- linaro-toolchain - lists.linaro.org

by David Gilbert

Hi, I've been looking at some basic libc routine optimisation and have a curious problem with memset and wondered if anyone can offer some insights. Some graphs and links to code are on https://wiki.linaro.org/WorkingGroups/ToolChain/Benchmarks/InitialMemset I've written a simple memset in both a with and without Neon variety and tested them on a Beagle(C4) and a Panda board and I'm finding that the Neon version is faster than the non-neon version (a bit) on the Beagle but a LOT slower on the Panda - and I'd like to understand why it's slower than the non-neon version - I'm guessing it's some form of cache interaction. The graphs on that page are all generated by timing a loop that repeatedly memsets the same area of memory; the X axis is the size of the memset. Prior to the test loop the area is read into cache (I came to the conclusion the A8 didn't write allocate?). There are two variants of the graphs - absolute in MB/s on Y, and a relative set (below the absolute) that are relative to the performance of the libc routines. (The ones below those pairs are just older versions). if you look at the top left graph on that page you can see that on the Beagle (left) my Neon routine beats my Thumb routine a bit (both beating libc). If you look on the top right you see the Panda performance with my Thumb code being the fastest and generally following libc, but the Neon code (red line) topping out at about 2.5GB/s which is substantially below the peak of the libc and ARM code. The core loop of the Neon code (see the bzr link for the full thing) is: 4: subs r4,r4,#32 vst2.8 {d0,d1,d2,d3}, [ r3:256 ]! bne 4b while the core of the non-Neon version is: 4: subs r4,r4,#16 stmia r3!,{r1,r5,r6,r7} bne 4b I've also tried vst1 and vstm in the neon loop and it still won't match the non-Neon version. All suggestions welcome, plus I'd appreciate if anyone can suggest which particular limit it's hitting - does anyone have figures for the theoretical bus and L1 and L2 write bandwidths for a Panda (and Beagle) ? Thanks in advance, Dave

15 years

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Draft of next weeks public review

by Michael Hope

Hi there. I've uploaded a draft of the slides and notes for next weeks public review at: http://bazaar.launchpad.net/~linaro-toolchain-wg/+junk/publicreview1105/fil… 'Toolchain Public Review 11.05.odp' is a set of slides I'll talk to. The first 15-20 minutes will go through these to describe our focus and goals and how they tie together the blueprints and priorities. The rest of the session will go through the current blueprints and priorities. See: Toolchain Blueprints (short).pdf for the summary version and: Toolchain Blueprints (long).pdf for the long version. The long version is interesting if you can't find a particular tool or technology. It may be small enough to be called out as a single work item. These are only a draft, but I realised I haven't shared the plans with the rest of the group very well and Monday's meeting won't be the best. I'm on holiday tomorrow but feel free to send me any comments, -- Michael

15 years

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Reviewing blueprints for the TSC

by Michael Hope

Hi there. I've been going through the blueprints in preparation for next weeks TSC review. The top level topics are good, and I'd like to have the rest of the engineering blueprints checked over and updated to match what we talked about at the summit. Ira, could you please create blueprints for the areas you plan to look into? Anything that will take longer than a month should have a blueprint. It's worth having a catch-all blueprint for anything left over. Please add these as a dependency to: https://blueprints.launchpad.net/linaro/+spec/tr-toolchain-neon-performance Zach, could you check: https://blueprints.launchpad.net/linaro-toolchain-misc/+spec/ltrace-support https://blueprints.launchpad.net/linaro/+spec/tr-toolchain-openocd Peter, there's a whole range of QEMU ones that could do with a pass over. For more about the review, see: https://wiki.linaro.org/Releases/1105/PublicPlanReview For a list of all of the toolchain blueprints, see: http://ex.seabright.co.nz/helpers/blueprints#toolchain -- Michael

15 years

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Mixed vector sizes

by Ira Rosen

Hi, I started to look into mixed vector sizes (in the same loop). My main reason for this was to allow widening and narrowing instructions, that have different vector sizes for src and dest, to work properly. My example was widen_mult (int = short * short), I thought its implementation was not optimal. But now that I have a working GCC mainline for ARM, I see that it works just fine. short ub[], uc[]; int c[]; for (i = 0; i < n; i++) c[i] = ub[i] * ua[i]; is compiled as: .L11: add r1, r1, #1 vldmia r4!, {d18-d19} cmp r5, r1 vldmia ip!, {d16-d17} vmull.s16 q10, d18, d16 vstr d20, [r3, #-32] vstr d21, [r3, #-24] vmull.s16 q8, d19, d17 vstr d16, [r3, #-16] vstr d17, [r3, #-8] add r3, r3, #32 bhi .L11 which looks good to me at least from the vmull point of view. Does anyone have an example when mixed vector size instructions are not used properly? Another reason for mixed sizes could be cases where only part of the loop can be vectorized with the wider vectors. I don't know how common this is. Are there any other reasons to implement mixed vector sizes? I understand that this can be a useful feature, I am just not sure it's the most important one. Thanks, Ira

15 years

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Backport criteria

by Michael Hope

I've been going through the ChangeLog for the release and am having trouble justifying some of the changes brought in. In particular: * -fstrict-volatile-bitfields, which is more appropriate for bare metal/kernel code * Cortex-M4 support * C locale support in libstdc++-v3 The march/mcpu clean up is OK but marginal. Our focus is time based performance on the Cortex-A series with an implied applications over kernel/bare metal. This is a very narrow view, but every non-performance line of code we bring in can also bring in a bug. Any thoughts? For those who are looking at using our toolchain, is earlier access to other toolchain improvements interesting? -- Michael

15 years

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Upstream GCC feature freeze

by Andrew Stubbs

Hi all, As you may or may not know, upstream GCC has now entered 'stage 3' of it's development cycle. This will last until spring. This means that they are only accepting bug fixes and documentation improvements. New features and any performance improvements must wait until GCC 4.6 branches, prior to release, and GCC 4.7 development opens. During this process, our usual preferred work flow (upstream first) will not work, so we'll have to do something else. Here's my proposal: * Create a new Launchpad branch for GCC 4.6. * Synchronize this branch with upstream regularly * once per week, perhaps. * Try to get upstream approval for all new patches in the usual way * on the understanding that they won't be applied until stage 1 * bug fixes are unaffected and may commit as usual. * Commit all pending patches to our own 4.6 branch * and backport them to our 4.5, branch, of course. * Usual "no test regressions" policy applies to our own patches * but beware regressions from merges from upstream. * we may want to track the clean 4.6 test results for comparison This is little different to what we do with the 4.5 release branch now. Thoughts? Andrew

15 years

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Linaro GCC 4.5 2010-11 released

by Michael Hope

The Linaro Toolchain Working Group is pleased to announce the latest release of Linaro GCC 4.5. Linaro GCC 4.5 is the fourth release in the 4.5 series. Based off the latest GCC 4.5.1+svn164911, it includes many ARM-focused performance improvements and bug fixes. Interesting changes include: * Various NEON related fixes * Performance improvements * A clean up of some of the testsuite test cases * An updated version of the __sync multicore primitives * Improvements in data packing when optimising for size * C locale support in libstdc++-v3 This release adds the new option -fstrict-volatile-bitfields and enables it by default on ARM. See doc/invoke.texi for more information. The source tarball is available from: https://launchpad.net/gcc-linaro/+milestone/4.5-2010.11-0 Downloads are available from the Linaro GCC page on Launchpad: https://launchpad.net/gcc-linaro Note that there were no changes to the 4.4 series. -- Michael

15 years

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Linaro GDB 7.2 2010.11-0 released

by Michael Hope

The Linaro Toolchain Working Group is pleased to announce the release of Linaro GDB 7.2. Linaro GDB 7.2 2010.11-0 is the second release in the 7.2 series. Based off the latest GDB 7.2, it includes a number of ARM-focused bug fixes and enhancements. This release concentrates on the GDB test suite and tidies up a number of failures. The source tarball is available at: https://launchpad.net/gdb-linaro/+milestone/7.2-2010.11-0 More information on Linaro GDB is available at: https://launchpad.net/gdb-linaro -- Michael

15 years

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Auto-detection of vector size for NEON

by Ira Rosen

Hi, It looks like it's enough to implement targetm.vectorize. autovectorize_vector_sizes for NEON in order to enable initial auto-detection of vector size. With the attached patch and -mvectorize-with-neon-quad flag, the vectorizer first tries to vectorize for 128 bit, and if this fails, it tries to vectorize for 64 bit. For example, in the attached testcase number of iterations is too small for 128 bit (first 2 iterations have to be peeled in order to align the array accesses), but is sufficient for 64 bit (the accesses are aligned here). I'd appreciate your comments on the patch, and I also have a few questions: 1. Why the default vector size is 64? 2. Where is the place of NEON vectorization tests? I found NEON tests with intrinsics at gcc.target/arm, is that the right place? 3. According to gcc.dg/vect/vect.exp the only flag that is used for NEON (in addition to target independent flags) is -ffast-math. Is that enough? Thanks, Ira ChangeLog: * config/arm/arm.c (arm_autovectorize_vector_sizes): New function. (TARGET_VECTORIZE_AUTOVECTORIZE_VECTOR_SIZES): Define. Index: config/arm/arm.c =================================================================== --- config/arm/arm.c (revision 166032) +++ config/arm/arm.c (working copy) @@ -246,6 +246,7 @@ static bool arm_builtin_support_vector_misalignmen const_tree type, int misalignment, bool is_packed); +static unsigned int arm_autovectorize_vector_sizes (void); /* Table of machine attributes. */ @@ -391,6 +392,9 @@ static const struct default_options arm_option_opt #define TARGET_VECTOR_MODE_SUPPORTED_P arm_vector_mode_supported_p #undef TARGET_VECTORIZE_PREFERRED_SIMD_MODE #define TARGET_VECTORIZE_PREFERRED_SIMD_MODE arm_preferred_simd_mode +#undef TARGET_VECTORIZE_AUTOVECTORIZE_VECTOR_SIZES +#define TARGET_VECTORIZE_AUTOVECTORIZE_VECTOR_SIZES \ + arm_autovectorize_vector_sizes #undef TARGET_MACHINE_DEPENDENT_REORG #define TARGET_MACHINE_DEPENDENT_REORG arm_reorg @@ -23223,6 +23227,12 @@ arm_expand_sync (enum machine_mode mode, } } +static unsigned int +arm_autovectorize_vector_sizes (void) +{ + return TARGET_NEON_VECTORIZE_QUAD ? 16 | 8 : 0; +} + static bool arm_vector_alignment_reachable (const_tree type, bool is_packed) { test: #define N 5 unsigned int ub[N+2] = {1,1,6,39,12,18,14}; unsigned int uc[N+2] = {2,3,4,11,6,7,1}; void main1 () { int i; unsigned int udiff = 2; unsigned int umax = 10; for (i = 0; i < N; i++) { /* Summation. */ udiff += (ub[i+2] - uc[i]); /* Maximum. */ umax = umax < uc[i+2] ? uc[i+2] : umax; } }

15 years

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Weekly meeting at 0900 UTC today

by Michael Hope

Hi there. Just a reminder that today's call is the first at the new time of 0900 UTC which is 9am in the UK, 10am in Germany, 11am in Israel, and 5pm in China. I've updated the meetings page at: https://wiki.linaro.org/WorkingGroups/ToolChain/Meetings with the new details. -- Michael

15 years

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Help on merging two patches

by Yao Qi

Hi, I am backporint some patches from FSF mainline, which may improve Linaro 4.5 gcc on thumb2 speed. The first one is done by Richard E. "Improve optimization to transform TST into LSLS" http://gcc.gnu.org/ml/gcc-patches/2010-06/msg02518.html After it applied to Linaro 4.5 tree, EEMBC speed number downgrades, while code size is reduced to some extent. The code difference is like this, 6801 ldr r1, [r0, #0] f831 3013 ldrh.w r3, [r1, r3, lsl #1] -f413 6f00 tst.w r3, #2048 ; 0x800 -f43f af41 beq.w cc <t_run_test+0xcc> +0518 lsls r0, r3, #20 +f57f af44 bpl.w cc <t_run_test+0xcc> 4610 mov r0, r2 After reading cortex-a8 TRM, I can't find exact timing cycles of lsls. Under Chung-Lin's help, we feel that lsls should be slower than tst, but don't have any evidence to prove. If any people is familiar with arm microarch, help is welcome. If our assumption is correct, we may can change this patch to an optimization specific to size only. The second patch is Bernd's "Fix an if statement in arm_rtx_costs_1" http://gcc.gnu.org/ml/gcc-patches/2010-07/msg02096.html After this patch applied, EEMBC benchmark number is not changed. Shall we merge this patch to linaro 4.5 tree? I am inclined to merge it, but if you have concerns on this patch, let us discuss here. -- Yao Qi CodeSourcery yao(a)codesourcery.com (650) 331-3385 x739

15 years

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Problems with BeagleBoard installation

by Ira Rosen

Hi, I am trying to install my new BeagleBoard xM. I started with https://wiki.linaro.org/Releases/DailyBuilds, but it failed to write to the SD card giving some errors about QEMU (I can try that again if the error messages are important). Dave Martin also gave me this link https://wiki.linaro.org/Source/ImageInstallation, but it doesn't exist anymore. So, I tried these instructions: https://wiki.linaro.org/Source/VexpressImageInstallation with http://snapshots.linaro.org/10.11-daily/linaro-headless/20101107/0/images/t… http://jameswestby.net:8080/view/Hardware%20Packs/job/linaro-omap3%20hwpack… --dev beagle). The installation of the SD card went ok, but the board doesn't work. During boot I see the printings as in the attached file and it doesn't go any further. I hope someone can help me with that, because otherwise I am at dead end... Thanks, Ira

15 years

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Changing meetings

by Michael Hope

Hi there. I plan to change the Toolchain WG meetings due to daylight savings and to better cover the US. The Monday meeting will be at 0900 UTC which is 9 am in the UK, 10 am in central Europe, and 5 pm in Beijing. The standup calls will be merged into one at 1800 UTC on Wednesday which is 6 pm in the UK, 7 pm in central Europe, and 1 pm on the US East Coast. I don't expect China to call in as it's a quite unreasonable time. I'll update the invites and wiki page to reflect this. We'll start the new times next week, so Monday the 8th will be the first meeting at the new time. -- Michael

15 years

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Links

by Michael Hope

This is more for the people in the sprint... http://ex.seabright.co.nz/helpers/blueprints#toolchain http://ex.seabright.co.nz/helpers/planner -- Michael

15 years

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Report of thumb2 tuning investigation

by Yao Qi

The gaol and plan of investigation has been described in [1] In the plan, this task is divided into three parts, 1) patch backport, 2) regression fix, and 3) exploration and study other ARM compilers. This report follow the same manner. 1. Patch backport. 8 patches are listed in [1]. Backport them to Linaro 4.5 tree will improve speed performance. Action/Recommendation: Backport them if speed improves. These patches are ones that I think they *should* improve speed, but "performance surprise" is not impossible. 2. Regression fix. So far (until r99399), Linaro GCC 4.5 is slower than FSF GCC 4.5.0 on some EEMBC benchmarks. Performance regression is introduced by four commits, r99324,r99330,r99369,r99380, see details in [2]. Action/Recommendation: Figure out why speed regression is introduced, and try to fix it. One cent here is that how to avoid speed regression. I do believe that sometimes regression is unavoidable, but it is better if can track them, and keep them manageable. 3. Exploration and study other ARM compilers. In this part, I don't find any possible thumb-2 specific improvements. However, loop optimization and instruction scheduling should be improved on ARM. (This statement may be true to all ports, or even all compilers) Some tickets are opened for this part, LP:660644 Missed optimization opportunities LP:662692 Inner loop in autcor00 can be optimized better LP:656957 LP:645267 Improve code generation on switch statement LP:663793 Tune Swing Modulo Scheduling or Selective Scheduling for ARM LP:656373 Try -fsched-pressure for ARM I have to admit that instruction scheduling is quite hard, but if we can do something here, that will be great. I've put it in "performance-insdie-gcc" session on UDS. Let us talk about it a little there next week. During this investigation, I also find LTO or "whole-program optimization" is useful to some EEMBC benchmarks. (I didn't run LTO/WPO at all, but I got this when read source of benchmarks) [1] Plan of CS304: Thumb2 tuning investigation. http://lists.linaro.org/pipermail/linaro-toolchain/2010-October/000300.html [2] https://wiki.linaro.org/YaoQi/Sandbox/Thumb2SpeedOptimization -- Yao Qi CodeSourcery yao(a)codesourcery.com (650) 331-3385 x739

15 years, 1 month

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finding in 4.5.2 20101003 (Linaro) [release 4.5-2010.10-0]

by leonid.moiseichuk＠nokia.com

Hello, Sorry I cannot find a bugzilla. One issue detected during compilation of ffmpeg, it looks like gcc -DHAVE_AV_CONFIG_H -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -I. -I"/home/ldm/misc/tc/suite/cases/ffmpeg/ffmpeg" -O3 -D_ISOC99_SOURCE -D_POSIX_C_SOURCE=200112 -std=c99 -fomit-frame-pointer -Wdeclaration-after-statement -Wall -Wno-switch -Wdisabled-optimization -Wpointer-arith -Wredundant-decls -Wno-pointer-sign -Wcast-qual -Wwrite-strings -Wtype-limits -Wundef -fno-math-errno -fno-signed-zeros -c -o libavcodec/dsputil.o libavcodec/dsputil.c /tmp/ccK6jdSP.s: Assembler messages: /tmp/ccK6jdSP.s:34764: Error: VFP/Neon double precision register expected -- `vmovl.s16 q3,s12' /tmp/ccK6jdSP.s:34781: Error: VFP/Neon double precision register expected -- `vmovl.s16 q2,s8' /tmp/ccK6jdSP.s:34798: Error: VFP/Neon double precision register expected -- `vmovl.s16 q1,s4' /tmp/ccK6jdSP.s:34800: Error: VFP/Neon double precision register expected -- `vmovl.s16 q0,s0' /tmp/ccK6jdSP.s:34820: Error: VFP/Neon double precision register expected -- `vmovl.s16 q8,s8' The sources, assembler and test case are attached. With best wishes, Leonid

15 years, 1 month

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2010-10-18 Toolchain WG minutes

by Michael Hope

Minutes from the 2010-10-18 call are here: https://wiki.linaro.org/WorkingGroups/ToolChain/Meetings/2010-10-18 I'm afraid I lost the recording of this call so the minutes are incomplete Minutes from the 2010-10-20 standup call are here: https://wiki.linaro.org/WorkingGroups/ToolChain/Meetings/2010-10-18 The calls focused on the upcoming summit. -- Michael

15 years, 1 month

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Linaro@UDS sessions

by Michael Hope

Hi there. I've updated the list of potential Summit sessions based on yesterdays call. Could people please check the Sessions table on https://wiki.linaro.org/WorkingGroups/ToolChain/Meetings/2010-10-18 and flesh out the agenda for sessions that have your name against them. The agenda should be five to ten discussion points, preferably of things that are not well understood and could use input from the group. There's a good discussion on what to expect at a Summit here: http://oubiwann.blogspot.com/2010/10/q-the-ubuntu-developer-summits.html You can check the already-approved sessions here: http://summit.ubuntu.com/uds-n/ Feel free to join in to any other sessions you might find interesting. There will be quite a few people with diverse backgrounds there, including ~80 people from Linaro, ~400 from Ubuntu, ~200 from the community, and ~200 remote. The overlap between Toolchain and Ubuntu interests might not be great, so I'll make sure a common work room is available for idle time. -- Michael

15 years, 1 month

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Flavoured toolchains anyone?

by Marcin Juszkiewicz

Some of Linaro developers works with ARM devices older then ARMv7-a architecture. Other people experiments with hard-float ABI. Each of them has to rebuild toolchain for own use and that means playing with components to have them build properly. But it is no more - I made some patches and armel-cross-toolchain-base since 1.53 version + newer source packages for gcc-4.[45]-armel-cross have support for "debian/flavour" file which allows to set some flags related to toolchain build. So far supported things are: - ARM architecture - float ABI - FPU mode - Thumb mode This feature is not merged into regular Ubuntu packages yet as this is work in progress which needs to be cleaned first. http://people.linaro.org/~hrw/armel-cross-toolchain/ has all source packages needed. Regards, -- JID: hrw(a)jabber.org Website: http://marcin.juszkiewicz.com.pl/ LinkedIn: http://www.linkedin.com/in/marcinjuszkiewicz

15 years, 1 month

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Flavoured toolchains anyone?

by Marcin Juszkiewicz

Some of Linaro developers works with ARM devices older then ARMv7-a architecture. Other people experiments with hard-float ABI. Each of them has to rebuild toolchain for own use and that means playing with components to have them build properly. But it is no more - I made some patches and armel-cross-toolchain-base since 1.53 version + newer source packages for gcc-4.[45]-armel-cross have support for "debian/flavour" file which allows to set some flags related to toolchain build. So far supported things are: - ARM architecture - float ABI - FPU mode - Thumb mode This feature is not merged into regular Ubuntu packages yet as this is work in progress which needs to be cleaned first. http://people.linaro.org/~hrw/armel-cross-toolchain/ has all source packages needed. Regards, -- JID: hrw(a)jabber.org Website: http://marcin.juszkiewicz.com.pl/ LinkedIn: http://www.linkedin.com/in/marcinjuszkiewicz

15 years, 1 month

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NEON vectorization: use of specialized load/store instructions

by Julian Brown

I meant to send this to the "external" Linaro toolchain mailing list, not the internal CS one. Apologies to those who receive it twice! In a follow-up message, Joseph Myers pointed out a post he'd written previously on the same subject: http://gcc.gnu.org/ml/gcc-patches/2010-06/msg00409.html In further followups (at the risk of misrepresenting Joseph & Paul Brook's opinions!), there seemed to be general agreement that a scheme something like that outlined below, with "permuting" loads/stores and some way of handling multiple in-register layouts for vectors seems like it will be a necessary addition to the vectorizer, going forward. Julian Begin forwarded message: Date: Thu, 7 Oct 2010 16:45:17 +0100 From: Julian Brown <julian(a)codesourcery.com> To: Ira Rosen <IRAR(a)il.ibm.com> Cc: Tejas Belagod <Tejas.Belagod(a)arm.com>, Linaro List <gnu-linaro-tools(a)codesourcery.com> Subject: [gnu-linaro-tools] NEON vectorization: use of specialized load/store instructions Hi, We're having some system issues, so I thought I'd take the chance to write down some things I've been thinking about re: utilising the NEON load/store instructions more effectively. I've also attempted to summarize the problems with big-endian mode. All unverified as of yet, so please take with a pinch of salt :-). Comments appreciated. It's been a while since I last thought about some of this stuff... Cheers, Julian Use of specialized load instructions ==================================== To provide good support for NEON's element and structure load/store instructions, GCC lacks support for a couple of key features: 1. A good way of representing a set of two, three or four vector registers (either D- or Q-sized), possibly with non-unit stride. 2. A generalised mapping between memory locations and lane numbers. To start with point 1: currently the element and structure load/store instructions are only supported via intrinsics. These are specified to load and store as if going via an array embedded in a union, i.e.: typedef struct int8x8x2_t { int8x8_t val[2]; } int8x8x2_t; __extension__ static __inline int8x8x2_t __attribute__ ((__always_inline__)) vld2_s8 (const int8_t * __a) { union { int8x8x2_t __i; __builtin_neon_ti __o; } __rv; __rv.__o = __builtin_neon_vld2v8qi ((const __builtin_neon_qi *) __a); return __rv.__i; } Even for a trivial test program, e.g.: #include <arm_neon.h> int foo (int8_t *x) { int8x8x2_t result = vld2_s8 (x); return vget_lane_s8 (result.val[0], 1); } We will generate code like so: sub sp, sp, #32 vld2.8 {d16-d17}, [r0] mov r3, sp vstmia sp, {d16-d17} add ip, sp, #16 ldmia r3, {r0, r1, r2, r3} stmia ip, {r0, r1, r2, r3} fldd d16, [sp, #16] vmov.s8 r0, d16[1] add sp, sp, #32 bx lr I.e., rather than being used directly, the registers loaded by vld2 will always be spilled to the stack then reloaded. This obviously reduces the usefulness of these intrinsics by a large factor. With some planning, it'd be good to find a powerful enough solution to this problem so that the same representation for multiple registers can be used by the autovectorizer as well as the intrinsic-handling code. (One difficulty is that the "foo.val[X]" interface should still be available to user code. There's probably no need for "val" to literally be an array, though other representations would require front-end changes). Assuming it's hard for the register allocator to deal with highly-constrained situations like requiring four consecutive registers, one (ugly) possibility might be to run a pass before register allocation, looking for "big" multi-register vectors and pre-allocating them to hard registers. Even using a fixed allocation of a single set of registers (e.g. make it so that all multi-reg loads/stores larger than a Q register must use d0-d7, or whatever) would probably give better code than what we produce at present, in most cases. Now, point 2. To start with, an aside: AIUI, there is currently an assumption in the vectoriser code that increasing element numbers in vector registers correspond to increasing addresses when those registers are loaded from and stored to memory (as if the vector was a short array, or alternatively as if a union of the vector register and an array of element-types had the same numberings for lanes and array indices corresponding to the same elements). Unfortunately that is only true for NEON in little-endian mode: in big-endian mode, the story is more complicated, for reasons I will try to explain. To remain compliant with the soft-float variant of the ARM EABI, we must pass vector register arguments in ARM registers (or the stack), not vector registers. This means that we must be very careful with the ordering of elements for values passed to functions. Consider the trivial function: int __attribute__((noinline)) qux (int16x8_t x) { x = vaddq_s16 (x, x); return vgetq_lane_s16 (x, 1); } This is compiled by GCC to the following (slightly unimpressively): vmov d18, r1, r0 @ v8hi vmov d19, r3, r2 vmov d20, r1, r0 @ v8hi vmov d21, r3, r2 vadd.i16 q8, q9, q10 vmov.s16 r0, d16[1] bx lr Which may then be called like, e.g.: ldmia sp, {r0-r3} blx qux So: notice that we're careful that when vector values are transferred from NEON registers to core registers, the same result will be transferred to/from memory when we use ldm/stm (core registers) or vldm/vstm (vector registers) -- i.e. we might use "vldm rX, {d18-d19}", storing d18 and d19 in consecutive increasing addresses, or "ldmia rX, {r0-r3}", again with consecutive registers in increasing memory locations, and we get the same outcome. The fact that we can use the multiple-register loads/stores is also important for spilling/reloading between vector and core registers, which inevitably happens occasionally. Notice also that when we call the above function like so: typedef union { int16x8_t quadvec; int16_t half[8]; } u; int foo (int8_t *x) { u bar; int i; for (i = 0; i < 8; i++) bar.half[i] = i; qux (bar.quadvec); } The value returned from "qux" is NOT 2 (1+1), as it would be if we were accessing the value at index 1 in the superimposed array in the union "u". The vgetq_lane_s16 call still interprets the array as if it had been loaded in little-endian element order. But we don't get the result we would have if the vector had been interpreted in purely big-endian order either (i.e. 12, 6+6)! In fact from the perspective of the element numbering used by vgetq_lane_s16, the vector elements we see for each of the (equal) operands of the "vadd" instruction in the qux function are: equiv. core register lane number (at function entry) value ----------- -------------------- ----- [0] high part of r1 3 [1] low part of r1 2 [2] high part of r0 1 [3] low part of r0 0 [4] high part of r3 7 [5] low part of r3 6 [6] high part of r2 5 [7] low part of r2 4 So the value returned will be 2+2, 4. Now, coming back to the vectorizer. Current practice means that increasing element numbers should correspond to increasing memory locations: i.e., that "array ordering" is in effect, just as in the call to vgetq_lane_s16 in the above example. This leads to an anomaly: it means that when the vectorizer asks for a particular element, it will generally get a different one. Most of the time we get away with this, since the vectorizer mostly deals with "opaque" vectors which are operated on element-wise: i.e. we only deal with data at the granularity of whole vectors, so it doesn't matter which order the elements are in. The ARM implementations of reduction operations fortuitously calculate the results across all elements simultaneously, so when one of those elements is extracted, we still get the right answer. One notable exception to this though is the movmisalign<mode> patterns: these are implemented using the vld1 and vst1 instructions, which load elements in "array" order (increasing elements from increasing memory locations), even in big-endian mode. Since vectors loaded using those instructions are "incompatible" with the above scheme, such misaligned accesses are simply disabled in big-endian mode. Of course, generally, sticking with the current non-solution in big-endian mode is not sustainable (and is probably already broken in various cases). So it might be worth thinking about whether supporting big-endian mode properly, as well as handling the more complex load and store element/structure instructions, can be done using some generalised solution. I'm thinking (without having much idea about how feasible such an idea is) of something along the lines of a function (in the mathematical sense) attached to each vector value manipulated by the vectorizer, to map that value's element numberings to and from memory offsets. So then the quad-word vector of 16-bit elements discussed above would look like, in big-endian mode: foo, {6, 4, 2, 0, 14, 12, 10, 8} Whereas in little-endian mode (or in big-endian mode, for vectors loaded using vld1), it would look like: foo, {0, 2, 4, 6, 8, 10, 12, 14} And then, perhaps more interestingly, a vector loaded using e.g. a "multiple 3-element structures" load, vld3.16 {d1, d2, d3}, [rN] Might look like (in either endianness, assuming we can represent a vector of such size in our hypothetical scheme): foo, {0, 6, 12, 18, 2, 8, 14, 20, 4, 10, 16, 22} Though it's not clear that such a scheme would be powerful enough to represent the whole range of element/structure loads/stores available (you'd probably need to be able to specify skipped or don't-care elements to do that, at least).

15 years, 1 month

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Plan of CS304: Thumb2 tuning investigation

by Yao Qi

First of all, the goal of this work is about investigation on speed improvement on linaro gcc 4.5. Finally, the output/result of this work is to list all possible recommendations/actions to improve speed on linaro 4.5. Comments to this plan are welcome. So far, we can improve speed in three ways, 1. Backport patches from FSF GCC 4.6. Note that we don't want to backport the whole 4.6. 2. Benchmark with FSF GCC 4.5.0. Fix performance regressions if there are on linaro gcc 4.5. Output is the reason of performance regression, or even further, give recommendations on how to fix it. 3. Study the code generated by other ARM compilers, and give recommendations on how to improve GCC to do better job. I'll describe these three ways in details in the following sections, - Backport patches from FSF GCC 4.6 I went through gcc-patches archive, and select several patches that are helpful to code improvements. 1 ifcvt optimization. Target independent. http://gcc.gnu.org/ml/gcc-patches/2010-04/msg00832.html 2 redundant register move for sign extending. Thumb2. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43137 3. PR 45335 Use ldrd and strd to access two consecutive words. Not yet approved. http://gcc.gnu.org/ml/gcc-patches/2010-09/msg00059.html 4. Fix an if statement in arm_rtx_costs_1. http://gcc.gnu.org/ml/gcc-patches/2010-07/msg02096.html 5. Reduce code duplication for Thumb2 move patterns http://gcc.gnu.org/ml/gcc-patches/2010-07/msg00624.html 6. ARM ldm/stm peepholes http://gcc.gnu.org/ml/gcc-patches/2010-07/msg00512.html 7. PR44999 Replace "and r0, r0, #255" with uxtb in thumb2 http://gcc.gnu.org/ml/gcc-patches/2010-07/msg01700.html 8. Improve optimization to transform TST into LSLS http://gcc.gnu.org/ml/gcc-patches/2010-06/msg02518.html 9. Fix bswap patterns for ARM / Thumb and Thumb2. http://gcc.gnu.org/ml/gcc-patches/2010-01/msg01238.html - Fix speed regression I found speed regression on EEMBC on linaro 4.5, compared with FSF GCC 4.5.0, and I'll investigate why speed regression happens on these cases. Here is a table below about speed regression compared between FSF GCC 4.5.0 and Linaro GCC 4.5 (revno:99398) O2 O3 puwmod01, -5.5 -3.5 bitmnp01, -7.9 -0.7 routelookup, -6.4 -8.2 conven00data_1, -7.2 -5.8 conven00data_2, -8.1 -7.3 conven00data_3, -6.6 -5.5 viterb00data_1, -1.7 +5.9 viterb00data_2, -4.3 +2.6 viterb00data_3, -2.3 +1.8 viterb00data_4, -5.3 -0.3 - Study the code generated by other ARM compilers. In this part, I'll study the binary generated by other ARM compilers, and try to teach GCC smart enough to do the same thing. This piece of work is quite open, and hard to estimate how much output we could get. -- Yao Qi CodeSourcery yao(a)codesourcery.com (650) 331-3385 x739

15 years, 1 month

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Gcc test suite failures

by Nicolas Pitre

People here might want to have a look at this bug: http://gcc.gnu.org/bugzilla/show_bg.cgi?id=45979 Note that the heap randomization feature added to the kernel was part of a Linaro security blueprint. Nicolas

15 years, 1 month

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Linaro Toolchain 2010.10 release

by Michael Hope

The Linaro Toolchain Working Group is pleased to announce the 2010.10 consolidation release including Linaro GCC 4.4, Linaro GCC 4.5, and the first version of Linaro GDB 7.2. Linaro GDB 7.2 2010.10-0 is the first release in the 7.2 series. Based off the latest GDB 7.2, it includes a number of ARM-focused bug fixes and enhancements. Interesting changes include: * Backtraces in Thumb-2 code are significantly improved * Much better prologue and epilogue parsing * Improved software watchpoint support * Many test suite tidy-ups Linaro GCC 4.5 is the third release in the 4.5 series. Based off the latest GCC 4.5.1+svn, it includes many ARM-focused performance improvements and bug fixes. Linaro GCC 4.4 is the fourth release in the 4.4 series. Based off the latest GCC 4.4.5, it fixes many of the issues found during building Ubuntu over the last few months. Interesting changes include: * Linaro GCC 4.4 is now based off FSF GCC 4.4.5 * Cortex A8 and Cortex A9 scheduler NEON improvements * Better code generation for constant addresses with inline assembly * Better code for copying small constant strings * Various correctness improvements Downloads are available from the Linaro GCC and GDB pages on Launchpad: https://launchpad.net/gcc-linaro https://launchpad.net/gdb-linaro -- Michael

15 years, 1 month

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DCC device driver and gdb server

by Øyvind Harboe

Hi all, I was wondering someone knows about a ARM DCC (debug communications channel) device driver. The idea is to run gdbserver on /dev/dcc such that application debugging does not hog a serial/ethernet port. I'd modify OpenOCD to forward the DCC onto a TCP/IP port to connect GDB to the gdbserver. -- Øyvind Harboe US toll free 1-866-980-3434 / International +47 51 63 25 00 http://www.zylin.com/zy1000.html ARM7 ARM9 ARM11 XScale Cortex JTAG debugger and flash programmer

15 years, 1 month

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