2014 ARM Forum

1. 1
ARMv8: Advantages for Android
Ian Rickards

2. Contents
 Why 64-bit in mobile?
 64-bit support in Android™ Lollipop
 Where next?
 Summary
2

3. The ARM® 64-bit Architecture : ARMv8-A
3
 Full native 32-bit execution, side-by-side
with 64-bit
 New, modern, A64 instruction set
architecture (ISA)
 Double the number (and size) of registers
 New instructions for both A32 and A64
AArch32 AArch64
T32 + A32
Crypto
Advanced SIMD
Scalar FP
ARMv8-A
ISA
A64 ISA

4. Why 64-bit in mobile?
4
Performance through
architecture
 Cleaner instruction set architecture
 Hard-float ABI by default in ARMv8-A
 More registers, less stack spillage
 Cheaper function calls
 Up to 16x crypto acceleration
 Preparation for larger memory devices

5. 64-bit support in Android Lollipop
5
 64-bit support for ARM®
 32-bit & 64-bit apps exist in the same build
 Also, introduces the ART runtime
Source : http://www.android.com/

6. What does L mean for developers?
6
 Pure Java apps get ARMv8-A benefit for
free via ART
 32-bit NDK apps run without change, and
at full performance
 Rebuild NDK code with
APP_ABI="arm64-v8a” to take full
advantage of A64
 Interworking rules mean Java apps run as
32-bits if they call 32-bit NDK code

7. What is ART?
7
 ART is a replacement for Dalvik
 AOT vs JIT (ahead of time - i.e. at install)
 Redesigned to be better on multi-core
systems
 Fits well with big.LITTLE™ technology
Measured on Nexus 7 with Dalvik/ART Preview on 4.4
200%
100%
0%
Quadrant CPU Linpack MT
Relative to Dalvik JIT
Dalvik ART

8. ART on ARMv8-A: performance features
8
 Utilizes the modern A64 ISA for 64-bit
apps
 Single-cycle instructions for Java long &
double types
 Uses hard-float ABI
 32-bit object references - no 64-bit
pointer penalty
Rocket by Luis Prado from the Noun Project

9. Considerations for native developers
9
 Porting C code to 64-bit is the same as for
any other architecture
 Review your feature detection code when
moving to 64-bit
 Assembly code needs to be ported to the
more efficient A64 ISA
 NEON™ changes can be simply
recompiled if written using compiler
intrinsics
Change graphic

10. NEON Intrinsics
 Include intrinsics header file (ACLE standard)
10
#include <arm_neon.h>
 Use special NEON data types which
correspond to D and Q registers, e.g.
int8x8_t D-register 8x 8-bit values
int16x4_t D-register 4x 16-bit values
int32x4_t Q-register 4x 32-bit values
 Use NEON intrinsics versions of instructions
vin1 = vld1q_s32(ptr);
vout = vaddq_s32(vin1, vin2);
vst1q_s32(vout, ptr);
 Strongly typed!
 Use vreinterpret_s16_s32( ) to change the type
Fully compatible with AArch64
tmp1 = vmull_u8(vreinterpret_u8_u32(va0), v16_y); // tmp1 = [tmp2 = vmull_u8(vreinterpret_u8_u32(va1), vy); // tmp2 = [static inline void Filter_32_opaque_neon(unsigned x, unsigned y,
SkPMColor a00, SkPMColor a01,
SkPMColor a10, SkPMColor a11,
SkPMColor *dst) {
uint8x8_t vy, vconst16_8, v16_y, vres;
uint16x4_t vx, vconst16_16, v16_x, tmp;
uint32x2_t va0, va1;
uint16x8_t tmp1, tmp2;
vy = vdup_n_u8(y); // duplicate y into vy
vconst16_8 = vmov_n_u8(16); // set up constant in vconst16_v16_y = vsub_u8(vconst16_8, vy); // v16_y = 16-y
va0 = vdup_n_u32(a00); // duplicate a00
va1 = vdup_n_u32(a10); // duplicate a10
va0 = vset_lane_u32(a01, va0, 1); // set top to a01
va1 = vset_lane_u32(a11, va1, 1); // set top to a11

11. Compatibility
 C/instrinsics will port with no effort
 Asm requires reworking of .s file
(mostly cosmetic, but can take
advantage of additional registers)
 AArch64 NEON optimization in
progress
11
 ARM & Linaro working on key Android
libraries using intrinsics
 ffmpeg AArch64 NEON decoders (asm)
 X264 AArch64 NEON encoder (asm)
AArch64 NEON coding
technique
Compatible?
Vectorized “C” Fully compatible
Intrinsics
(“arm_neon.h”)
Fully compatible
Asm (.s) Some porting required
Library routines Yes, if library available

12. Performance – Native
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30%
25%
20%
15%
10%
5%
0%
Single Thread Multithreaded
AArch64 improvement
over AArch32
AnTuTu 32/64bit CPU Test v5.0
Measured on Juno (2x Cortex-A57, 4x Cortex-A53)
30%
25%
20%
15%
10%
5%
0%
bionic
AArch64 improvement
over AArch32
bionic-benchmarks

13. Performance – ART
13
Quadrant 2.0
30%
25%
20%
15%
10%
5%
Measured on Juno (2x Cortex-A57, 4x Cortex-A53)
30%
25%
20%
15%
10%
5%
0%
AArch64 improvement
over AArch32
CPU Score
0%
AArch64 improvement
over AArch32
Linpack
Multi-threaded

14. Want to know more?
 Join us in the Android group at
Connected Community!
14
 http://community.arm.com/groups/android-community
 ARMv8-A Porting Guide:
 http://community.arm.com/docs/DOC-
8453
 Taming ARMv8-A NEON: from theory
to benchmark results
 http://youtu.be/ixuDntaSnHI?list=UUIVqQ
KxCyQLJS6xvSmfndLA
 Porting & optimizing for 64-bit, a compiler
perspective
 http://www.linaro.org/assets/common/campus
-party-presentation-Sept_2013.pdf
 https://www.youtube.com/watch?v=epzYErIIx
0Y
 An OSX perspective of the 32-64-bit
transition
 https://developer.apple.com/library/mac/docu
mentation/Darwin/Conceptual/64bitPorting/in
tro/intro.html

15. Summary
 The ARMv8-A architecture makes the
difference for mobile and 64-bit
 Android Lollipop provides multi-arch
support enabling both 32/64-bit
applications
 Performance gains for those taking
advantage of the ARMv8-A architecture
 Come join us at Connected Community
15

16. 16
Thank You
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