Fixed nested if statements (#88)

* Fixed nested if statements

* feedback

* Removed a line

Author: VZout

rustc_codegen_spirv/src/linker/structurizer.rs

@@ -24,14 +24,9 @@ fn find_block_index_from_id(blocks: &[Block], id: &Word) -> usize {
     panic!("Failed to find block from id");
 }
 
-fn get_possible_merge_positions(
-    sess: Option<&Session>,
-    blocks: &[Block],
-    start: Word,
-) -> Vec<usize> {
+fn get_possible_merge_positions(blocks: &[Block], start: Word) -> Vec<usize> {
     let mut retval = Vec::new();
     let mut next: VecDeque<Word> = VecDeque::new();
-    let mut processed = Vec::new();
     next.push_back(start);
 
     while let Some(front) = next.pop_front() {
@@ -43,19 +38,28 @@ fn get_possible_merge_positions(
             retval.push(find_block_index_from_id(blocks, &new_edges[0]));
         }
 
-        if processed.contains(&front) {
-            if let Some(sess) = sess {
-                sess.err("Loops are unsupported");
-            }
-            break;
-        } else {
-            processed.push(front);
+        next.extend(new_edges);
+    }
+
+    retval
+}
+
+fn block_is_loop(blocks: &[Block], start: Word) -> bool {
+    let mut next: VecDeque<Word> = VecDeque::new();
+    next.push_back(start);
+
+    while let Some(front) = next.pop_front() {
+        let block_idx = find_block_index_from_id(blocks, &front);
+        let new_edges = outgoing_edges(&blocks[block_idx]);
+
+        if new_edges.contains(&start) {
+            return true;
         }
 
         next.extend(new_edges);
     }
 
-    retval
+    false
 }
 
 fn ends_in_return(block: &Block) -> bool {
@@ -72,6 +76,12 @@ pub fn insert_selection_merge_on_conditional_branch(
 
     // Find conditional branch
     for (bi, block) in blocks.iter().enumerate() {
+        if block_is_loop(blocks, block.label_id().unwrap()) {
+            if let Some(sess) = sess {
+                sess.err("Loops are unsupported");
+            }
+        }
+
         for (ii, inst) in block.instructions.iter().enumerate() {
             if inst.class.opcode == Op::BranchConditional {
                 branch_conditional_ops.push((bi, ii));
@@ -85,8 +95,8 @@ pub fn insert_selection_merge_on_conditional_branch(
         if out.len() != 2 {
             panic!("Viktor missunderstood something");
         }
-        let a_nexts = get_possible_merge_positions(sess, blocks, out[0]);
-        let b_nexts = get_possible_merge_positions(sess, blocks, out[1]);
+        let a_nexts = get_possible_merge_positions(blocks, out[0]);
+        let b_nexts = get_possible_merge_positions(blocks, out[1]);
 
         // Check for a matching possible merge position.
         let mut first_merge = None;