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April 11, 2013 [Issue 9920] New: [Optimizer] Use mul/imul for integer division by constant | ||||
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http://d.puremagic.com/issues/show_bug.cgi?id=9920 Summary: [Optimizer] Use mul/imul for integer division by constant Product: D Version: D2 Platform: All OS/Version: All Status: NEW Severity: enhancement Priority: P2 Component: DMD AssignedTo: nobody@puremagic.com ReportedBy: dmitry.olsh@gmail.com --- Comment #0 from Dmitry Olshansky <dmitry.olsh@gmail.com> 2013-04-11 08:44:06 PDT --- Created an attachment (id=1208) fast divison by constant for uint values It's a common knowledge and is totally expected for any modern compiler to re-write divisions by constant to double word width _multiplication_ by a specific constant followed by shift. DMD still DOESN'T DO it. Attached is an example of this optimization done via meta-programming in D along with test and a benchmark. On average (on my PC) the mul version is around 3 times faster then compiler-generated div. On LDC and GDC results are that compiler-generate version is a bit faster. Compiler can easily do a better job especially with 64bit values (as 2x64 accumulator is completely unaccessible for the programmer). For full description of one such technique see Agner Fog's manuals on assembly optimizations: http://www.agner.org/optimize/optimizing_assembly.pdf See the chapter 16. "Problematic instructions", section on DIV/IDIV is 16.9. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
April 11, 2013 [Issue 9920] [Optimizer] Use mul/imul for integer division by constant | ||||
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Posted in reply to Dmitry Olshansky | http://d.puremagic.com/issues/show_bug.cgi?id=9920 bearophile_hugs@eml.cc changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |bearophile_hugs@eml.cc --- Comment #1 from bearophile_hugs@eml.cc 2013-04-11 11:00:21 PDT --- Dupe of 5607 ? (Even if it's a dupe you have good code in attach) -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
April 11, 2013 [Issue 9920] [Optimizer] Use mul/imul for integer division by constant | ||||
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Posted in reply to Dmitry Olshansky | http://d.puremagic.com/issues/show_bug.cgi?id=9920 --- Comment #2 from Dmitry Olshansky <dmitry.olsh@gmail.com> 2013-04-11 11:16:38 PDT --- (In reply to comment #1) > Dupe of 5607 ? 5607 is a special case of this enhancement. This one is generalization since any division by constant can be optimized. Using small divisors only allows some more specialized varations of this optimization (less accurate but enough for numbers in question). > > (Even if it's a dupe you have good code in attach) I'd say this one supersedes 5607, so how about merge your example here and close 5607 ? (the 2 have to be merged anyway since the root cause is the same) -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
April 11, 2013 [Issue 9920] [Optimizer] Use mul/imul for integer division by constant | ||||
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Posted in reply to Dmitry Olshansky | http://d.puremagic.com/issues/show_bug.cgi?id=9920 --- Comment #3 from bearophile_hugs@eml.cc 2013-04-11 11:33:55 PDT --- (In reply to comment #2) > any division by constant can be optimized. I didn't know this. > I'd say this one supersedes 5607, so how about merge your example here and close 5607 ? (the 2 have to be merged anyway since the root cause is the same) If you think this supersedes 5607, then copy what you think should be copied, and close it. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
April 11, 2013 [Issue 9920] [Optimizer] Use mul/imul for integer division by constant | ||||
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Posted in reply to Dmitry Olshansky | http://d.puremagic.com/issues/show_bug.cgi?id=9920 --- Comment #4 from Dmitry Olshansky <dmitry.olsh@gmail.com> 2013-04-11 12:28:08 PDT --- *** Issue 5607 has been marked as a duplicate of this issue. *** -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
April 11, 2013 [Issue 9920] [Optimizer] Use mul/imul for integer division by constant | ||||
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Posted in reply to Dmitry Olshansky | http://d.puremagic.com/issues/show_bug.cgi?id=9920 --- Comment #5 from Dmitry Olshansky <dmitry.olsh@gmail.com> 2013-04-11 12:33:09 PDT --- Another data point taken from issue 5607 on div/mod heavy program seems to confirm my estimate of 3x+ speed difference with mul-trick applied. The program is bound on div/mul operaiton so other differences of GCC vs DMC codegen has little effect. --------------------------------- Timings, n = 100_000_000: GCC: 3.13 s DMD: 11.69 s dmd -O -release -inline testd.d gcc -O3 -s testc.c -o testc.exe 32 bit GCC 4.5.2 DMD 2.052beta --------------------------------- // D2 code import std.c.stdio: printf; int main() { uint total = 0; uint i; for (i = 0; i < 100000000; i++) { uint n = i; uint res = n % 10; n /= 10; while (n != 0) { res = (n % 10) + (10 * res); n /= 10; } total += res; } printf("%u\n", total); // 461784529 return 0; } --------------------------------- // C code #include "stdio.h" typedef unsigned long uint; int main() { uint total = 0; uint i; for (i = 0; i < 100000000; i++) { uint n = i; uint res = n % 10; n /= 10; while (n != 0) { res = (n % 10) + (10 * res); n /= 10; } total += res; } printf("%u\n", total); // 461784529 return 0; } --------------------------------- DMD inner loop: L1C: lea EBX,[EBX*4][EBX] mov EAX,ESI mov ECX,0Ah xor EDX,EDX add EBX,EBX div ECX add EBX,EDX test EAX,EAX mov ESI,EAX jne L1C --------------------------------- GCC inner loop: L7: movl %ecx, %eax mull %esi leal (%ebx,%ebx,4), %ebx shrl $3, %edx leal (%edx,%edx,4), %eax addl %eax, %eax subl %eax, %ecx testl %edx, %edx leal (%ecx,%ebx,2), %ebx movl %edx, %ecx jne L7 -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
April 14, 2013 [Issue 9920] [Optimizer] Use mul/imul for integer division by constant | ||||
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Posted in reply to Dmitry Olshansky | http://d.puremagic.com/issues/show_bug.cgi?id=9920 Walter Bright <bugzilla@digitalmars.com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |bugzilla@digitalmars.com Version|D2 |D1 & D2 --- Comment #6 from Walter Bright <bugzilla@digitalmars.com> 2013-04-14 01:19:01 PDT --- https://github.com/D-Programming-Language/dmd/pull/1893 -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
April 16, 2013 [Issue 9920] [Optimizer] Use mul/imul for integer division by constant | ||||
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Posted in reply to Dmitry Olshansky | http://d.puremagic.com/issues/show_bug.cgi?id=9920 yebblies <yebblies@gmail.com> changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |yebblies@gmail.com --- Comment #7 from yebblies <yebblies@gmail.com> 2013-04-16 15:18:14 EST --- D2 commit: https://github.com/D-Programming-Language/dmd/commit/ccb0c9b37d3340f63cf52148b540e282bcdc3ba4 -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
April 16, 2013 [Issue 9920] [Optimizer] Use mul/imul for integer division by constant | ||||
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Posted in reply to Dmitry Olshansky | http://d.puremagic.com/issues/show_bug.cgi?id=9920 --- Comment #8 from bearophile_hugs@eml.cc 2013-04-16 10:18:30 PDT --- (In reply to comment #5) > Timings, n = 100_000_000: > GCC: 3.13 s > DMD: 11.69 s The new timings are acceptable: GCC: 3.03 s DMD: 3.74 s dmd -O -release -noboundscheck -inline gcc -Ofast -flto -s gcc V.4.8 The asm of the D version below seems to contains a bit too many "mov": --------------------------------- New DMD inner loop: L28: lea EBX, [EBX*4][EBX] mov EAX, ESI mov EDX, 0CCCCCCCDh mul EDX shr EDX, 3 mov ECX, ESI imul EAX, EDX, 0Ah sub ECX, EAX mov EAX, EDX add EBX, EBX mov EDX, ECX add EBX, EDX test EAX, EAX mov ESI, EAX jne L28 --------------------------------- GCC: L5: movl %edi, %eax mull %esi movl %edx, %ebx shrl $3, %ebx leal (%ebx,%ebx,4), %eax movl %ebx, %ecx addl %eax, %eax movl %edi, %ebx subl %eax, %ebx testl %ecx, %ecx je L2 .p2align 4,,7 L4: movl %ecx, %eax mull %esi leal (%ebx,%ebx,4), %ebx shrl $3, %edx leal (%edx,%edx,4), %eax addl %eax, %eax subl %eax, %ecx testl %edx, %edx leal (%ecx,%ebx,2), %ebx movl %edx, %ecx jne L4 addl $1, %edi addl %ebx, 12(%esp) cmpl $100000000, %edi jne L5 --------------------------------- -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
April 16, 2013 [Issue 9920] [Optimizer] Use mul/imul for integer division by constant | ||||
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Posted in reply to Dmitry Olshansky | http://d.puremagic.com/issues/show_bug.cgi?id=9920 Walter Bright <bugzilla@digitalmars.com> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|NEW |RESOLVED Resolution| |FIXED -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- |
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