Under the old 3-step process (ncvlog+ncelab+ncsim), I could compile the testbench once. Then call ncsim multiple times to run the sim again(without recompiling.)
How do I do that with irun? What are the proper command-line switches?
I'll also add that, if you haven't updated the source code, using the exact same irun command line as used to compile the first time will behave the way you've described. irun will compare the timestamps of the source code files and if they are earlier than the snapshot, they won't be recompiled. If nothing is recompiled, the elaborator won't be run again either.
However, iIf you change the compiler's command-line options, then irun will recognize that and recompile when necessary.
The -r and -R options that Stewart mentioned are especially useful if you have changed a source file, but want to hold off recompiling 'til later.