[Issue 11821] New: dmd backend: redundant x86 instruction in a simple loop

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=11821

           Summary: dmd backend: redundant x86 instruction in a simple
                    loop
           Product: D
           Version: D2
          Platform: x86
               URL: http://forum.dlang.org/thread/nfobptpqpiueelhehbfy@for
                    um.dlang.org
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: gassa at mail.ru


--- Comment #0 from Ivan Kazmenko <gassa at mail.ru> 2013-12-26 02:12:34 PST ---
I am trying to figure out why win32 executables compiled from D source by dmd
are usually somewhat slower than similar win32 programs compiled from C++
source by, for example, mingw-gcc.

I believe I found a relatively simple case where dmd puts a redundant
instruction into the object code.

I have this simple D program:

-----
immutable int MAX_N = 1_000_000;
int main () {
    int [MAX_N] a;
    foreach (i; 0..MAX_N)
        a[i] = i;
    return a[7];
}
-----

The assembly (dmd -O -release -inline -noboundscheck, then obj2asm) has the
following piece corresponding to the cycle:

-----
L2C:        mov    -03D0900h[EDX*4][EBP],EDX
        mov    ECX,EDX
        inc    EDX
        cmp    EDX,0F4240h
        jb    L2C
-----

Here, the second line "mov ECX, EDX" does not seem to serve any purpose at all.
 If this observation is correct, this instruction is an indication of a bug in
code generation, and fixing that bug may improve performance in more general
case.

The "return a[7]" part is to assure the whole loop need not be optimized out. 
The ldmd2 compiler reportedly does that when no return is present.  DMD however
does not, however that is irrelevant to this issue.

Previous discussion:
http://forum.dlang.org/thread/nfobptpqpiueelhehbfy@forum.dlang.org

Will attach source and disassembly in comments.

Ivan Kazmenko.

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