[tip:x86/asm] x86: Align jump targets to 1-byte boundaries

tip-bot for Ingo Molnar
2015-05-15T09:41:07Z

Commit-ID: be6cb02779ca74d83481f017db21578cfe92891c
Gitweb: http://git.kernel.org/tip/be6cb02779ca74d83481f017db21578cfe92891c
Author: Ingo Molnar <mingo@kernel.org>
AuthorDate: Fri, 10 Apr 2015 14:08:46 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 15 May 2015 11:04:28 +0200

x86: Align jump targets to 1-byte boundaries

The following NOP in a hot function caught my attention:

> 5a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)

That's a dead NOP that bloats the function a bit, added for the
default 16-byte alignment that GCC applies for jump targets.

I realize that x86 CPU manufacturers recommend 16-byte jump
target alignments (it's in the Intel optimization manual),
to help their relatively narrow decoder prefetch alignment
and uop cache constraints, but the cost of that is very
significant:

text data bss dec filename
12566391 1617840 1089536 15273767 vmlinux.align.16-byte
12224951 1617840 1089536 14932327 vmlinux.align.1-byte

By using 1-byte jump target alignment (i.e. no alignment at all)
we get an almost 3% reduction in kernel size (!) - and a
probably similar reduction in I$ footprint.

Now, the usual justification for jump target alignment is the
following:

- modern decoders tend to have 16-byte (effective) decoder
prefetch windows. (AMD documents it higher but measurements
suggest the effective prefetch window on curretn uarchs is
still around 16 bytes)

- on Intel there's also the uop-cache with cachelines that have
16-byte granularity and limited associativity.

- older x86 uarchs had a penalty for decoder fetches that crossed
16-byte boundaries. These limits are mostly gone from recent
uarchs.

So if a forward jump target is aligned to cacheline boundary then
prefetches will start from a new prefetch-cacheline and there's
higher chance for decoding in fewer steps and packing tightly.

But I think that argument is flawed for typical optimized kernel
code flows: forward jumps often go to 'cold' (uncommon) pieces
of code, and aligning cold code to cache lines does not bring a
lot of advantages (they are uncommon), while it causes
collateral damage:

- their alignment 'spreads out' the cache footprint, it shifts
followup hot code further out

- plus it slows down even 'cold' code that immediately follows 'hot'
code (like in the above case), which could have benefited from the
partial cacheline that comes off the end of hot code.

But even in the cache-hot case the 16 byte alignment brings
disadvantages:

- it spreads out the cache footprint, possibly making the code
fall out of the L1 I$.

- On Intel CPUs, recent microarchitectures have plenty of
uop cache (typically doubling every 3 years) - while the
size of the L1 cache grows much less aggressively. So
workloads are rarely uop cache limited.

The only situation where alignment might matter are tight
loops that could fit into a single 16 byte chunk - but those
are pretty rare in the kernel: if they exist they tend
to be pointer chasing or generic memory ops, which both tend
to be cache miss (or cache allocation) intensive and are not
decoder bandwidth limited.

So the balance of arguments strongly favors packing kernel
instructions tightly versus maximizing for decoder bandwidth:
this patch changes the jump target alignment from 16 bytes
to 1 byte (tightly packed, unaligned).

Acked-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/20150410120846.GA17101@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/Makefile | 3 +++
1 file changed, 3 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index c7c3187..ca17e5f 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -77,6 +77,9 @@ else
KBUILD_AFLAGS += -m64
KBUILD_CFLAGS += -m64

+ # Align jump targets to 1 byte, not the default 16 bytes:
+ KBUILD_CFLAGS += -falign-jumps=1
+
# Don't autogenerate traditional x87 instructions
KBUILD_CFLAGS += $(call cc-option,-mno-80387)
KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)