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  • Can I use Duff's Device on an array in C?

    - by Ben Fossen
    I have a loop here and I want to make it run faster. I am passing in a large array. I recently heard of Duff's Device can it be applied to this for loop? any ideas? for (i = 0; i < dim; i++) { for (j = 0; j < dim; j++) { dst[RIDX(dim-1-j, i, dim)] = src[RIDX(i, j, dim)]; } }

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  • most efficient method of turning multiple 1D arrays into columns of a 2D array

    - by Ty W
    As I was writing a for loop earlier today, I thought that there must be a neater way of doing this... so I figured I'd ask. I looked briefly for a duplicate question but didn't see anything obvious. The Problem: Given N arrays of length M, turn them into a M-row by N-column 2D array Example: $id = [1,5,2,8,6] $name = [a,b,c,d,e] $result = [[1,a], [5,b], [2,c], [8,d], [6,e]] My Solution: Pretty straight forward and probably not optimal, but it does work: <?php // $row is returned from a DB query // $row['<var>'] is a comma separated string of values $categories = array(); $ids = explode(",", $row['ids']); $names = explode(",", $row['names']); $titles = explode(",", $row['titles']); for($i = 0; $i < count($ids); $i++) { $categories[] = array("id" => $ids[$i], "name" => $names[$i], "title" => $titles[$i]); } ?> note: I didn't put the name = value bit in the spec, but it'd be awesome if there was some way to keep that as well.

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  • approximating log10[x^k0 + k1]

    - by Yale Zhang
    Greetings. I'm trying to approximate the function Log10[x^k0 + k1], where .21 < k0 < 21, 0 < k1 < ~2000, and x is integer < 2^14. k0 & k1 are constant. For practical purposes, you can assume k0 = 2.12, k1 = 2660. The desired accuracy is 5*10^-4 relative error. This function is virtually identical to Log[x], except near 0, where it differs a lot. I already have came up with a SIMD implementation that is ~1.15x faster than a simple lookup table, but would like to improve it if possible, which I think is very hard due to lack of efficient instructions. My SIMD implementation uses 16bit fixed point arithmetic to evaluate a 3rd degree polynomial (I use least squares fit). The polynomial uses different coefficients for different input ranges. There are 8 ranges, and range i spans (64)2^i to (64)2^(i + 1). The rational behind this is the derivatives of Log[x] drop rapidly with x, meaning a polynomial will fit it more accurately since polynomials are an exact fit for functions that have a derivative of 0 beyond a certain order. SIMD table lookups are done very efficiently with a single _mm_shuffle_epi8(). I use SSE's float to int conversion to get the exponent and significand used for the fixed point approximation. I also software pipelined the loop to get ~1.25x speedup, so further code optimizations are probably unlikely. What I'm asking is if there's a more efficient approximation at a higher level? For example: Can this function be decomposed into functions with a limited domain like log2((2^x) * significand) = x + log2(significand) hence eliminating the need to deal with different ranges (table lookups). The main problem I think is adding the k1 term kills all those nice log properties that we know and love, making it not possible. Or is it? Iterative method? don't think so because the Newton method for log[x] is already a complicated expression Exploiting locality of neighboring pixels? - if the range of the 8 inputs fall in the same approximation range, then I can look up a single coefficient, instead of looking up separate coefficients for each element. Thus, I can use this as a fast common case, and use a slower, general code path when it isn't. But for my data, the range needs to be ~2000 before this property hold 70% of the time, which doesn't seem to make this method competitive. Please, give me some opinion, especially if you're an applied mathematician, even if you say it can't be done. Thanks.

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  • when is java faster than c++ (or when is JIT faster then precompiled)?

    - by kostja
    I have heard that under certain circumstances, Java programs or rather parts of java programs are able to be executed faster than the "same" code in C++ (or other precompiled code) due to JIT optimizations. This is due to the compiler being able to determine the scope of some variables, avoid some conditionals and pull similar tricks at runtime. Could you give an (or better - some) example, where this applies? And maybe outline the exact conditions under which the compiler is able to optimize the bytecode beyond what is possible with precompiled code? NOTE : This question is not about comparing Java to C++. Its about the possibilities of JIT compiling. Please no flaming. I am also not aware of any duplicates. Please point them out if you are.

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  • Alternate User select interface in django admin to reduce page size on large site?

    - by David Eyk
    I have a Django-based site with roughly 300,000 User objects. Admin pages for objects with a ForeignKey field to User take a very long time to load as the resulting form is about 6MB in size. Of course, the resulting dropdown isn't particularly useful, either. Are there any off-the-shelf replacements for handling this case? I've been googling for a snippet or a blog entry, but haven't found anything yet. I'd like to have a smaller download size and a more usable interface.

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  • How to index a date column with null values?

    - by Heinz Z.
    How should I index a date column when some rows has null values? We have to select rows between a date range and rows with null dates. We use Oracle 9.2 and higher. Options I found Using a bitmap index on the date column Using an index on date column and an index on a state field which value is 1 when the date is null Using an index on date column and an other granted not null column My thoughts to the options are: to 1: the table have to many different values to use an bitmap index to 2: I have to add an field only for this purpose and to change the query when I want to retrieve the null date rows to 3: locks tricky to add an field to an index which is not really needed What is the best practice for this case? Thanks in advance Some infos I have read: Oracle Date Index When does Oracle index null column values?

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  • Oracle Sql Query taking a day long to return results using dblink

    - by Suresh S
    Guys i have the following oracle sql query that gives me the monthwise report between the dates. Basically for nov month i want sum of values between the dates 01nov to 30 nov. The table tha is being queried is residing in another database and accesssed using dblink. The DT columns is of NUMBER type (for ex 20101201) .The execution of the query is taking a day long and not completed. kindly suggest me , if their is any optimisation that can be suggested to my DBA on the dblink, or any tuning that can be done on the query , or rewriting the same. SELECT /*+ PARALLEL (A 8) */ TO_CHAR(TRUNC(TRUNC(SYSDATE,'MM')- 1,'MM'),'MONYYYY') "MONTH", TYPE AS "TYPE", COLUMN, COUNT (DISTINCT A) AS "A_COUNT", COUNT (COLUMN) AS NO_OF_COLS, SUM (DURATION) AS "SUM_DURATION", SUM (COST) AS "COST" FROM **A@LN_PROD A** WHERE DT >=TO_NUMBER(TO_CHAR(TRUNC(TRUNC(SYSDATE,'MM')-1,'MM'),'YYYYMMDD')) AND DT < TO_NUMBER(TO_CHAR(TRUNC(TRUNC(SYSDATE,'MM'),'MM'),'YYYYMMDD')) GROUP BY TYPE, COLUMN

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  • Should not a tail-recursive function also be faster?

    - by Balint Erdi
    I have the following Clojure code to calculate a number with a certain "factorable" property. (what exactly the code does is secondary). (defn factor-9 ([] (let [digits (take 9 (iterate #(inc %) 1)) nums (map (fn [x] ,(Integer. (apply str x))) (permutations digits))] (some (fn [x] (and (factor-9 x) x)) nums))) ([n] (or (= 1 (count (str n))) (and (divisible-by-length n) (factor-9 (quot n 10)))))) Now, I'm into TCO and realize that Clojure can only provide tail-recursion if explicitly told so using the recur keyword. So I've rewritten the code to do that (replacing factor-9 with recur being the only difference): (defn factor-9 ([] (let [digits (take 9 (iterate #(inc %) 1)) nums (map (fn [x] ,(Integer. (apply str x))) (permutations digits))] (some (fn [x] (and (factor-9 x) x)) nums))) ([n] (or (= 1 (count (str n))) (and (divisible-by-length n) (recur (quot n 10)))))) To my knowledge, TCO has a double benefit. The first one is that it does not use the stack as heavily as a non tail-recursive call and thus does not blow it on larger recursions. The second, I think is that consequently it's faster since it can be converted to a loop. Now, I've made a very rough benchmark and have not seen any difference between the two implementations although. Am I wrong in my second assumption or does this have something to do with running on the JVM (which does not have automatic TCO) and recur using a trick to achieve it? Thank you.

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  • Quickest way to write to file in java

    - by user1097772
    I'm writing an application which compares directory structure. First I wrote an application which writes gets info about files - one line about each file or directory. My soulution is: calling method toFile Static PrintWriter pw = new PrintWriter(new BufferedWriter( new FileWriter("DirStructure.dlis")), true); String line; // info about file or directory public void toFile(String line) { pw.println(line); } and of course pw.close(), at the end. My question is, can I do it quicker? What is the quickest way? Edit: quickest way = quickest writing in the file

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  • unroll nested for loops in C++

    - by Hristo
    How would I unroll the following nested loops? for(k = begin; k != end; ++k) { for(j = 0; j < Emax; ++j) { for(i = 0; i < N; ++i) { if (j >= E[i]) continue; array[k] += foo(i, tr[k][i], ex[j][i]); } } } I tried the following, but my output isn't the same, and it should be: for(k = begin; k != end; ++k) { for(j = 0; j < Emax; ++j) { for(i = 0; i+4 < N; i+=4) { if (j >= E[i]) continue; array[k] += foo(i, tr[k][i], ex[j][i]); array[k] += foo(i+1, tr[k][i+1], ex[j][i+1]); array[k] += foo(i+2, tr[k][i+2], ex[j][i+2]); array[k] += foo(i+3, tr[k][i+3], ex[j][i+3]); } if (i < N) { for (; i < N; ++i) { if (j >= E[i]) continue; array[k] += foo(i, tr[k][i], ex[j][i]); } } } } I will be running this code in parallel using Intel's TBB so that it takes advantage of multiple cores. After this is finished running, another function prints out what is in array[] and right now, with my unrolling, the output isn't identical. Any help is appreciated. Thanks, Hristo

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  • Verifying compiler optimizations in gcc/g++ by analyzing assembly listings

    - by Victor Liu
    I just asked a question related to how the compiler optimizes certain C++ code, and I was looking around SO for any questions about how to verify that the compiler has performed certain optimizations. I was trying to look at the assembly listing generated with g++ (g++ -c -g -O2 -Wa,-ahl=file.s file.c) to possibly see what is going on under the hood, but the output is too cryptic to me. What techniques do people use to tackle this problem, and are there any good references on how to interpret the assembly listings of optimized code or articles specific to the GCC toolchain that talk about this problem?

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  • How to make Visual C++ 9 not emit code that is actually never called?

    - by sharptooth
    My native C++ COM component uses ATL. In DllRegisterServer() I call CComModule::RegisterServer(): STDAPI DllRegisterServer() { return _Module.RegisterServer(FALSE); // <<< notice FALSE here } FALSE is passed to indicate to not register the type library. ATL is available as sources, so I in fact compile the implementation of CComModule::RegisterServer(). Somewhere down the call stack there's an if statement: if( doRegisterTypeLibrary ) { //<< FALSE goes here // do some stuff, then call RegisterTypeLib() } The compiler sees all of the above code and so it can see that in fact the if condition is always false, yet when I inspect the linker progress messages I see that the reference to RegisterTypeLib() is still there, so the if statement is not eliminated. Can I make Visual C++ 9 perform better static analysis and actually see that some code is never called and not emit that code?

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  • Optimizing an embedded SELECT query in mySQL

    - by Crazy Serb
    Ok, here's a query that I am running right now on a table that has 45,000 records and is 65MB in size... and is just about to get bigger and bigger (so I gotta think of the future performance as well here): SELECT count(payment_id) as signup_count, sum(amount) as signup_amount FROM payments p WHERE tm_completed BETWEEN '2009-05-01' AND '2009-05-30' AND completed > 0 AND tm_completed IS NOT NULL AND member_id NOT IN (SELECT p2.member_id FROM payments p2 WHERE p2.completed=1 AND p2.tm_completed < '2009-05-01' AND p2.tm_completed IS NOT NULL GROUP BY p2.member_id) And as you might or might not imagine - it chokes the mysql server to a standstill... What it does is - it simply pulls the number of new users who signed up, have at least one "completed" payment, tm_completed is not empty (as it is only populated for completed payments), and (the embedded Select) that member has never had a "completed" payment before - meaning he's a new member (just because the system does rebills and whatnot, and this is the only way to sort of differentiate between an existing member who just got rebilled and a new member who got billed for the first time). Now, is there any possible way to optimize this query to use less resources or something, and to stop taking my mysql resources down on their knees...? Am I missing any info to clarify this any further? Let me know... EDIT: Here are the indexes already on that table: PRIMARY PRIMARY 46757 payment_id member_id INDEX 23378 member_id payer_id INDEX 11689 payer_id coupon_id INDEX 1 coupon_id tm_added INDEX 46757 tm_added, product_id tm_completed INDEX 46757 tm_completed, product_id

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  • How to store static content across branches in a single location in version control

    - by Shravan
    [Just a random thought] I have a pdf doc that is downloaded when the user clicks on 'help' on my website. Now, this is a pretty huge document and is saved in version control (SVN) and is thus copied for all branches that exist in SVN. This is static content and something that developers are not working on, and does not change often. Is there a more efficient way to store it (that would not hamper local deployments) that would make SVN checkouts and updates relatively faster. I know the benefit we get is not huge, this is something that came to my head none the less.

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  • Optimizing Vector elements swaps using CUDA

    - by Orion Nebula
    Hi all, Since I am new to cuda .. I need your kind help I have this long vector, for each group of 24 elements, I need to do the following: for the first 12 elements, the even numbered elements are multiplied by -1, for the second 12 elements, the odd numbered elements are multiplied by -1 then the following swap takes place: Graph: because I don't yet have enough points, I couldn't post the image so here it is: http://www.freeimagehosting.net/image.php?e4b88fb666.png I have written this piece of code, and wonder if you could help me further optimize it to solve for divergence or bank conflicts .. //subvector is a multiple of 24, Mds and Nds are shared memory _shared_ double Mds[subVector]; _shared_ double Nds[subVector]; int tx = threadIdx.x; int tx_mod = tx ^ 0x0001; int basex = __umul24(blockDim.x, blockIdx.x); Mds[tx] = M.elements[basex + tx]; __syncthreads(); // flip the signs if (tx < (tx/24)*24 + 12) { //if < 12 and even if ((tx & 0x0001)==0) Mds[tx] = -Mds[tx]; } else if (tx < (tx/24)*24 + 24) { //if >12 and < 24 and odd if ((tx & 0x0001)==1) Mds[tx] = -Mds[tx]; } __syncthreads(); if (tx < (tx/24)*24 + 6) { //for the first 6 elements .. swap with last six in the 24elements group (see graph) Nds[tx] = Mds[tx_mod + 18]; Mds [tx_mod + 18] = Mds [tx]; Mds[tx] = Nds[tx]; } else if (tx < (tx/24)*24 + 12) { // for the second 6 elements .. swp with next adjacent group (see graph) Nds[tx] = Mds[tx_mod + 6]; Mds [tx_mod + 6] = Mds [tx]; Mds[tx] = Nds[tx]; } __syncthreads(); Thanks in advance ..

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  • Will the compiler optimize escaping an inner loop?

    - by BCS
    The code I have looks like this (all uses of done shown): bool done = false; for(int i = 0; i < big; i++) { ... for(int j = 0; j < wow; j++) { ... if(foo(i,j)) { done = true; break; } ... } if(done) break; ... } will any compilers convert it to this: for(int i = 0; i < big; i++) { ... for(int j = 0; j < wow; j++) { ... if(foo(i,j)) goto __done; // same as a labeled break if we had it ... } ... } __done:;

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  • Nodes set of the same type with if-test. Make it less.

    - by Kalinin
    How to make the code more beautiful (compact)? <xsl:template match="part"> <table class="part"> <xsl:if test="name != ''"> <tr> <td>????????</td><td><xsl:value-of select="name"/></td> </tr> </xsl:if> <xsl:if test="model != ''"> <tr> <td>??????</td><td><xsl:value-of select="model"/></td> </tr> </xsl:if> <xsl:if test="year != ''"> <tr> <td>???</td><td><xsl:value-of select="year"/></td> </tr> </xsl:if> <xsl:if test="glass_type != ''"> <tr> <td>???</td><td><xsl:value-of select="glass_type"/></td> </tr> </xsl:if> <xsl:if test="scancode != ''"> <tr> <td>???????</td><td><xsl:value-of select="scancode"/></td> </tr> </xsl:if> <xsl:if test="eurocode != ''"> <tr> <td>???????</td><td><xsl:value-of select="eurocode"/></td> </tr> </xsl:if> <xsl:if test="coment != ''"> <tr> <td>???????????</td><td><xsl:value-of select="coment"/></td> </tr> </xsl:if> <xsl:if test="glass_size != ''"> <tr> <td>??????</td><td><xsl:value-of select="glass_size"/></td> </tr> </xsl:if> <xsl:if test="vendor != ''"> <tr> <td>?????????????</td><td><xsl:value-of select="vendor"/></td> </tr> </xsl:if> <xsl:if test="trademark != ''"> <tr> <td>???????? ?????</td><td><xsl:value-of select="trademark"/></td> </tr> </xsl:if> <xsl:if test="fprice != ''"> <tr> <td>????</td><td><xsl:value-of select="fprice"/></td> </tr> </xsl:if> </table> </xsl:template> Update: i wrote: <my:translations xmlns:my="my:my"> <w e="name" r="????????"/> <w e="model" r="??????"/> <w e="year" r="???"/> <w e="glass_type" r="???"/> <w e="scancode" r="???????"/> <w e="eurocode" r="???????"/> <w e="comment" r="???????????"/> <w e="glass_size" r="??????"/> <w e="vendor" r="?????????????"/> <w e="trademark" r="???????? ?????"/> <w e="fprice" r="????"/> </my:translations> <xsl:value-of select="//w/@r"/> And have no result from this code. Is it normal? And how can i get new element w?

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  • Hierarchical Hibernate, how many queries are executed?

    - by ghost1
    So I've been dealing with a home brew DB framework that has some seriously flaws, the justification for use being that not using an ORM will save on the number of queries executed. If I'm selecting all possibile records from the top level of a joinable object hierarchy, how many separate calls to the DB will be made when using an ORM (such as Hibernate)? I feel like calling bullshit on this, as joinable entities should be brought down in one query , right? Am I missing something here? note: lazy initialization doesn't matter in this scenario as all records will be used.

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  • MySQL subqueries

    - by swamprunner7
    Can we do this query without subqueries? SELECT login, post_n, (SELECT SUM(vote) FROM votes WHERE votes.post_n=posts.post_n)AS votes, (SELECT COUNT(comments.post_n) FROM comments WHERE comments.post_n=posts.post_n)AS comments_count FROM users, posts WHERE posts.id=users.id AND (visibility=2 OR visibility=3) ORDER BY date DESC LIMIT 0, 15 tables: Users: id, login Posts: post_n, id, visibility Votes: post_n, vote id — it`s user id, Users the main table.

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  • Is there a faster TList implementation ?

    - by dmauric.mp
    My application makes heavy use of TList, so I was wondering if there are any alternative implementations that are faster or optimized for particular use case. I know of RtlVCLOptimize.pas 2.77, which has optimized implementations of several TList methods. But I'd like to know if there is anything else out there. I also don't require it to be a TList descendant, I just need the TList functionality regardless of how it's implemented. It's entirely possible, given the rather basic functionality TList provides, that there is not much room for improvement, but would still like to verify that, hence this question.

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  • Is this implementation truely tail-recursive?

    - by CFP
    Hello everyone! I've come up with the following code to compute in a tail-recursive way the result of an expression such as 3 4 * 1 + cos 8 * (aka 8*cos(1+(3*4))) The code is in OCaml. I'm using a list refto emulate a stack. type token = Num of float | Fun of (float->float) | Op of (float->float->float);; let pop l = let top = (List.hd !l) in l := List.tl (!l); top;; let push x l = l := (x::!l);; let empty l = (l = []);; let pile = ref [];; let eval data = let stack = ref data in let rec _eval cont = match (pop stack) with | Num(n) -> cont n; | Fun(f) -> _eval (fun x -> cont (f x)); | Op(op) -> _eval (fun x -> cont (op x (_eval (fun y->y)))); in _eval (fun x->x) ;; eval [Fun(fun x -> x**2.); Op(fun x y -> x+.y); Num(1.); Num(3.)];; I've used continuations to ensure tail-recursion, but since my stack implements some sort of a tree, and therefore provides quite a bad interface to what should be handled as a disjoint union type, the call to my function to evaluate the left branch with an identity continuation somehow irks a little. Yet it's working perfectly, but I have the feeling than in calling the _eval (fun y->y) bit, there must be something wrong happening, since it doesn't seem that this call can replace the previous one in the stack structure... Am I misunderstanding something here? I mean, I understand that with only the first call to _eval there wouldn't be any problem optimizing the calls, but here it seems to me that evaluation the _eval (fun y->y) will require to be stacked up, and therefore will fill the stack, possibly leading to an overflow... Thanks!

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  • When compiling programs to run inside a VM, what should march and mtune be set to?

    - by Russ
    With VMs being slave to whatever the host machine is providing, what compiler flags should be provided to gcc? I would normally think that -march=native would be what you would use when compiling for a dedicated box, but the fine detail that -march=native is going to as indicated in this article makes me extremely wary of using it. So... what to set -march and -mtune to inside a VM? For a specific example... My specific case right now is compiling python (and more) in a linux guest inside a KVM-based "cloud" host that I have no real control over the host hardware (aside from 'simple' stuff like CPU GHz m CPU count, and available RAM). Currently, cpuinfo tells me I've got an "AMD Opteron(tm) Processor 6176" but I honestly don't know (yet) if that is reliable and whether the guest can get moved around to different architectures on me to meet the host's infrastructure shuffling needs (sounds hairy/unlikely). All I can really guarantee is my OS, which is a 64-bit linux kernel where uname -m yields x86_64.

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  • Javascriptlibrary more efficient than Rickshaw for realtime visualizations

    - by dan kutz
    I want to visualize data as time-series graphs on mobile devices(tablets) and therefore stumbled upon rickshaw, which is based on D3. First I must say I was a little bit confused when I realized that realtime in web design is defined totally different to realtime in engineering which has fixed(and often very short) timeframes. Anyway my aim is to visualize the data as fast as possible, and on older tablets visualization with rickshaw is quite slow. Can anybody recommend another library, which may be more efficient in rendering? Or is there no way out and I have to go native? regards Dan.

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