linux-kernel - [tip:x86/asm] x86: Pack loops tightly as well

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Message-ID: <tip-24b0af706205ba49cd139913f92fea837a5724a7@git.kernel.org>
Date:	Fri, 15 May 2015 02:40:08 -0700
From:	tip-bot for Ingo Molnar <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	aswin@...com, tim.c.chen@...ux.intel.com, jason.low2@...com,
	hpa@...or.com, brgerst@...il.com, peterz@...radead.org,
	a.p.zijlstra@...llo.nl, dvlasenk@...hat.com, luto@...capital.net,
	dave@...olabs.net, torvalds@...ux-foundation.org,
	tglx@...utronix.de, paulmck@...ux.vnet.ibm.com, bp@...en8.de,
	mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip:x86/asm] x86: Pack loops tightly as well

Commit-ID:  24b0af706205ba49cd139913f92fea837a5724a7
Gitweb:     http://git.kernel.org/tip/24b0af706205ba49cd139913f92fea837a5724a7
Author:     Ingo Molnar <mingo@...nel.org>
AuthorDate: Fri, 10 Apr 2015 14:30:18 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 15 May 2015 11:17:11 +0200

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 <dvlasenk@...hat.com>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Aswin Chandramouleeswaran <aswin@...com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Jason Low <jason.low2@...com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Link: http://lkml.kernel.org/r/20150410123017.GB19918@gmail.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5c7edf9..8c7cc44 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -83,6 +83,9 @@ else
         # Pack functions tightly as well:
         KBUILD_CFLAGS += -falign-functions=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)
--
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