interp: fix phi instruction

I've discovered a bug in the implementation of the PHI instruction in
the interp package. This commit fixes the bug.

I've found this issue while investigating an issue with maps after
running interp per package.

interp/interpreter.go

@@ -33,6 +33,50 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 	lastBB := -1 // last basic block is undefined, only defined after a branch
 	var operands []value
 	for instIndex := 0; instIndex < len(bb.instructions); instIndex++ {
+		if instIndex == 0 {
+			// This is the start of a new basic block.
+			// There may be PHI nodes that need to be resolved. Resolve all PHI
+			// nodes before continuing with regular instructions.
+			// PHI nodes need to be treated specially because they can have a
+			// mutual dependency:
+			//   for.loop:
+			//     %a = phi i8 [ 1, %entry ], [ %b, %for.loop ]
+			//     %b = phi i8 [ 3, %entry ], [ %a, %for.loop ]
+			// If these PHI nodes are processed like a regular instruction, %a
+			// and %b are both 3 on the second iteration of the loop because %b
+			// loads the value of %a from the second iteration, while it should
+			// load the value from the previous iteration. The correct behavior
+			// is that these two values swap each others place on each
+			// iteration.
+			var phiValues []value
+			var phiIndices []int
+			for _, inst := range bb.phiNodes {
+				var result value
+				for i := 0; i < len(inst.operands); i += 2 {
+					if int(inst.operands[i].(literalValue).value.(uint32)) == lastBB {
+						incoming := inst.operands[i+1]
+						if local, ok := incoming.(localValue); ok {
+							result = locals[fn.locals[local.value]]
+						} else {
+							result = incoming
+						}
+						break
+					}
+				}
+				if r.debug {
+					fmt.Fprintln(os.Stderr, indent+"phi", inst.operands, "->", result)
+				}
+				if result == nil {
+					panic("could not find PHI input")
+				}
+				phiValues = append(phiValues, result)
+				phiIndices = append(phiIndices, inst.localIndex)
+			}
+			for i, value := range phiValues {
+				locals[phiIndices[i]] = value
+			}
+		}
+
 		inst := bb.instructions[instIndex]
 		operands = operands[:0]
 		isRuntimeInst := false
@@ -121,26 +165,6 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 			if r.debug {
 				fmt.Fprintln(os.Stderr, indent+"switch", operands, "->", currentBB)
 			}
-		case llvm.PHI:
-			var result value
-			for i := 0; i < len(inst.operands); i += 2 {
-				if int(inst.operands[i].(literalValue).value.(uint32)) == lastBB {
-					incoming := inst.operands[i+1]
-					if local, ok := incoming.(localValue); ok {
-						result = locals[fn.locals[local.value]]
-					} else {
-						result = incoming
-					}
-					break
-				}
-			}
-			if r.debug {
-				fmt.Fprintln(os.Stderr, indent+"phi", inst.operands, "->", result)
-			}
-			if result == nil {
-				panic("could not find PHI input")
-			}
-			locals[inst.localIndex] = result
 		case llvm.Select:
 			// Select is much like a ternary operator: it picks a result from
 			// the second and third operand based on the boolean first operand.