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digitalmars.D.bugs - [Issue 9920] New: [Optimizer] Use mul/imul for integer division by constant

reply d-bugmail puremagic.com writes:
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.

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next sibling parent d-bugmail puremagic.com writes:
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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)

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--- 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: -------
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--- 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: -------
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--- 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. ***

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--- 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

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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

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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

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--- 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: -------
Apr 16 2013
prev sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


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