From 2e23ddbe7ba48857fbdcf014da3251f1decca00d Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Tue, 25 Apr 2023 04:49:47 +0100
Subject: [PATCH 1/2] CBE: minor optimizations to output source

---
 src/codegen/c.zig | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/src/codegen/c.zig b/src/codegen/c.zig
index 903d2449fd..30227d747e 100644
--- a/src/codegen/c.zig
+++ b/src/codegen/c.zig
@@ -4291,9 +4291,13 @@ fn airBlock(f: *Function, inst: Air.Inst.Index) !CValue {
     }
 
     try f.object.indent_writer.insertNewline();
-    // label might be unused, add a dummy goto
-    // label must be followed by an expression, add an empty one.
-    try writer.print("goto zig_block_{d};\nzig_block_{d}: (void)0;\n", .{ block_id, block_id });
+
+    // noreturn blocks have no `br` instructions reaching them, so we don't want a label
+    if (!f.air.typeOfIndex(inst).isNoReturn()) {
+        // label must be followed by an expression, include an empty one.
+        try writer.print("zig_block_{d}:;\n", .{block_id});
+    }
+
     return result;
 }
 
@@ -4345,7 +4349,7 @@ fn lowerTry(
             else
                 try f.writeCValueMember(writer, err_union, .{ .identifier = "error" });
         }
-        try writer.writeByte(')');
+        try writer.writeAll(") ");
 
         try genBodyResolveState(f, inst, liveness_condbr.else_deaths, body, false);
         try f.object.indent_writer.insertNewline();
@@ -4417,7 +4421,11 @@ fn airBitcast(f: *Function, inst: Air.Inst.Index) !CValue {
 
     const local = try f.allocLocal(inst, dest_ty);
 
+    // If the assignment looks like 'x = x', we don't need it
+    const can_elide = operand == .local and operand.local == local.new_local;
+
     if (operand_ty.isAbiInt() and dest_ty.isAbiInt()) {
+        if (can_elide) return local;
         const src_info = dest_ty.intInfo(target);
         const dest_info = operand_ty.intInfo(target);
         if (src_info.signedness == dest_info.signedness and
@@ -4432,6 +4440,7 @@ fn airBitcast(f: *Function, inst: Air.Inst.Index) !CValue {
     }
 
     if (dest_ty.isPtrAtRuntime() and operand_ty.isPtrAtRuntime()) {
+        if (can_elide) return local;
         try f.writeCValue(writer, local, .Other);
         try writer.writeAll(" = (");
         try f.renderType(writer, dest_ty);
@@ -5463,6 +5472,12 @@ fn airUnwrapErrUnionErr(f: *Function, inst: Air.Inst.Index) !CValue {
     const error_ty = error_union_ty.errorUnionSet();
     const payload_ty = error_union_ty.errorUnionPayload();
     const local = try f.allocLocal(inst, inst_ty);
+
+    if (!payload_ty.hasRuntimeBits() and operand == .local and operand.local == local.new_local) {
+        // The store will be 'x = x'; elide it.
+        return local;
+    }
+
     const writer = f.object.writer();
     try f.writeCValue(writer, local, .Other);
     try writer.writeAll(" = ");
@@ -5560,6 +5575,12 @@ fn airWrapErrUnionErr(f: *Function, inst: Air.Inst.Index) !CValue {
 
     const writer = f.object.writer();
     const local = try f.allocLocal(inst, inst_ty);
+
+    if (repr_is_err and err == .local and err.local == local.new_local) {
+        // The store will be 'x = x'; elide it.
+        return local;
+    }
+
     if (!repr_is_err) {
         const a = try Assignment.start(f, writer, payload_ty);
         try f.writeCValueMember(writer, local, .{ .identifier = "payload" });

From 3df2f356eba9b0882ee3fa09704aae7dc173f3d4 Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Wed, 26 Apr 2023 01:43:55 +0100
Subject: [PATCH 2/2] Sema: avoid emitting sequential dbg_stmt instructions

Often, a `dbg_stmt` ends up being associated with no real code because
whatever it referred to was eliminated by semantic analysis. In these
cases, Sema can replace the last `dbg_stmt` with the new one to avoid
redundant AIR instructions which at best are nops and at worst cause
backends to emit useless info (e.g. CBE does this).
---
 src/Sema.zig | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/Sema.zig b/src/Sema.zig
index 199bdc43ab..8e4a82ccb5 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -5852,6 +5852,19 @@ fn zirDbgStmt(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!voi
     if (block.is_comptime or sema.mod.comp.bin_file.options.strip) return;
 
     const inst_data = sema.code.instructions.items(.data)[inst].dbg_stmt;
+
+    if (block.instructions.items.len != 0) {
+        const idx = block.instructions.items[block.instructions.items.len - 1];
+        if (sema.air_instructions.items(.tag)[idx] == .dbg_stmt) {
+            // The previous dbg_stmt didn't correspond to any actual code, so replace it.
+            sema.air_instructions.items(.data)[idx].dbg_stmt = .{
+                .line = inst_data.line,
+                .column = inst_data.column,
+            };
+            return;
+        }
+    }
+
     _ = try block.addInst(.{
         .tag = .dbg_stmt,
         .data = .{ .dbg_stmt = .{