From 52648e83c9a6b9f7fc3dd272d4d10175e93aa62a Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Sun, 17 May 2015 07:56:54 +0200 Subject: x86: Pack loops tightly as well Packing loops tightly (-falign-loops=1) is beneficial to code size: text data bss dec filename 12566391 1617840 1089536 15273767 vmlinux.align.16-byte 12224951 1617840 1089536 14932327 vmlinux.align.1-byte 11976567 1617840 1089536 14683943 vmlinux.align.1-byte.funcs-1-byte 11903735 1617840 1089536 14611111 vmlinux.align.1-byte.funcs-1-byte.loops-1-byte Which reduces the size of the kernel by another 0.6%, so the the total combined size reduction of the alignment-packing patches is ~5.5%. The x86 decoder bandwidth and caching arguments laid out in: be6cb02779ca ("x86: Align jump targets to 1-byte boundaries") apply to loop alignment as well. Furtermore, modern CPU uarchs have a loop cache/buffer that is a L0 cache before even any uop cache, covering a few dozen most recently executed instructions. This loop cache generally does not have the 16-byte alignment restrictions of the uop cache. Now loop alignment can still be beneficial if: - a loop is cache-hot and its surroundings are not. - if the loop is so cache hot that the instruction flow becomes x86 decoder bandwidth limited But loop alignment is harmful if: - a loop is cache-cold - a loop's surroundings are cache-hot as well - two cache-hot loops are close to each other - if the loop fits into the loop cache - if the code flow is not decoder bandwidth limited and I'd argue that the latter five scenarios are much more common in the kernel, as our hottest loops are typically: - pointer chasing: this should fit into the loop cache in most cases and is typically data cache and address generation limited - generic memory ops (memset, memcpy, etc.): these generally fit into the loop cache as well, and are likewise data cache limited. So this patch packs loop addresses tightly as well. Acked-by: Denys Vlasenko Cc: Andy Lutomirski Cc: Aswin Chandramouleeswaran Cc: Borislav Petkov Cc: Brian Gerst Cc: Davidlohr Bueso Cc: H. Peter Anvin Cc: Jason Low Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Tim Chen Link: http://lkml.kernel.org/r/20150410123017.GB19918@gmail.com Signed-off-by: Ingo Molnar --- arch/x86/Makefile | 3 +++ 1 file changed, 3 insertions(+) diff --git a/arch/x86/Makefile b/arch/x86/Makefile index ca17e5fcd125..57996ee840dd 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -80,6 +80,9 @@ else # Align jump targets to 1 byte, not the default 16 bytes: KBUILD_CFLAGS += -falign-jumps=1 + # Pack loops tightly as well: + KBUILD_CFLAGS += -falign-loops=1 + # Don't autogenerate traditional x87 instructions KBUILD_CFLAGS += $(call cc-option,-mno-80387) KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387) -- cgit v1.2.3