digitalmars.D.bugs - [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
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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Apr 11 2013
d-bugmail puremagic.com
http://d.puremagic.com/issues/show_bug.cgi?id=9920
bearophile_hugs eml.cc changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |bearophile_hugs eml.cc
Dupe of 5607 ?
(Even if it's a dupe you have good code in attach)
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http://d.puremagic.com/issues/show_bug.cgi?id=9920 11:16:38 PDT ---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: -------
Apr 11 2013
http://d.puremagic.com/issues/show_bug.cgi?id=9920any 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: -------
Apr 11 2013
http://d.puremagic.com/issues/show_bug.cgi?id=9920 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: -------
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http://d.puremagic.com/issues/show_bug.cgi?id=9920
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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Apr 11 2013
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
01:19:01 PDT ---
https://github.com/D-Programming-Language/dmd/pull/1893
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Apr 14 2013
http://d.puremagic.com/issues/show_bug.cgi?id=9920
yebblies <yebblies gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |yebblies gmail.com
D2 commit:
https://github.com/D-Programming-Language/dmd/commit/ccb0c9b37d3340f63cf52148b540e282bcdc3ba4
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Apr 15 2013
http://d.puremagic.com/issues/show_bug.cgi?id=9920Timings, n = 100_000_000: GCC: 3.13 s DMD: 11.69 sThe 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
http://d.puremagic.com/issues/show_bug.cgi?id=9920
Walter Bright <bugzilla digitalmars.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution| |FIXED
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Apr 16 2013