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• ARM inline asm secrets

  6 juillet 2010, par Mans — ARM, Compilers

  Although I generally recommend against using GCC inline assembly, preferring instead pure assembly code in separate files, there are occasions where inline is the appropriate solution. Should one, at a time like this, turn to the GCC documentation for guidance, one must be prepared for a degree of disappointment. As it happens, much of the inline asm syntax is left entirely undocumented. This article attempts to fill in some of the blanks for the ARM target.
  

  Constraints

  Each operand of an inline asm block is described by a constraint string encoding the valid representations of the operand in the generated assembly. For example the “r” code denotes a general-purpose register. In addition to the standard constraints, ARM allows a number of special codes, only some of which are documented. The full list, including a brief description, is available in the constraints.md file in the GCC source tree. The following table is an extract from this file consisting of the codes which are meaningful in an inline asm block (a few are only useful in the machine description itself).

  f	Legacy FPA registers f0-f7.
  t	The VFP registers s0-s31.
  v	The Cirrus Maverick co-processor registers.
  w	The VFP registers d0-d15, or d0-d31 for VFPv3.
  x	The VFP registers d0-d7.
  y	The Intel iWMMX co-processor registers.
  z	The Intel iWMMX GR registers.
  l	In Thumb state the core registers r0-r7.
  h	In Thumb state the core registers r8-r15.
  j	A constant suitable for a MOVW instruction. (ARM/Thumb-2)
  b	Thumb only. The union of the low registers and the stack register.
  I	In ARM/Thumb-2 state a constant that can be used as an immediate value in a Data Processing instruction. In Thumb-1 state a constant in the range 0 to 255.
  J	In ARM/Thumb-2 state a constant in the range -4095 to 4095. In Thumb-1 state a constant in the range -255 to -1.
  K	In ARM/Thumb-2 state a constant that satisfies the I constraint if inverted. In Thumb-1 state a constant that satisfies the I constraint multiplied by any power of 2.
  L	In ARM/Thumb-2 state a constant that satisfies the I constraint if negated. In Thumb-1 state a constant in the range -7 to 7.
  M	In Thumb-1 state a constant that is a multiple of 4 in the range 0 to 1020.
  N	Thumb-1 state a constant in the range 0 to 31.
  O	In Thumb-1 state a constant that is a multiple of 4 in the range -508 to 508.
  Pa	In Thumb-1 state a constant in the range -510 to +510
  Pb	In Thumb-1 state a constant in the range -262 to +262
  Ps	In Thumb-2 state a constant in the range -255 to +255
  Pt	In Thumb-2 state a constant in the range -7 to +7
  G	In ARM/Thumb-2 state a valid FPA immediate constant.
  H	In ARM/Thumb-2 state a valid FPA immediate constant when negated.
  Da	In ARM/Thumb-2 state a const_int, const_double or const_vector that can be generated with two Data Processing insns.
  Db	In ARM/Thumb-2 state a const_int, const_double or const_vector that can be generated with three Data Processing insns.
  Dc	In ARM/Thumb-2 state a const_int, const_double or const_vector that can be generated with four Data Processing insns. This pattern is disabled if optimizing for space or when we have load-delay slots to fill.
  Dn	In ARM/Thumb-2 state a const_vector which can be loaded with a Neon vmov immediate instruction.
  Dl	In ARM/Thumb-2 state a const_vector which can be used with a Neon vorr or vbic instruction.
  DL	In ARM/Thumb-2 state a const_vector which can be used with a Neon vorn or vand instruction.
  Dv	In ARM/Thumb-2 state a const_double which can be used with a VFP fconsts instruction.
  Dy	In ARM/Thumb-2 state a const_double which can be used with a VFP fconstd instruction.
  Ut	In ARM/Thumb-2 state an address valid for loading/storing opaque structure types wider than TImode.
  Uv	In ARM/Thumb-2 state a valid VFP load/store address.
  Uy	In ARM/Thumb-2 state a valid iWMMX load/store address.
  Un	In ARM/Thumb-2 state a valid address for Neon doubleword vector load/store instructions.
  Um	In ARM/Thumb-2 state a valid address for Neon element and structure load/store instructions.
  Us	In ARM/Thumb-2 state a valid address for non-offset loads/stores of quad-word values in four ARM registers.
  Uq	In ARM state an address valid in ldrsb instructions.
  Q	In ARM/Thumb-2 state an address that is a single base register.

  Operand codes

  Within the text of an inline asm block, operands are referenced as %0, %1 etc. Register operands are printed as rN, memory operands as [rN, #offset], and so forth. In some situations, for example with operands occupying multiple registers, more detailed control of the output may be required, and once again, an undocumented feature comes to our rescue.

  Special code letters inserted between the % and the operand number alter the output from the default for each type of operand. The table below lists the more useful ones.

  c	An integer or symbol address without a preceding # sign
  B	Bitwise inverse of integer or symbol without a preceding #
  L	The low 16 bits of an immediate constant
  m	The base register of a memory operand
  M	A register range suitable for LDM/STM
  H	The highest-numbered register of a pair
  Q	The least significant register of a pair
  R	The most significant register of a pair
  P	A double-precision VFP register
  p	The high single-precision register of a VFP double-precision register
  q	A NEON quad register
  e	The low doubleword register of a NEON quad register
  f	The high doubleword register of a NEON quad register
  h	A range of VFP/NEON registers suitable for VLD1/VST1
  A	A memory operand for a VLD1/VST1 instruction
  y	S register as indexed D register, e.g. s5 becomes d2[1]

• I Really Like My New EeePC

  29 août 2010, par Multimedia Mike — General

  Fair warning : I’m just going to use this post to blather disconnectedly about a new-ish toy.

  I really like my new EeePC. I was rather enamored with the original EeePC 701 from late 2007, a little box with a tiny 7″ screen that is credited with kicking off the netbook revolution. Since then, Asus has created about a hundred new EeePC models.

  Since I’m spending so much time on a train these days, I finally took the plunge to get a better netbook. I decided to stay loyal to Asus and their Eee lineage and got the highest end EeePC they presently offer (which was still under US$500)– the EeePC 1201PN. The ’12′ in the model number represents a 12″ screen size and the rest of the specs are commensurately as large. Indeed, it sort of blurs the line between netbook and full-blown laptop.

  
  
  

  Incidentally, after I placed the order for the 1201PN nearly 2 months ago, and I mean the very literal next moment, this Engadget headline came across announcing the EeePC 1215N. My new high-end (such as it is) computer purchase was immediately obsoleted ; I thought that only happened in parody. (As of this writing, the 1215N still doesn’t appear to be shipping, though.)

  It’s a sore point among Linux aficionados that Linux was used to help kickstart the netbook trend but that now it’s pretty much impossible to find Linux pre-installed on a netbook. So it is in this case. This 1201PN comes with Windows 7 Home Premium installed. This is a notable differentiator from most netbooks which only have Windows 7 Home Starter, a.k.a., the Windows 7 version so crippled that it doesn’t even allow the user to change the background image.

  I wished to preserve the Windows 7 installation (you never know when it will come in handy) and dual boot Linux. I thought I would have to use the Windows partition tool to divide work some magic. Fortunately, the default installation already carved the 250 GB HD in half ; I was able to reformat the second partition and install Linux. The details are a little blurry, but I’m pretty sure one of those external USB optical drives shown in my last post actually performed successfully for this task. Lucky break.

  
  
  The EeePC 1201PN, EeePC 701, Belco Alpha-400, and even a comparatively gargantuan Sony Vaio full laptop– all of the portable computers in the household
  

  So I got Ubuntu 10.04 Linux installed in short order. This feels like something of a homecoming for me. You see, I used Linux full-time at home from 1999-2006. In 2007, I switched to using Windows XP full-time, mostly because my home use-case switched to playing a lot of old, bad computer games. By the end of 2008, I had transitioned to using the Mac Mini that I had originally purchased earlier that year for running FATE cycles. That Mac served as my main home computer until I purchased the 1201PN 2 months ago.

  Mostly, I have this overriding desire for computers to just work, at least in their basic functions. And that’s why I’m so roundly impressed with the way Linux handles right out of the box. Nearly everything on the 1201PN works in Linux. The video, the audio, the wireless networking, the webcam, it all works out of the box. I had to do the extra installation step to get the binary nVidia drivers installed but even that’s relatively seamless, especially compared to “the way things used to be” (drop to a prompt, run some binary installer from the prompt as root, watch it fail in arcane ways because the thing is only certified to run on one version of one Linux distribution). The 1201PN, with its nVidia Ion2 graphics, is able to drive both its own 1366×768 screen simultaneously with an external monitor running at up on 2560×1600.

  The only weird hiccup in the whole process was that I had a little trouble with the special volume keys on the keyboard (specifically, the volume up/down/mute keys didn’t do anything). But I quickly learned that I had to install some package related to ACPI and they magically started to do the right thing. Now I get to encounter the Linux Flash Player bug where modifying volume via those special keys forces fullscreen mode to exit. Adobe really should fix that.

  Also, trackpad multitouch gestures don’t work right away. Based on my reading, it is possible to set those up in Linux. But it’s largely a preference thing– I don’t care much for multitouch. This creates a disparity when I use Windows 7 on the 1201PN which is configured per default to use multitouch.

  
  
  The same 4 laptops stacked up
  

  So, in short, I’m really happy with this little machine. Traditionally, I have had absolutely no affinity for laptops/notebooks/portable computers at all even if everyone around was always completely enamored with the devices. What changed for me ? Well for starters, as a long-time Linux user, I was used to having to invest in very specific, carefully-researched hardware lest I not be able to use it under the Linux OS. This was always a major problem in the laptop field which typically reign supreme in custom, proprietary hardware components. These days, not so much, and these netbooks seem to contain well-supported hardware. Then there’s the fact that laptops always cost so much more than similarly capable desktop systems and that I had no real reason for taking a computer with me when I left home. So my use case changed, as did the price point for relatively low-power laptops/netbooks.

  Data I/O geek note : The 1201PN is capable of wireless-N networking — as many netbooks seem to have — but only 100 Mbit ethernet. I wondered why it didn’t have gigabit ethernet. Then I remembered that 100 Mbit ethernet provides 11-11.5 Mbytes/sec of transfer speed which, in my empirical experience, is approximately the maximum write speed of a 5400 RPM hard drive– which is what the 1201PN possesses.

• VP8 for Real-time Video Applications

  15 février 2011, par noreply@blogger.com (John Luther)

  With the growing interest in videoconferencing on the web platform, it’s a good time to explore the features of VP8 that make it an exceptionally good codec for real-time applications like videoconferencing.

  VP8 Design History & Features

  Real-time applications were a primary use case when VP8 was designed. The VP8 encoder has features specifically engineered to overcome the challenges inherent in compressing and transmitting real-time video data.
  

  • Processor-adaptive encoding. 16 encoder complexity levels automatically (or manually) adjust encoder features such as motion search strategy, quantizer optimizations, and loop filtering strength.
  • Encoder can be configured to use a target percentage of the host CPU.
    Ability to measure the time taken to encode each frame and adjust encoder complexity dynamically to keep the encoding time per frame constant
  • Robust error recovery (packet retransmission, forward error correction, recovery frame/new keyframe requests)
  • Temporal scalability (i.e., a single video bitstream that can degrade as needed depending on a participant’s available bandwidth)
  • Highly efficient decoding performance on low-power devices. Conventional video technology has grown to a state of complexity where dedicated hardware chips are needed to make it work well. With VP8, software-based solutions have proven to meet customer needs without requiring specialized hardware.

  For a more information about real-time video features in VP8, see the slide presentation by WebM Project engineer Paul Wilkins (PDF file).

  Commercially Available Products

  Millions of people around the world have been using VP7/8 for video chat for years. VP8 is deployed in some of today’s most popular consumer videoconferencing applications, including Skype (group video calling), Sightspeed, ooVoo and Logitech Vid. All of these vendors are active WebM project supporters. VP8’s predecessor, VP7, has been used in Skype video calling since 2005 and is supported in the new Skype app for iPhone. Other real-time VP8 implementations are coming soon, including ooVoo, and VP8 will play a leading role in Google’s plans for real-time applications on the web platform.

  Real-time applications will be extremely important as the web platform matures. The WebM community has made significant improvements in VP8 for real-time use cases since our launch and will continue to do so in the future.

  John Luther is Product Manager of the WebM Project.