| msg278355 - (view) |
Author: Serhiy Storchaka (serhiy.storchaka) *  |
Date: 2016-10-09 12:32 |
| The expression for testing that some index is in half-open range from 0 to limit can be written as index >= 0 && index < limit or as (size_t)index < (size_t)limit The latter form generates simpler machine code. This is idiomatic code, it is used in many C and C++ libraries (including C++ stdlib implementations). It already is used in CPython (in deque implementation). Proposed patch rewrites index range checks in more efficient way. The patch was generated automatically by coccinelle script, and then manually cleaned up. |
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| msg278397 - (view) |
Author: Raymond Hettinger (rhettinger) * |
Date: 2016-10-10 02:17 |
| Don't change the code in the collections module. While semantically valid, the change obfuscates the code. The meaning of maxlen < 0 is that there is no maximum length. I don't want this test hidden; otherwise, I would have changed it long ago as was done elsewhere in the module where it made sense. Elsewhere, consider making a macro with clear name and comment (as was done with NEEDS_TRIM) in the collections module. Otherwise, you're just leaving behind constipated and tricky code with no indication of why a signed variable is being coerced to unsigned. |
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| msg278408 - (view) |
Author: Stefan Krah (skrah) * |
Date: 2016-10-10 10:05 |
| Serhiy, could you please take out _decimal.c? I thought we had been through that before. :) :) :) |
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| msg278410 - (view) |
Author: Stefan Krah (skrah) * |
Date: 2016-10-10 11:55 |
| The same applies to memoryview.c and _testbuffer.c -- I doubt that there's any measurable speed benefit and the clarity is reduced (I also don't want macros there). |
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| msg278469 - (view) |
Author: Stefan Krah (skrah) * |
Date: 2016-10-11 11:05 |
| In the following program, with gcc-5.3 doit() is significantly faster than doit2() in 64-bit Linux: ================================================================ #include <stdint.h> int doit(int64_t index, int64_t nitems) { return index < 0 | |
index >= nitems; } int doit2(int64_t index, int64_t nitems) { return (uint64_t)index >= (uint64_t)nitems; } int main(void) { int count, i; for (i = 0; i < 1000000000; i++) { count += doit(832921, i); } return count; } ================================================================ |
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| msg278471 - (view) |
Author: Stefan Krah (skrah) * |
Date: 2016-10-11 11:45 |
| The difference in favor of doit() is even more pronounced with this loop (also sorry for the uninitialized variable, but that does not make a difference for benchmarking): ===================================== for (i = 0; i < 10000; i++) { for (j = 0; j < 10000; j++) { count += doit(i, j); } } ====================================== |
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| msg278472 - (view) |
Author: Марк Коренберг (socketpair) * |
Date: 2016-10-11 11:45 |
| I have tested. performace differs in about of two times. gcc -O3 0.22 nanoseconds per comparison. vs 0.58 nanoseconds per comparison. Does it cost a time that we spent to discuss here ? |
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| msg278473 - (view) |
Author: Stefan Krah (skrah) * |
Date: 2016-10-11 11:47 |
| Which version is faster in your tests? |
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| msg278474 - (view) |
Author: Марк Коренберг (socketpair) * |
Date: 2016-10-11 11:47 |
| Much more conveniet way is to use unsigned variables in appropriate places. |
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| msg278475 - (view) |
Author: Марк Коренберг (socketpair) * |
Date: 2016-10-11 11:49 |
| oh shi....doit() i.e. return index < 0 | |
index >= nitems; is faster! |
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| msg278476 - (view) |
Author: Марк Коренберг (socketpair) * |
Date: 2016-10-11 11:50 |
| Without optimisation in compiler (-O0) speed is the same. |
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| msg278477 - (view) |
Author: Марк Коренберг (socketpair) * |
Date: 2016-10-11 11:51 |
| -O2 -- the same speed too! -O3 really helps. |
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| msg278478 - (view) |
Author: Марк Коренберг (socketpair) * |
Date: 2016-10-11 11:52 |
| $ gcc --version gcc (Ubuntu 5.4.0-6ubuntu1~16.04.2) 5.4.0 20160609 |
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| msg278479 - (view) |
Author: Stefan Krah (skrah) * |
Date: 2016-10-11 11:54 |
| That matches my results as well: -O2 gives about the same speed, with -O3 doit() has a huge advantage. I'm not sure this is an optimization at all. |
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| msg278481 - (view) |
Author: Serhiy Storchaka (serhiy.storchaka) * |
Date: 2016-10-11 13:05 |
| In your example functions are inlined. If prohibit inlining, the second function is faster. $ gcc -O3 -o -2.c $ time ./issue28397 0 real 0m8.097s user 0m7.992s sys 0m0.012s $ time ./issue28397 1 real 0m5.467s user 0m5.436s sys 0m0.024s |
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| msg278484 - (view) |
Author: Марк Коренберг (socketpair) * |
Date: 2016-10-11 13:24 |
| $ gcc -O3 -DDOIT=doit ./zzz.c -o zzz && time ./zzz real 0m1.675s user 0m1.672s sys 0m0.000s $ gcc -O3 -DDOIT=doit2 ./zzz.c -o zzz && time ./zzz real 0m1.657s user 0m1.656s sys 0m0.000s ==================================================== #include <stdint.h> static int __attribute__((noinline)) doit(int64_t index, int64_t nitems) { return index < 0 | |
index >= nitems; } static int __attribute__((noinline)) doit2(int64_t index, int64_t nitems) { return (uint64_t)index >= (uint64_t)nitems; } int main(void) { int count=0, i; for (i = 0; i < 1000000000; i++) { count += DOIT(832921, i); } return count; } |
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| msg278485 - (view) |
Author: Stefan Krah (skrah) * |
Date: 2016-10-11 13:30 |
| On 64-bit Linux there's no difference: $ ./usr/bin/gcc -O3 -o -2 -2.c $ time ./issue28397-2 0 real 0m2.486s user 0m2.424s sys 0m0.014s $ time ./issue28397-2 1 real 0m2.433s user 0m2.422s sys 0m0.008s Also, most of the time "index < 0 | |
index >= nitems" *is* inlined, and it was at least three times faster here. I guess the general point is that such micro-optimizations are unpredictable on modern architectures and modern compilers. Note that the fast inlined version used SSE instructions. |
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| msg278486 - (view) |
Author: STINNER Victor (vstinner) * |
Date: 2016-10-11 13:32 |
| Serhiy: "The latter form generates simpler machine code." Since this issue is an optimization, can you please provide results of Python benchmarks? Maybe run the performance benchmark suite? I just released performance 0.3 with new benchmarks and less bugs! ;-) http://github.com/python/performance |
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| msg278489 - (view) |
Author: Serhiy Storchaka (serhiy.storchaka) * |
Date: 2016-10-11 14:25 |
| Unlikely this optimization have measurable affect on benchmarks. I opened this issue because this optimization already was applied to deque (), and it is easy to write a script for applying it to all code. But since there are doubts about this optimization, I withdraw my patch. |
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| msg278491 - (view) |
Author: STINNER Victor (vstinner) * |
Date: 2016-10-11 14:39 |
| Serhiy Storchaka: "I opened this issue because this optimization already was applied to deque ()" Ah, change 1e89094998b2 written by Raymond Hettinger last year, Raymond who wrote (): "Don't change the code in the collections module. While semantically valid, the change obfuscates the code." :-) To stay consistent, I suggest to revert the useless micro-optimization 1e89094998b2. Python uses Py_ssize_t instead of size_t to support "i >= 0" checks", it's a deliberate choice to catch bugs. |
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| msg278492 - (view) |
Author: Serhiy Storchaka (serhiy.storchaka) * |
Date: 2016-10-11 14:50 |
| Raymond extracted his optimization into separate function and commented it. |
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| msg327467 - (view) |
Author: Serhiy Storchaka (serhiy.storchaka) * |
Date: 2018-10-10 12:03 |
| See also PR 9784 where Raymond shown assempler code generated for two variants. There is the similar difference on 64-bit platform with GCC 7.3: $ gcc -O3 -o -2.c $ time ./issue28397-2 0 real 0m2,740s user 0m2,739s sys 0m0,000s $ time ./issue28397-2 1 real 0m2,449s user 0m2,449s sys 0m0,000s But with GCC 8.2 there is not a difference. $ time ./issue28397-2 0 real 0m2,498s user 0m2,498s sys 0m0,000s $ time ./issue28397-2 1 real 0m2,500s user 0m2,496s sys 0m0,004s Both versions generate the same code for tested functions, there are only minor differences in the main() function (some independent instructions are reordered). I don't know what is the cause of such difference. |
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