Basic DCE pass.

src/alloy/compiler/passes/dead_code_elimination_pass.cc

@@ -0,0 +1,123 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2013 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#include <alloy/compiler/passes/dead_code_elimination_pass.h>
+
+using namespace alloy;
+using namespace alloy::compiler;
+using namespace alloy::compiler::passes;
+using namespace alloy::hir;
+
+
+DeadCodeEliminationPass::DeadCodeEliminationPass() :
+    Pass() {
+}
+
+DeadCodeEliminationPass::~DeadCodeEliminationPass() {
+}
+
+int DeadCodeEliminationPass::Run(FunctionBuilder* builder) {
+  // ContextPromotion/DSE will likely leave around a lot of dead statements.
+  // Code generated for comparison/testing produces many unused statements and
+  // with proper use analysis it should be possible to remove most of them:
+  // After context promotion/simplification:
+  //   v33.i8 = compare_ult v31.i32, 0
+  //   v34.i8 = compare_ugt v31.i32, 0
+  //   v35.i8 = compare_eq v31.i32, 0
+  //   store_context +300, v33.i8
+  //   store_context +301, v34.i8
+  //   store_context +302, v35.i8
+  //   branch_true v35.i8, loc_8201A484
+  // After DSE:
+  //   v33.i8 = compare_ult v31.i32, 0
+  //   v34.i8 = compare_ugt v31.i32, 0
+  //   v35.i8 = compare_eq v31.i32, 0
+  //   branch_true v35.i8, loc_8201A484
+  // After DCE:
+  //   v35.i8 = compare_eq v31.i32, 0
+  //   branch_true v35.i8, loc_8201A484
+
+  // 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
+  // go to zero and we recursively kill up the graph. This is kind of
+  // nasty in that we walk randomly and scribble all over memory, but (I think)
+  // it's better than doing passes over all instructions until we quiesce.
+  // To prevent our iteration from getting all messed up we just replace
+  // all removed ops with NOP and then do a single pass that removes them
+  // all.
+
+  const OpcodeInfo* nop = builder->GetNopOpcode();
+
+  bool any_removed = false;
+  Block* block = builder->first_block();
+  while (block) {
+    Instr* i = block->instr_tail;
+    while (i) {
+      Instr* prev = i->prev;
+
+      const OpcodeInfo* opcode = i->opcode;
+      uint32_t signature = opcode->signature;
+      if (!(opcode->flags & OPCODE_FLAG_VOLATILE) &&
+          i->dest && !i->dest->use_head) {
+        // Has no uses and is not volatile. This instruction can die!
+        MakeNopRecursive(nop, i);
+        any_removed = true;
+      }
+
+      i = prev;
+    }
+
+    block = block->next;
+  }
+
+  // Remove all nops.
+  if (any_removed) {
+    Block* block = builder->first_block();
+    while (block) {
+      Instr* i = block->instr_head;
+      while (i) {
+        Instr* next = i->next;
+        if (i->opcode->num == OPCODE_NOP) {
+          // Nop - remove!
+          i->Remove();
+        }
+        i = next;
+      }
+      block = block->next;
+    }
+  }
+
+  return 0;
+}
+
+void DeadCodeEliminationPass::MakeNopRecursive(
+    const OpcodeInfo* nop, Instr* i) {
+  i->opcode = nop;
+  i->dest->def = NULL;
+  i->dest = NULL;
+
+#define MAKE_NOP_SRC(n) \
+  if (i->src##n##_use) { \
+    Value::Use* use = i->src##n##_use; \
+    Value* value = i->src##n##.value; \
+    i->src##n##_use = NULL; \
+    i->src##n##.value = NULL; \
+    value->RemoveUse(use); \
+    if (!value->use_head) { \
+      /* Value is now unused, so recursively kill it. */ \
+      if (value->def && value->def != i) { \
+        MakeNopRecursive(nop, value->def); \
+      } \
+    } \
+  }
+  MAKE_NOP_SRC(1);
+  MAKE_NOP_SRC(2);
+  MAKE_NOP_SRC(3);
+}