From c9a153c7978b363a252f83878f75bd875fe6ae5e Mon Sep 17 00:00:00 2001
From: Jakub Konka <kubkon@jakubkonka.com>
Date: Sat, 11 Mar 2023 09:29:53 +0100
Subject: [PATCH] x86_64: add .dead pseudo-instruction to mark an unused MIR
 instruction

---
 src/arch/x86_64/CodeGen.zig | 16 ++++++++--------
 src/arch/x86_64/Emit.zig    |  2 ++
 src/arch/x86_64/Mir.zig     |  4 ++++
 3 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/src/arch/x86_64/CodeGen.zig b/src/arch/x86_64/CodeGen.zig
index fabca1d848..feb78ca7fc 100644
--- a/src/arch/x86_64/CodeGen.zig
+++ b/src/arch/x86_64/CodeGen.zig
@@ -630,8 +630,8 @@ fn gen(self: *Self) InnerError!void {
         // TODO During semantic analysis, check if there are no function calls. If there
         // are none, here we can omit the part where we subtract and then add rsp.
         const backpatch_stack_sub = try self.addInst(.{
-            .tag = .nop,
-            .ops = .none,
+            .tag = .dead,
+            .ops = undefined,
             .data = undefined,
         });
 
@@ -657,8 +657,8 @@ fn gen(self: *Self) InnerError!void {
 
         // Push callee-preserved regs that were used actually in use.
         const backpatch_push_callee_preserved_regs = try self.addInst(.{
-            .tag = .nop,
-            .ops = .none,
+            .tag = .dead,
+            .ops = undefined,
             .data = undefined,
         });
 
@@ -688,8 +688,8 @@ fn gen(self: *Self) InnerError!void {
 
         // Pop saved callee-preserved regs.
         const backpatch_pop_callee_preserved_regs = try self.addInst(.{
-            .tag = .nop,
-            .ops = .none,
+            .tag = .dead,
+            .ops = undefined,
             .data = undefined,
         });
 
@@ -701,8 +701,8 @@ fn gen(self: *Self) InnerError!void {
 
         // Maybe add rsp, x if required. This is backpatched later.
         const backpatch_stack_add = try self.addInst(.{
-            .tag = .nop,
-            .ops = .none,
+            .tag = .dead,
+            .ops = undefined,
             .data = undefined,
         });
 
diff --git a/src/arch/x86_64/Emit.zig b/src/arch/x86_64/Emit.zig
index 6e92e81882..161c436323 100644
--- a/src/arch/x86_64/Emit.zig
+++ b/src/arch/x86_64/Emit.zig
@@ -137,6 +137,8 @@ pub fn lowerMir(emit: *Emit) InnerError!void {
 
             .push_regs => try emit.mirPushPopRegisterList(.push, inst),
             .pop_regs => try emit.mirPushPopRegisterList(.pop, inst),
+
+            .dead => {},
         }
     }
 
diff --git a/src/arch/x86_64/Mir.zig b/src/arch/x86_64/Mir.zig
index 5de8ad1410..4cde4dd240 100644
--- a/src/arch/x86_64/Mir.zig
+++ b/src/arch/x86_64/Mir.zig
@@ -163,6 +163,10 @@ pub const Inst = struct {
         /// Pop registers
         /// Uses `payload` payload with data of type `SaveRegisterList`.
         pop_regs,
+
+        /// Tombstone
+        /// Emitter should skip this instruction.
+        dead,
     };
 
     pub const Ops = enum(u8) {