[PATCH] x86: Turn off GCC branch probability heuristics

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> Btw., totally off topic, the following NOP caught my attention:
> 
> >   5a:	66 0f 1f 44 00 00    	nopw   0x0(%rax,%rax,1)
> 
> That's a dead NOP that boats the function a bit, added for the 16 byte 
> alignment of one of the jump targets.

Another thing caught my attention (and I'm hijacking the RCU thread 
again): GCC's notion of how to place branches seems somewhat random, 
and rather bloaty.

So I tried the experiment below on an x86 defconfig, turning off GCC's 
branch heuristics, and it's rather surprising:

     text           data     bss      dec         filename
 12566447        1617840 1089536 15273823         vmlinux.fguess-branch-probability
 11923593        1617840 1089536 14630969         vmlinux.fno-guess-branch-probability

That's an 5.4% code size improvement!

So maybe we should try this, as it results in much more predictable 
(and more compact!) code by default - and allows us to shape loops and 
branches in a natural fashion: by their placement, and also via 
likely()/unlikely() hints when absolutely necessary.

Thoughts?

Thanks,

	Ingo


=================>
From: Ingo Molnar <mingo@xxxxxxxxxx>
Date: Sat, 11 Apr 2015 11:16:30 +0200
Subject: [PATCH] x86: Turn off GCC branch probability heuristics

Not-Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5ba2d9ce82dc..7c12b3f56915 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -81,6 +81,9 @@ else
         KBUILD_CFLAGS += $(call cc-option,-mno-80387)
         KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)
 
+        # Don't guess branch probabilities, follow the code and unlikely()/likely() hints:
+        KBUILD_CFLAGS += -fno-guess-branch-probability
+
 	# Use -mpreferred-stack-boundary=3 if supported.
 	KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)
 
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