fix: dropkeep not being emited correctly

Signed-off-by: Henry <mail@henrygressmann.de>

Author: explodingcamera

crates/cli/src/load.rs

@@ -73,7 +73,7 @@ fn load_module_from_bytes(input: &str, bytes: &[u8]) -> Result<LoadedModule> {
 
     #[cfg(not(feature = "wat"))]
     if input != "-" && has_extension(input, "wat") {
-        bail!("wat support is not enabled in this build")
+        bail!("wat support is not enabled in this build");
     }
 
     #[cfg(feature = "wat")]

crates/parser/src/visit.rs

@@ -1017,7 +1017,8 @@ impl FunctionBuilder<'_> {
 
     /// Emits the stack-shaping instruction required by a branch.
     fn emit_dropkeep(&mut self, base: ValueCounts, keep: ValueCounts) {
-        if base.is_empty() && keep.is_empty() {
+        let target = ValueCounts { c32: base.c32 + keep.c32, c64: base.c64 + keep.c64, c128: base.c128 + keep.c128 };
+        if self.lane_counts == target {
             return;
         }
         self.instructions.push(Instruction::DropKeep((base, keep).into()));

crates/tinywasm/tests/wasm-custom/compression-control-flow.wast

@@ -0,0 +1,43 @@
+(module
+  ;; Reduced from miniz_oxide's Huffman-table builder, which reached a bounds
+  ;; panic during compression. Branches to empty targets skipped DropKeep, so
+  ;; a stale operand remained on the stack and corrupted a later loop.
+  (func (export "branch-drops-retained") (result i32)
+    (local $iteration i32)
+    (local $count i32)
+    (local $result i32)
+    block $done
+      loop $outer
+        block $scan-done
+          i32.const 7
+          i32.const 20
+          local.get $iteration
+          select
+          local.get $iteration
+          i32.eqz
+          if
+            br $scan-done
+          end
+          i32.const 2
+          local.set $count
+          loop $inner
+            local.get $count
+            i32.const 1
+            i32.sub
+            local.tee $count
+            br_if $inner
+          end
+          local.set $result
+          br $done
+        end
+        local.get $iteration
+        i32.const 1
+        i32.add
+        local.set $iteration
+        br $outer
+      end
+    end
+    local.get $result)
+)
+
+(assert_return (invoke "branch-drops-retained") (i32.const 7))