Does Xcode's objective-c compiler optimize for bit shifts?

Question

Does the objective-c compiler in Xcode know better, or is it faster if I use bit shift for multiplications and divisions by powers of 2?

NSInteger parentIndex = index >> 1; // integer division by 2

Answer 1

Isn't this a bit 1980's? Don't processors run these instructions in the same time these days? I remember back in my 68000 days when a div was 100+ cycles and a shift only 3 or 4... not sure this is the case any more as processors have moved on.

Why don't you get the compiler to generate the assembler file and have a look what it's generating and run some benchmarks.

I found this on the web which may help you... although it's for 'C' I think most of the options will be the same.

Q: How can I peek at the assembly code generated by GCC?

Q: How can I create a file where I can see the C code and its assembly
   translation together?

A: Use the -S (note: capital S) switch to GCC, and it will emit the assembly code to a file with a .s extension. For example, the following command:

gcc -O2 -S -c foo.c

will leave the generated assembly code on the file foo.s.

If you want to see the C code together with the assembly it was converted to, use a command line like this:

gcc -c -g -Wa,-a,-ad [other GCC options] foo.c > foo.lst

which will output the combined C/assembly listing to the file foo.lst.

If you need to both get the assembly code and to compile/link the program, you can either give the -save-temps option to GCC (which will leave all the temporary files including the .s file in the current directory), or use the -Wa,aln=foo.s option which instructs the assembler to output the assembly translation of the C code (together with the hex machine code and some additional info) to the file named after the =.