[PATCH] x86: Micro-optimization in nhmex_rbox_get_constraint

[PATCH] x86: Micro-optimization in nhmex_rbox_get_constraint

[Rasmus Villemoes]

Flipping the LSB doesn't require four lines of code. This shaves a few
bytes of the generated code, including a branch.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 arch/x86/kernel/cpu/perf_event_intel_uncore.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_uncore.c b/arch/x86/kernel/cpu/perf_event_intel_uncore.c
index 65bbbea..e009f3c 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_uncore.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_uncore.c
@@ -2946,10 +2946,7 @@ again:
 		 * extra registers. If we failed to take an extra
 		 * register, try the alternative.
 		 */
-		if (idx % 2)
-			idx--;
-		else
-			idx++;
+		idx ^= 1;
 		if (idx != reg1->idx % 6) {
 			if (idx == 2)
 				config1 >>= 8;
-- 
1.9.2

Re: [PATCH] x86: Micro-optimization in nhmex_rbox_get_constraint

[Peter Zijlstra]

On Thu, Jun 19, 2014 at 03:15:31PM +0200, Rasmus Villemoes wrote:
> Flipping the LSB doesn't require four lines of code. This shaves a few
> bytes of the generated code, including a branch.
> 
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>

Thanks!

[tip:perf/core] perf/x86: Micro-optimize nhmex_rbox_get_constraint()

[tip-bot for Rasmus Villemoes]

Commit-ID:  2172c1f5aa58310784f358ca20fdddfcdc2a0d7b
Gitweb:     http://git.kernel.org/tip/2172c1f5aa58310784f358ca20fdddfcdc2a0d7b
Author:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
AuthorDate: Thu, 19 Jun 2014 15:15:31 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Sat, 5 Jul 2014 11:21:52 +0200

perf/x86: Micro-optimize nhmex_rbox_get_constraint()

Flipping the LSB doesn't require four lines of code. This shaves a few
bytes of the generated code, including a branch.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1403183731-15402-1-git-send-email-linux@rasmusvillemoes.dk
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/cpu/perf_event_intel_uncore.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_uncore.c b/arch/x86/kernel/cpu/perf_event_intel_uncore.c
index 65bbbea..e009f3c 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_uncore.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_uncore.c
@@ -2946,10 +2946,7 @@ again:
 		 * extra registers. If we failed to take an extra
 		 * register, try the alternative.
 		 */
-		if (idx % 2)
-			idx--;
-		else
-			idx++;
+		idx ^= 1;
 		if (idx != reg1->idx % 6) {
 			if (idx == 2)
 				config1 >>= 8;