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* [Bug c/37443] New: fast 64-bit divide by constant 32-bit @ 2008-09-09 13:59 ajrobb at bigfoot dot com 2008-09-13 12:28 ` [Bug middle-end/37443] " rguenth at gcc dot gnu dot org ` (4 more replies) 0 siblings, 5 replies; 7+ messages in thread From: ajrobb at bigfoot dot com @ 2008-09-09 13:59 UTC (permalink / raw) To: gcc-bugs consider the code fragment: uint64_t slow(uint64_t x) { return x / 1220703125u; } This can be replaced by: uint64_t fast(uint64_t x) { uint32_t a = ((x >> 32) * 1270091284u) >> 32; uint32_t b = ((x & 0xffffffffu) * 3777893186u) >> 32; return ((x >> 32) * 3777893186ull + a + b) >> 30; } The 'fast' code runs 50% faster than 'slow'. However, removing the redundant multiplies (see my earlier bug - fixed in 4.4 trunk) and tidying up storage, I can use the following assembler to run nearly 100% faster than 'slow': .p2align 4,,15 .globl _fast .def _fast; .scl 2; .type 32; .endef _fast: pushl %ebx movl $1270091284, %eax mull 12(%esp) movl $-517074110, %eax movl %edx, %ebx mull 8(%esp) movl $-517074110, %eax movl %edx, %ecx mull 12(%esp) addl %ebx, %ecx popl %ebx adcl $0, %edx addl %ecx, %eax adcl $0, %edx shrdl $30, %edx, %eax shrl $30, %edx ret NOTE: the 2 multipliers are derived using 96-bit arithmetic: d = 1220703125u -517074110 = 0xffe12e1342u = (((1u << 94) + d - 1) / d) >> 32 1270091284 = (((1u << 94) + d - 1) / d) & 0xffffffffu -- Summary: fast 64-bit divide by constant 32-bit Product: gcc Version: 4.2.3 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: ajrobb at bigfoot dot com GCC build triplet: i686-pc-cygwin GCC host triplet: i686-pc-cygwin GCC target triplet: i686-pc-cygwin http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37443 ^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug middle-end/37443] fast 64-bit divide by constant 32-bit 2008-09-09 13:59 [Bug c/37443] New: fast 64-bit divide by constant 32-bit ajrobb at bigfoot dot com @ 2008-09-13 12:28 ` rguenth at gcc dot gnu dot org 2008-09-16 0:18 ` ajrobb at bigfoot dot com ` (3 subsequent siblings) 4 siblings, 0 replies; 7+ messages in thread From: rguenth at gcc dot gnu dot org @ 2008-09-13 12:28 UTC (permalink / raw) To: gcc-bugs -- rguenth at gcc dot gnu dot org changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Ever Confirmed|0 |1 Keywords| |missed-optimization Last reconfirmed|0000-00-00 00:00:00 |2008-09-13 12:26:41 date| | http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37443 ^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug middle-end/37443] fast 64-bit divide by constant 32-bit 2008-09-09 13:59 [Bug c/37443] New: fast 64-bit divide by constant 32-bit ajrobb at bigfoot dot com 2008-09-13 12:28 ` [Bug middle-end/37443] " rguenth at gcc dot gnu dot org @ 2008-09-16 0:18 ` ajrobb at bigfoot dot com 2008-09-16 7:16 ` [Bug middle-end/37443] fast 64-bit divide by constant on 32-bit platform ajrobb at bigfoot dot com ` (2 subsequent siblings) 4 siblings, 0 replies; 7+ messages in thread From: ajrobb at bigfoot dot com @ 2008-09-16 0:18 UTC (permalink / raw) To: gcc-bugs ------- Comment #1 from ajrobb at bigfoot dot com 2008-09-16 00:17 ------- I have tested my initial routine with more values and it turns out that the short-cut of omitting the least significant multiplication is wrong. I added the 4th multiply to generate the full 128-bit precision. I also took the multiplier used by gcc 4.2.3 on x86_64-linux-gnu (ubuntu) (4056481920730334085) and (28+64) bit shift to achieve the division by 1220703125u (largest 32-bit power of 5): uint64_t fast(uint64_t x) { static const uint64_t fact = 4056481920730334085ull; uint32_t c = ((x & 0xffffffffu) * (fact & 0xffffffffu)) >> 32; uint64_t a = ((x & 0xffffffffu) * (fact >> 32) + c); /* * Split the addition of the two middle 64-bit intermediate results * into 2 stages to avoid possible overflow. * (not actually an issue with this value of 'fact') */ uint32_t b = ((x >> 32) * (fact & 0xffffffffu) + (a & 0xffffffffu)) >> 32; return ((x >> 32) * (fact >> 32) + (a >> 32) + b) >> 28; } After hand coding the assembler on 32-bit x86, the extra multiply increased the run time from 15s to 17s - still much faster than 'slow' (64-bit divide API call). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37443 ^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug middle-end/37443] fast 64-bit divide by constant on 32-bit platform 2008-09-09 13:59 [Bug c/37443] New: fast 64-bit divide by constant 32-bit ajrobb at bigfoot dot com 2008-09-13 12:28 ` [Bug middle-end/37443] " rguenth at gcc dot gnu dot org 2008-09-16 0:18 ` ajrobb at bigfoot dot com @ 2008-09-16 7:16 ` ajrobb at bigfoot dot com 2008-09-20 17:34 ` ajrobb at bigfoot dot com 2008-09-21 2:41 ` ajrobb at bigfoot dot com 4 siblings, 0 replies; 7+ messages in thread From: ajrobb at bigfoot dot com @ 2008-09-16 7:16 UTC (permalink / raw) To: gcc-bugs ------- Comment #2 from ajrobb at bigfoot dot com 2008-09-16 07:15 ------- I changed the summary as it is equally applicable to all 64-bit constant divides on 32-bit x86 that are already compiled as multiplies on 64-bit x64. It might be worth implementing as another API call, replacing the 64-bit divisor with a 64-bit multiplier and a shift count. -- ajrobb at bigfoot dot com changed: What |Removed |Added ---------------------------------------------------------------------------- Summary|fast 64-bit divide by |fast 64-bit divide by |constant 32-bit |constant on 32-bit platform http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37443 ^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug middle-end/37443] fast 64-bit divide by constant on 32-bit platform 2008-09-09 13:59 [Bug c/37443] New: fast 64-bit divide by constant 32-bit ajrobb at bigfoot dot com ` (2 preceding siblings ...) 2008-09-16 7:16 ` [Bug middle-end/37443] fast 64-bit divide by constant on 32-bit platform ajrobb at bigfoot dot com @ 2008-09-20 17:34 ` ajrobb at bigfoot dot com 2008-09-21 2:41 ` ajrobb at bigfoot dot com 4 siblings, 0 replies; 7+ messages in thread From: ajrobb at bigfoot dot com @ 2008-09-20 17:34 UTC (permalink / raw) To: gcc-bugs ------- Comment #3 from ajrobb at bigfoot dot com 2008-09-20 17:32 ------- Created an attachment (id=16369) --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=16369&action=view) proposed 32-bit API calls for 64-bit constant divison I have attached a sample assembler code for a API calls for the 2 types of 64-bit division through multiplication: mul0shift - for multiplier with 64 or fewer significant bits mul1shift - for multiplier with 65 significant bits (implicit high bit) -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37443 ^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug middle-end/37443] fast 64-bit divide by constant on 32-bit platform 2008-09-09 13:59 [Bug c/37443] New: fast 64-bit divide by constant 32-bit ajrobb at bigfoot dot com ` (3 preceding siblings ...) 2008-09-20 17:34 ` ajrobb at bigfoot dot com @ 2008-09-21 2:41 ` ajrobb at bigfoot dot com 4 siblings, 0 replies; 7+ messages in thread From: ajrobb at bigfoot dot com @ 2008-09-21 2:41 UTC (permalink / raw) To: gcc-bugs ------- Comment #4 from ajrobb at bigfoot dot com 2008-09-21 02:39 ------- Created an attachment (id=16370) --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=16370&action=view) proposed 32-bit API calls for 64-bit constant divison The original attachment did not handle shift greater than 31. -- ajrobb at bigfoot dot com changed: What |Removed |Added ---------------------------------------------------------------------------- Attachment #16369|0 |1 is obsolete| | http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37443 ^ permalink raw reply [flat|nested] 7+ messages in thread
[parent not found: <bug-37443-4@http.gcc.gnu.org/bugzilla/>]
* [Bug middle-end/37443] fast 64-bit divide by constant on 32-bit platform [not found] <bug-37443-4@http.gcc.gnu.org/bugzilla/> @ 2021-08-15 11:28 ` pinskia at gcc dot gnu.org 0 siblings, 0 replies; 7+ messages in thread From: pinskia at gcc dot gnu.org @ 2021-08-15 11:28 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37443 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Build|i686-pc-cygwin | Host|i686-pc-cygwin | --- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> --- Must be a cost model issue because I can get /10u working but not /1220703125u (in GCC 11+). ^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2021-08-15 11:28 UTC | newest] Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2008-09-09 13:59 [Bug c/37443] New: fast 64-bit divide by constant 32-bit ajrobb at bigfoot dot com 2008-09-13 12:28 ` [Bug middle-end/37443] " rguenth at gcc dot gnu dot org 2008-09-16 0:18 ` ajrobb at bigfoot dot com 2008-09-16 7:16 ` [Bug middle-end/37443] fast 64-bit divide by constant on 32-bit platform ajrobb at bigfoot dot com 2008-09-20 17:34 ` ajrobb at bigfoot dot com 2008-09-21 2:41 ` ajrobb at bigfoot dot com [not found] <bug-37443-4@http.gcc.gnu.org/bugzilla/> 2021-08-15 11:28 ` pinskia at gcc dot gnu.org
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