skip body extraction when the loop has 0 iterations (#57)

circom/tests/subcmps/conv_map2idx_C.circom

@@ -0,0 +1,55 @@
+pragma circom 2.0.0;
+
+// REQUIRES: circom
+// RUN: rm -rf %t && mkdir %t && %circom --llvm -o %t %s | sed -n 's/.*Written successfully:.* \(.*\)/\1/p' | xargs cat | FileCheck %s --enable-var-scope
+
+template SegmentMulFix(nWindows) {
+    signal input e[nWindows];
+}
+
+template EscalarMulFix() {
+    //Needs at least 2 subcomp to trigger the crash
+    component segments[2];
+    for (var s = 0; s < 2; s++) {
+
+        // s = 0, nseg = 9, nWindows = 9
+        // s = 1, nseg = 4, nWindows = 6
+        var nseg = (s == 0) ? 9 : 4;
+        var nWindows = (s == 0) ? 9 : 6;
+
+        segments[s] = SegmentMulFix(nWindows);
+
+        // Needs this split loop to trigger the crash
+        for (var i = 0; i < nseg; i++) {
+            //Runs 9 times for s=0
+            //Runs 4 times for s=1
+            segments[s].e[i] <-- 999;
+        }
+        for (var i = nseg; i < nWindows; i++) {
+            //Runs 0 times for s=0      //this is the case where the extracted body is generated but shouldn't be!
+            //Runs 2 times for s=1
+            segments[s].e[i] <-- 888;
+        }
+    }
+}
+
+component main = EscalarMulFix();
+
+//CHECK-NOT: ..generated..loop.body.
+//
+//CHECK-LABEL: define void @EscalarMulFix_2_run([0 x i256]* %0)
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 999
+//CHECK: store i256 888
+//CHECK: store i256 888

circuit_passes/src/passes/loop_unroll/mod.rs

@@ -64,7 +64,7 @@ impl<'d> LoopUnrollPass<'d> {
         }
 
         let mut block_body = vec![];
-        if EXTRACT_LOOP_BODY_TO_NEW_FUNC && recorder.is_safe_to_move() {
+        if EXTRACT_LOOP_BODY_TO_NEW_FUNC && recorder.is_safe_to_move() && recorder.get_iter() > 0 {
             // If the loop body contains more than one instruction, extract it into a new
             // function and generate 'recorder.get_iter()' number of calls to that function.
             // Otherwise, just duplicate the body 'recorder.get_iter()' number of times.