[Vm-dev] Is bytecodePrimMultiply correct?

[Stefan Marr]

Dear all:

With the RoarVM, I was hitting a problem with Clang 3.0 (tags/Apple/clang-211.9).

It turned out that Clang does not like the current implementation of bytecodePrimMultiply.

From what I can tell, it should be identical with what I found in the SqueakVM sources.

RoarVM src: https://github.com/smarr/RoarVM/blob/master/vm/src/interpreter/interpreter_bytecodes.cpp#L316

    oop_int_t ri = rcvr.integerValue();
    oop_int_t ai = arg.integerValue();
    oop_int_t r  = ri * ai;
    if (ai == 0  ||  (r / ai  ==  ri    &&    Oop::isIntegerValue(r))) {
      internalPopThenPush(2, Oop::from_int(r));
      fetchNextBytecode();
      return;
    }

Squeak src:
http://squeakvm.org/cgi-bin/viewcvs.cgi/branches/Cog/src/vm/cointerp.c?rev=2497&view=auto

	rcvr = (rcvr >> 1);
	arg = (arg >> 1);
	result = rcvr * arg;
	if (((arg == 0)
	|| ((result / arg) == rcvr))
	 && ((result ^ (result << 1)) >= 0)) {
		/* begin internalPop:thenPush: */
		longAtPointerput((localSP += (2 - 1) * BytesPerWord), ((result << 1) | 1));
		/* begin fetchNextBytecode */
		currentBytecode = byteAtPointer(++localIP);
		goto l69;
	}


The problematic example is rcvr: 40453, arg: 864001.

In that case, the current implementation does not detect the overflow correctly when Clang is used with >= O1 optimization.

The big question for me now is, whether we got here a Clang compiler bug, or unspecified C behavior.


My quick fix is:

    oop_int_t ri = rcvr.integerValue();
    oop_int_t ai = arg.integerValue();
    long long result_with_overflow = (long long)ri * ai;
    if (ai == 0  ||  ((result_with_overflow >= -1073741824) && (result_with_overflow <= 1073741823)))   
    {
      internalPopThenPush(2, Oop::from_int(result_with_overflow));
      fetchNextBytecode();
      return;
    }

This works obviously, but I assume that the original code is a well considered speed optimizations.

Any thoughts?

Thanks
Stefan

-- 
Stefan Marr
Software Languages Lab
Vrije Universiteit Brussel
Pleinlaan 2 / B-1050 Brussels / Belgium
http://soft.vub.ac.be/~smarr
Phone: +32 2 629 2974
Fax:   +32 2 629 3525