From 5a58eb807a1a63eb5301f1b9bca2292b208528d9 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Mon, 6 Apr 2026 14:11:52 +0100
Subject: Seal blocks as soon as possible

---
 patio-to-cranelift/src/codegen/context.rs |  5 +++--
 patio-to-cranelift/src/codegen/mod.rs     | 14 ++++++++------
 2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/patio-to-cranelift/src/codegen/context.rs b/patio-to-cranelift/src/codegen/context.rs
index ddd50492..f7e5d4d0 100644
--- a/patio-to-cranelift/src/codegen/context.rs
+++ b/patio-to-cranelift/src/codegen/context.rs
@@ -92,6 +92,7 @@ impl CompilerContext {
         let entry_block = function_builder.create_block();
         function_builder.append_block_params_for_function_params(entry_block);
         function_builder.switch_to_block(entry_block);
+        function_builder.seal_block(entry_block);
 
         let mut context = CompilerFunctionContext {
             func_name_to_func_id: &self.func_name_to_func_id,
@@ -216,8 +217,8 @@ impl<'a> CompilerFunctionContext<'a> {
         self.function_builder.switch_to_block(block);
     }
 
-    pub(crate) fn seal_current_block(&mut self) {
-        self.function_builder.seal_block(self.current_block());
+    pub(crate) fn seal_block(&mut self, block: cranelift_codegen::ir::Block) {
+        self.function_builder.seal_block(block);
     }
 
     pub(crate) fn current_block(&self) -> cranelift_codegen::ir::Block {
diff --git a/patio-to-cranelift/src/codegen/mod.rs b/patio-to-cranelift/src/codegen/mod.rs
index 7fad42f5..798c0381 100644
--- a/patio-to-cranelift/src/codegen/mod.rs
+++ b/patio-to-cranelift/src/codegen/mod.rs
@@ -456,7 +456,8 @@ fn compile_if(if_: &PatioIf, context: &mut CompilerFunctionContext<'_>) {
         cranelift_else_block,
         &[],
     );
-    context.seal_current_block();
+    context.seal_block(cranelift_then_block);
+    context.seal_block(cranelift_else_block);
 
     context.switch_to_block(cranelift_then_block);
     let cranelift_then_results = compile_block(&if_.true_body, context);
@@ -464,7 +465,6 @@ fn compile_if(if_: &PatioIf, context: &mut CompilerFunctionContext<'_>) {
         cranelift_end_block,
         cranelift_block_args(cranelift_then_results).iter(),
     );
-    context.function_builder.seal_block(cranelift_then_block);
 
     context.switch_to_block(cranelift_else_block);
     let cranelift_else_results = compile_block(&if_.false_body, context);
@@ -472,8 +472,8 @@ fn compile_if(if_: &PatioIf, context: &mut CompilerFunctionContext<'_>) {
         cranelift_end_block,
         cranelift_block_args(cranelift_else_results).iter(),
     );
-    context.function_builder.seal_block(cranelift_else_block);
 
+    context.seal_block(cranelift_end_block);
     context.switch_to_block(cranelift_end_block);
 
     for (destination, cranelift_end_param) in if_.destinations.iter().zip(cranelift_end_params) {
@@ -645,12 +645,15 @@ fn compile_switch(switch: &PatioSwitch, context: &mut CompilerFunctionContext<'_
     for (case, case_block) in switch.cases.iter().zip(&case_blocks) {
         cranelift_switch.set_entry(case.value.into(), *case_block);
     }
-    context.seal_current_block();
     cranelift_switch.emit(
         &mut context.function_builder,
         cranelift_value,
         default_block,
     );
+    for case_block in &case_blocks {
+        context.seal_block(*case_block);
+    }
+    context.seal_block(default_block);
 
     // Compile the case and default blocks.
     for (case, case_block) in switch.cases.iter().zip(&case_blocks) {
@@ -660,7 +663,6 @@ fn compile_switch(switch: &PatioSwitch, context: &mut CompilerFunctionContext<'_
             .function_builder
             .ins()
             .jump(end_block, &cranelift_block_args(end_args));
-        context.seal_current_block();
     }
 
     {
@@ -670,9 +672,9 @@ fn compile_switch(switch: &PatioSwitch, context: &mut CompilerFunctionContext<'_
             .function_builder
             .ins()
             .jump(end_block, &cranelift_block_args(end_args));
-        context.seal_current_block();
     }
 
+    context.seal_block(end_block);
     context.switch_to_block(end_block);
     for (destination, cranelift_end_param) in switch.destinations.iter().zip(cranelift_end_params) {
         def_destination(destination, cranelift_end_param, context);
-- 
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