[Issue 9920] New: [Optimizer] Use mul/imul for integer division by constant

[d-bugmail at puremagic.com]

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 at puremagic.com
        ReportedBy: dmitry.olsh at gmail.com


--- Comment #0 from Dmitry Olshansky <dmitry.olsh at 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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