Re: Cracking DES with C++ is faster than Java?
From: Ernst Lippe (ernstl-at-planet-dot-nl_at_ignore.this)
Date: 04/28/04
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Date: Wed, 28 Apr 2004 16:04:30 +0200
On Wed, 28 Apr 2004 12:18:12 +0000, Andrew Swallow wrote:
> "Jerry Coffin" <jcoffin@taeus.com> wrote in message
> news:b2e4b04.0404271646.24e70541@posting.google.com...
> [snip]
>> If anything, it looks to me like thie situation is getting worse: I
>> don't think I've looked at anything in the last 5 years that was
>> nearly AS CLOSE to optimal as Control Data's FORTRAN IV compiler
>> typically produced 20+ years ago!
>>
> Computers can access fixed locations in memory
> faster that relative locations.
I assume that with "fixed location" you mean addresses that are known at link
time, when the executable is produced.
I don't think that your statement is true in general, it all depends on the
circumstances. One of the disadvantages of using absolute memory addresses is
that the size of instruction must always be larger than the number of bits in
the address space. Many CPU's have built-in support for addressing
small-sized offsets to a stack-pointer and in general the size of these
instructions is smaller than the size of the instructions that use absolute
addresses. When the instruction is longer, either it will take more time to
fetch it from memory, or it will decrease the effective number of instructions
that can be held in the instruction cache.
Even on CPU architectures where both types of instructions have the same size,
there may not be any difference in execution speed. For most modern CPU's
memory access is the main bottle-neck. Simple arithmetic operations to
registers, such as adding a small offset, are so fast relative to the time it
takes to fetch/store the contents of some address in memory, that there is no
difference. Also, virtually all modern computers have memory caches, and the
performance depends heavily on whether the contents are available in the
cache. But for the memory cache there is no difference between a fixed address
and an address that has been computed dynamically, the only thing that is
important if that address has been used recently.
> Recursion requires
> local variables to be stored on the stack, i.e. to
> be accessed using relative locations.
The overhead of allowing recursion (if any) is minimal on modern
CPU's. Anyhow, all modern programming languages (even Fortran) allow it, so
apparently language designers believe that the benefits are sufficiently
important.
>Data structures
> on the heap are even more complicated to access.
In most cases this statement is false, in most computers data structures on
the heap are simply identified by their address which is not more complicated
than using absolute addresses or locations on the stack. I have the feeling
that you believe that heaps only occur in languages, that have a relocating
garbage collector (see below), but it is standard usage to refer to the memory
area from which dynamically sized memory is allocated as "the heap". For
example, the malloc in C uses a heap.
> Dynamic storage is very bad, you can see the computer
> stop for several thousand million clock cycles whilst the
> garbage collector tries to find some more memory.
What do you mean by dynamic storage? The standard meaning of the term is
storage where the actual memory size of an object is not known at compile
time. This is pretty common in most computer languages, and in most cases this
is not at all a performance problem.
You are probably referring to relocating garbage collectors that can change
the actual memory addresses where objects are located. Traditionally, these
used to have the problems that you describe, but there have been important
improvements (generation scavenging, multi-threading) and most of these
problems have disappeared.
Anyhow, I don't think that your argument that fixed addresses are
faster is very relevant. Virtually all computer programs are written
with procedures that can accept varying arguments (that was already
the case for these old Fortran programs). So there are hardly any instances
where computers actually use fixed addresses, because most code
relies on variables that have been passed as arguments instead of
global variables.
Ernst Lippe
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