From 4db1b13e9865b5fdf471b0c78144728108021814 Mon Sep 17 00:00:00 2001
From: Ben Vanik <ben.vanik@gmail.com>
Date: Sat, 4 Jan 2014 11:56:05 -0800
Subject: [PATCH] DCE pass removes assigns.

---
 .../passes/dead_code_elimination_pass.cc      | 32 +++++++++++++++++++
 .../passes/dead_code_elimination_pass.h       |  1 +
 2 files changed, 33 insertions(+)

diff --git a/src/alloy/compiler/passes/dead_code_elimination_pass.cc b/src/alloy/compiler/passes/dead_code_elimination_pass.cc
index 9bc564c90..a9b7c7bdb 100644
--- a/src/alloy/compiler/passes/dead_code_elimination_pass.cc
+++ b/src/alloy/compiler/passes/dead_code_elimination_pass.cc
@@ -43,6 +43,12 @@ int DeadCodeEliminationPass::Run(HIRBuilder* builder) {
   //   v35.i8 = compare_eq v31.i32, 0
   //   branch_true v35.i8, loc_8201A484
 
+  // This also removes useless ASSIGNs:
+  //   v1 = v0
+  //   v2 = add v1, v1
+  // becomes:
+  //   v2 = add v0, v0
+
   // We process DCE by reverse iterating over instructions and looking at the
   // use count of the dest value. If it's zero, we can safely remove the
   // instruction. Once we do that, the use counts of any of the src ops may
@@ -67,6 +73,10 @@ int DeadCodeEliminationPass::Run(HIRBuilder* builder) {
         // Has no uses and is not volatile. This instruction can die!
         MakeNopRecursive(i);
         any_removed = true;
+      } else if (opcode == &OPCODE_ASSIGN_info) {
+        // Assignment. These are useless, so just try to remove by completely
+        // replacing the value.
+        ReplaceAssignment(i);
       }
 
       i = prev;
@@ -118,3 +128,25 @@ void DeadCodeEliminationPass::MakeNopRecursive(Instr* i) {
   MAKE_NOP_SRC(2);
   MAKE_NOP_SRC(3);
 }
+
+void DeadCodeEliminationPass::ReplaceAssignment(Instr* i) {
+  auto src = i->src1.value;
+  auto dest = i->dest;
+
+  auto use = dest->use_head;
+  while (use) {
+    auto use_instr = use->instr;
+    if (use_instr->src1.value == dest) {
+      use_instr->set_src1(src);
+    }
+    if (use_instr->src2.value == dest) {
+      use_instr->set_src2(src);
+    }
+    if (use_instr->src3.value == dest) {
+      use_instr->set_src3(src);
+    }
+    use = use->next;
+  }
+
+  i->Remove();
+}
diff --git a/src/alloy/compiler/passes/dead_code_elimination_pass.h b/src/alloy/compiler/passes/dead_code_elimination_pass.h
index 1d414d5c2..9a8cfc43a 100644
--- a/src/alloy/compiler/passes/dead_code_elimination_pass.h
+++ b/src/alloy/compiler/passes/dead_code_elimination_pass.h
@@ -27,6 +27,7 @@ public:
 
 private:
   void MakeNopRecursive(hir::Instr* i);
+  void ReplaceAssignment(hir::Instr* i);
 };