
-   bool "Optimize for performance (-O2)"
+   bool "Optimize for performance (-O3)"
    help
      This is the default optimization level for the kernel, building
-     with the "-O2" compiler flag for best performance and most
+     with the "-O3" compiler flag for best performance and most
      helpful compile-time warnings.

 config CC_OPTIMIZE_FOR_SIZE


Edited patch;
Code:
diff --git a/Makefile b/Makefile
--- a/Makefile
+++ b/Makefile
@@ -442,7 +442,7 @@ endif
 HOSTPKG_CONFIG   = pkg-config

 KBUILD_USERHOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
-          -O2 -fomit-frame-pointer -std=gnu11
+          -O3 -fomit-frame-pointer -std=gnu11 -flto=12 -fuse-linker-plugin -ftree-loop-optimize -fgraphite-identity -floop-nest-optimize -floop-parallelize-all -fdevirtualize-at-ltrans -fipa-pta -fno-semantic-interposition
 KBUILD_USERCFLAGS  := $(KBUILD_USERHOSTCFLAGS) $(USERCFLAGS)
 KBUILD_USERLDFLAGS := $(USERLDFLAGS)

@@ -474,7 +474,7 @@ endif
              -Wclippy::dbg_macro

 KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
-KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
+KBUILD_HOSTCXXFLAGS := -Wall -O3 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
 KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
          -Zallow-features= $(HOSTRUSTFLAGS)
 KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
@@ -757,7 +757,7 @@ KBUILD_CFLAGS       += $(call cc-disable-warning, format-overflow)
 KBUILD_CFLAGS  += $(call cc-disable-warning, address-of-packed-member)

 ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
-KBUILD_CFLAGS += -O2
+KBUILD_CFLAGS += -O3
 KBUILD_RUSTFLAGS += -Copt-level=2
 else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS += -Os
diff --git a/init/Kconfig b/init/Kconfig
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1401,10 +1401,10 @@ choice
    default CC_OPTIMIZE_FOR_PERFORMANCE

 config CC_OPTIMIZE_FOR_PERFORMANCE
-   bool "Optimize for performance (-O2)"
+   bool "Optimize for performance (-O3)"
    help
      This is the default optimization level for the kernel, building
-     with the "-O2" compiler flag for best performance and most
+     with the "-O3" compiler flag for best performance and most
      helpful compile-time warnings.

 config CC_OPTIMIZE_FOR_SIZE
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