Merge pull request #2683 from cfallin/br-table-unreachable

Fix bad jumptable block ref when DCE removes a block.

cranelift/codegen/src/ir/jumptable.rs

@@ -68,6 +68,11 @@ impl JumpTableData {
     pub fn iter_mut(&mut self) -> IterMut<Block> {
         self.table.iter_mut()
     }
+
+    /// Clears all entries in this jump table.
+    pub fn clear(&mut self) {
+        self.table.clear();
+    }
 }
 
 impl Display for JumpTableData {

cranelift/codegen/src/unreachable_code.rs

@@ -43,4 +43,17 @@ pub fn eliminate_unreachable_code(
         // Finally, remove the block from the layout.
         pos.func.layout.remove_block(block);
     }
+
+    // Remove all jumptable block-list contents that refer to unreachable
+    // blocks; the jumptable itself must have been unused (or used only in an
+    // unreachable block) if so. Note that we are not necessarily removing *all*
+    // unused jumptables, because that would require computing their
+    // reachability as well; we are just removing enough to clean up references
+    // to deleted blocks.
+    for jt_data in func.jump_tables.values_mut() {
+        let invalid_ref = jt_data.iter().any(|block| !domtree.is_reachable(*block));
+        if invalid_ref {
+            jt_data.clear();
+        }
+    }
 }

cranelift/filetests/filetests/isa/x64/unused_jt_unreachable_block.clif

@@ -0,0 +1,21 @@
+test compile
+target x86_64
+feature "experimental_x64"
+
+;; From: https://github.com/bytecodealliance/wasmtime/issues/2670
+
+function %f() system_v {
+    jt0 = jump_table [block1]
+
+block0:
+    return
+
+block1:
+    trap unreachable
+}
+
+; check:  pushq   %rbp
+; nextln: movq    %rsp, %rbp
+; nextln: movq    %rbp, %rsp
+; nextln: popq    %rbp
+; nextln: ret