🚀 go-pugleaf

Jens Schweikhardt (schweikh@itosun.ito.uni-stuttgart.de) wrote:
: In article <4i805f$47d@solutions.solon.com>,
: Dan T. Abell <dabell@quark.umd.edu> wrote:
[snip]
: >Do most compilers use plain ol' multiplication or do they use bit-shift  
: >operators.  Also, does the answer depend on the type of the array p?

: The answer is, as so often, "it depends".
: What the compiler will generate for an int array acces depends on

[Among other things...]

: - What the instruction set of the machine supports.
:   There may be machines that have instructions to compute
:   base+(index * somesize) [I bet there are!]

Intel 80386 and up have such a mode for 32-bit addressing.
Valid values of somesize are 1,2,4,8.

16-bit addressing is still stuck with bit shifting.

(According to the references I have.)

: If you want to be sure how your compiler does it, trying the -S option
: might give an assembler source of your C code. I once investigated it with
: gcc on an i486 machine with sizeof int = 4. It does use bit shifts
: to compute adresses in int arrays. You don't need to be an assembler
: guru to see what's going on in a small program.

I tried "result=int_array[index];" with gcc 2.7.2 (and 2.5.8) on an i486 
and got:

	movl index,%eax
	movl int_array(,%eax,4),%edx
	movl %edx,result

Which looks like at least the current gcc uses scaled indexes.

: To put it in a nutshell:
:   You only can be sure by looking at the compiler output.

: Bye, Jens

: -- 
: SIGSIG -- signature too long (core dumped)

--
Nathan Green			(ngreen@plains.nodak.edu)
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