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Enumerating array using for vs. foreach

  • Artigo

Interesting question over an internal alias today…

Questions:

What is the difference between enumerating an array using for versus foreach ?

What is the recommended practice and why?

 

Here is an example:

Assembly[] assemblies = AppDomain.CurrentDomain.GetAssemblies();

 

for (int i=0; i<assemblies.Length; i++)

      DoSomething(assemblies[i]);

 

foreach (Assembly a in assemblies)

      DoSomething(a);

 

Answer (from one of the perf architects on the CLR team):

The code gen is basically identical... guidance is to use foreach for clarity unless you plan to modify the contents of the array as you go, in which case for is required. Enumerators are not created in the array case. Note this analysis is not applicable to the collection class case, this is for arrays.

 

For more details see the "Enumeration Overhead" section in https://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnpag/html/scalenetchapt05.asp -- searching for "foreach" in that same document will give other illustrations and guidance.

 

using System;

class Test
{
static void Main(String[] args)
{
for (int i=0; i<args.Length;i++)
Console.WriteLine(args[i]);

 

  foreach (String s in args)
Console.WriteLine(s);
}
}

 

 

// standard prolog

G_M001_IG01:

IN0038: 000000 push EDI
IN0039: 000001 push ESI
IN0040: 000002 push EBX
IN0041: 000003 mov EDI, ECX   // the array gets stored in EDI on a long term basis

 

G_M001_IG02: ; offs=000005H

 

// this is the "for" case

// this is all a redundant length check as far as I can tell

G_M001_IG02:
IN0001: 000005 xor EBX, EBX // EBX is holding 'i' -- starts at zero
IN0002: 000007 cmp dword ptr [EDI+4], 0  // checking length of array to see if it is negative, it won't be
IN0003: 00000B jle SHORT G_M001_IG06

 

G_M001_IG03: ; offs=00000DH

 

G_M001_IG03:
IN0004: 00000D cmp EBX, dword ptr [EDI+4] // check the loop variable against array length
IN0005: 000010 jae SHORT G_M001_IG11
IN0006: 000012 mov ESI, gword ptr [EDI+4*EBX+12] // fetch the string from the array
IN0007: 000016 cmp gword ptr [classVar[0x5b9d1994]], 0 // test if the stream has been initialized
IN0008: 00001D jne SHORT G_M001_IG05

 

G_M001_IG04: ; offs=00001FH

 

G_M001_IG04:
IN0009: 00001F mov ECX, 1
IN0010: 000024 call System.Console.InitializeStdOutError(bool) // initialize stream

 

G_M001_IG05: ; offs=000029H

 

G_M001_IG05:
IN0011: 000029 mov ECX, gword ptr [classVar[0x5b9d1994]] // we now have the power to get the stdout stream into ECX
IN0012: 00002F mov EDX, ESI // get the string we saved before set it up as an arg
IN0013: 000031 call dword ptr [(reloc 0x3a80014)]System.IO.TextWriter.WriteLine(ref) // write the string
IN0014: 000037 add EBX, 1 // ebx has 'i' in it remember, upcount
IN0015: 00003A cmp dword ptr [EDI+4], EBX // test the current 'i' against the length
IN0016: 00003D jg SHORT G_M001_IG03 //loop until done

 

G_M001_IG06: ; offs=00003FH

 

// this is the "foreach" case

G_M001_IG06:
IN0017: 00003F xor ESI, ESI // ESI is holding the loop index -- starts at zero -- will not hurt codegen at all
IN0018: 000041 cmp dword ptr [EDI+4], 0 // same redundant length check
IN0019: 000045 jle SHORT G_M001_IG10

 

G_M001_IG07: ; offs=000047H

 

G_M001_IG07:
IN0020: 000047 cmp ESI, dword ptr [EDI+4] // testing index before entering loop as above
IN0021: 00004A jae SHORT G_M001_IG11
IN0022: 00004C mov EBX, gword ptr [EDI+4*ESI+12]
IN0023: 000050 cmp gword ptr [classVar[0x5b9d1994]], 0 // testing if we need to run the .cctor as above
IN0024: 000057 jne SHORT G_M001_IG09

 

G_M001_IG08: ; offs=000059H

 

G_M001_IG08:
IN0025: 000059 mov ECX, 1
IN0026: 00005E call System.Console.InitializeStdOutError(bool) // call inlined body of .cctor as above

 

G_M001_IG09: ; offs=000063H

 

G_M001_IG09:
IN0027: 000063 mov ECX, gword ptr [classVar[0x5b9d1994]] // get stream and write string as above
IN0028: 000069 mov EDX, EBX
IN0029: 00006B call dword ptr [(reloc 0x3a8002c)]System.IO.TextWriter.WriteLine(ref)
IN0030: 000071 add ESI, 1
IN0031: 000074 cmp dword ptr [EDI+4], ESI // same loop dynamics as above only using ESI instead of EBX
IN0032: 000077 jg SHORT G_M001_IG07

 

G_M001_IG10: ; offs=000079H

 

G_M001_IG10:
IN0033: __epilog:
000079 pop EBX // we are so outta here
00007A pop ESI
00007B pop EDI
IN0034: 00007C ret    

 

Notice that this is very specific for arrays (and in V1.1 and above enumerating over strings), this does not apply to enumerating over collections generally… Kevin has a good post on that…

Comments

  • Anonymous
    April 29, 2004
    Interesting post, but to gain Clarification ... I was under the impression that it is better to use For Loops of Foreach as a For loop can bypass a boundary check being emitted for every iteration ?

    Thanks.

  • Anonymous
    April 29, 2004
    Oops read the code and thats whats happening , although as far as the CLR is concerned I don't think the check is redundant, as logically a foreach doesn't know its at the end till it gets there ...

  • Anonymous
    April 29, 2004
    I'd like to know how you got the JITted x86 asm. Is there a relatively easy way to do this?

    I've tried doing this using cordbg, but I find it a very painful experience. Any tips?

  • Anonymous
    April 29, 2004
    I'd like to be able to foreach over an array in the reverse direction. I don't think there is any easier way than trying to get the for(...) loop correct...

  • Anonymous
    April 30, 2004
    mikeb : put a breakpoint into your program in VS, when it happens go into debug and choose Assembly ... you can even put break codes in the assembly :)

  • Anonymous
    May 02, 2004
    Mike -- You can also use windbg/cdb with the SOS extension... If you can get an IP within the method that you're interested in inspecting you can get the disassembly using "!u <address>". SOS will do it's best to pretty print the results (adding info to call sites where they can be resolved, and such...).

  • Anonymous
    June 30, 2004
    I'd like to know how you got the JITted x86 asm. Is there a relatively easy way to do this?

    I've tried doing this using cordbg, but I find it a very painful experience. Any tips?

  • Anonymous
    July 30, 2004
    Should be for example like this, the understanding that it could be better to give an example, because my understanding ability is bad, don't laugh at me ! !

  • Anonymous
    June 06, 2005
    Here are some resources to read up on the differences between 'for' and 'foreach'.
    Enumerating array...

  • Anonymous
    March 14, 2007
    for和foreach的效率问题是个老问题了，从网上看到的是众说纷纭，有说for效率高的也有说foreach效率高的，还有说测试方法有问题的；鉴于此，我就自己做了个试验证明一下，然后探究一下可能的原...

  • Anonymous
    March 14, 2007
    for和foreach 的效率问题是个老问题了，从网上看到的是众说纷纭，有说for效率高的也有说foreach效率高的，还有说测试方法有问题的；鉴于此，我就自己做了个试验证明一下，然后探究一下可能的原因。

  • Anonymous
    July 27, 2007
    for和foreach 的效率问题是个老问题了，从网上看到的是众说纷纭，有说for效率高的也有说foreach效率高的，还有说测试方法有问题的；通过实验发现for的效率比foreach高。

  • Anonymous
    January 21, 2009
    PingBack from http://www.keyongtech.com/640444-c-compiler-fails-to-optimize

  • Anonymous
    January 22, 2009
    PingBack from http://www.hilpers.fr/931652-for-each-en-partant-de/2