fix: always create the destination branch argument for the sentinel v…

…alue

when looking for the value definition in the predecessors. #51

This removes the remnants of the previously removed `ValueData::Alias` variant.
It does not matter if we find the definition in the predecessors since we cannot
swap all sentinel value uses with it. So this fix always makes the sentinel value into a
block argument.

frontend-wasm/src/ssa.rs

@@ -134,12 +134,6 @@ enum Call {
     FinishPredecessorsLookup(Value, Block),
 }
 
-/// Emit instructions to produce a zero value in the given type.
-fn emit_zero(_ty: &Type, //, mut cur: FuncCursor
-) -> Value {
-    todo!("emit zero value at the beginning of a block")
-}
-
 /// The following methods are the API of the SSA builder. Here is how it should be used when
 /// translating to Miden IR:
 ///
@@ -461,55 +455,18 @@ impl SSABuilder {
         let num_predecessors = self.predecessors(dest_block).len();
         // When this `Drain` is dropped, these elements will get truncated.
         let results = self.results.drain(self.results.len() - num_predecessors..);
-
-        let pred_val = {
-            let mut iter = results.as_slice().iter().filter(|&val| val != &sentinel);
-            if let Some(val) = iter.next() {
-                // This variable has at least one non-temporary definition. If they're all the same
-                // value, we can remove the block parameter and reference that value instead.
-                if iter.all(|other| other == val) {
-                    Some(*val)
-                } else {
-                    None
-                }
-            } else {
-                // The variable is used but never defined before. This is an irregularity in the
-                // code, but rather than throwing an error we silently initialize the variable to
-                // 0. This will have no effect since this situation happens in unreachable code.
-                if !dfg.is_block_inserted(dest_block) {
-                    dfg.append_block(dest_block);
-                }
-                self.side_effects
-                    .instructions_added_to_blocks
-                    .push(dest_block);
-                let zero = emit_zero(
-                    dfg.value_type(sentinel),
-                    // FuncCursor::new(func).at_first_insertion_point(dest_block),
-                );
-                Some(zero)
-            }
-        };
-
-        if let Some(pred_val) = pred_val {
-            // Here all the predecessors use a single value to represent our variable
-            // so we don't need to have it as a block argument.
-            dfg.remove_block_param(sentinel);
-            pred_val
-        } else {
-            // There is disagreement in the predecessors on which value to use so we have
-            // to keep the block argument.
-            let mut preds = self.ssa_blocks[dest_block].predecessors;
-            for (idx, &val) in results.as_slice().iter().enumerate() {
-                let pred = preds.get_mut(idx, &mut self.inst_pool).unwrap();
-                let branch = *pred;
-                assert!(
-                    dfg.insts[branch].opcode().is_branch(),
-                    "you have declared a non-branch instruction as a predecessor to a block!"
-                );
-                dfg.append_branch_destination_argument(branch, dest_block, val);
-            }
-            sentinel
+        // Keep the block argument.
+        let mut preds = self.ssa_blocks[dest_block].predecessors;
+        for (idx, &val) in results.as_slice().iter().enumerate() {
+            let pred = preds.get_mut(idx, &mut self.inst_pool).unwrap();
+            let branch = *pred;
+            assert!(
+                dfg.insts[branch].opcode().is_branch(),
+                "you have declared a non-branch instruction as a predecessor to a block!"
+            );
+            dfg.append_branch_destination_argument(branch, dest_block, val);
         }
+        sentinel
     }
 
     /// Returns the list of `Block`s that have been declared as predecessors of the argument.

tests/integration/expected/fib.hir

@@ -8,10 +8,29 @@ global external @gv2 : i32 = $0 { id = 2 };
 
 pub fn fib(i32) -> i32 {
 block0(v0: i32):
-    v2 = const.i32 1000 : i32;
-    v3 = cast v0 : u32;
-    v4 = cast v2 : u32;
-    v5 = lt v3, v4 : i1;
-    v6 = cast v5 : i32;
-    ret v6;
+    br block2(v0);
+
+block1(v1: i32):
+
+block2(v3: i32):
+    v4 = const.i32 3 : i32;
+    v5 = cast v3 : u32;
+    v6 = cast v4 : u32;
+    v7 = gt v5, v6 : i1;
+    v8 = cast v7 : i32;
+    v9 = neq v8, 0 : i1;
+    condbr v9, block4, block5;
+
+block3(v2: i32):
+
+block4:
+    v10 = const.i32 3 : i32;
+    v11 = cast v3 : u32;
+    v12 = cast v10 : u32;
+    v13 = div.checked v11, v12 : u32;
+    v14 = cast v13 : i32;
+    br block2(v14);
+
+block5:
+    ret v3;
 }

tests/integration/expected/fib.masm

@@ -602,18 +602,59 @@ export.load_dw
 end
 
 export.fib
+  dup.0
   dup.0
   push.2147483648
   u32.and
   eq.2147483648
   assertz
-  push.1000
+  push.3
   dup.0
   push.2147483648
   u32.and
   eq.2147483648
   assertz
-  u32.lt.checked
+  u32.gt.checked
+  u32.neq.0
+  push.1
+  while.true
+    if.true
+      dup.0
+      push.2147483648
+      u32.and
+      eq.2147483648
+      assertz
+      push.3
+      dup.0
+      push.2147483648
+      u32.and
+      eq.2147483648
+      assertz
+      u32.div.checked
+      dup.0
+      push.2147483648
+      u32.and
+      eq.2147483648
+      assertz
+      dup.0
+      dup.0
+      push.2147483648
+      u32.and
+      eq.2147483648
+      assertz
+      push.3
+      dup.0
+      push.2147483648
+      u32.and
+      eq.2147483648
+      assertz
+      u32.gt.checked
+      u32.neq.0
+      push.1
+    else
+      push.0
+    end
+  end
 
 end
 begin

tests/integration/expected/fib.wat

@@ -1,9 +1,21 @@
 (module
   (type (;0;) (func (param i32) (result i32)))
   (func $fib (;0;) (type 0) (param i32) (result i32)
-    local.get 0
-    i32.const 1000
-    i32.lt_u
+    loop (result i32) ;; label = @1
+      block ;; label = @2
+        local.get 0
+        i32.const 3
+        i32.gt_u
+        br_if 0 (;@2;)
+        local.get 0
+        return
+      end
+      local.get 0
+      i32.const 3
+      i32.div_u
+      local.set 0
+      br 0 (;@1;)
+    end
   )
   (table (;0;) 1 1 funcref)
   (memory (;0;) 16)

tests/rust-apps/fib/src/lib.rs

@@ -12,9 +12,9 @@ pub fn fib(n: u32) -> u32 {
     // }
     // a
 
-    if n < 1000 {
-        1
-    } else {
-        0
+    let mut a = n;
+    while a > 3 {
+        a = a / 3;
     }
+    a
 }