[spirv] Implement basic dead code elimination

Fixes invalid SPIR-V in Trails through Daybreak.

src/spirv/spirv_code_buffer.cpp

@@ -50,6 +50,18 @@ namespace dxvk {
   }
 
 
+  void SpirvCodeBuffer::append(const SpirvInstruction& ins) {
+    const size_t size = m_code.size();
+
+    m_code.resize(size + ins.length());
+
+    for (uint32_t i = 0; i < ins.length(); i++)
+      m_code[size + i] = ins.arg(i);
+
+    m_ptr += ins.length();
+  }
+
+
   void SpirvCodeBuffer::append(const SpirvCodeBuffer& other) {
     if (other.size() != 0) {
       const size_t size = m_code.size();

src/spirv/spirv_code_buffer.h

@@ -89,6 +89,14 @@ namespace dxvk {
      */
     uint32_t allocId();
     
+    /**
+     * \brief Appends an instruction
+     *
+     * Slightly faster than individually adding words.
+     * \param [in] ins Instruction
+     */
+    void append(const SpirvInstruction& ins);
+
     /**
      * \brief Merges two code buffers
      * 

src/spirv/spirv_module.cpp

@@ -15,7 +15,7 @@ namespace dxvk {
   }
   
   
-  SpirvCodeBuffer SpirvModule::compile() const {
+  SpirvCodeBuffer SpirvModule::compile() {
     SpirvCodeBuffer result;
     result.putHeader(m_version, m_id);
     result.append(m_capabilities);
@@ -28,7 +28,35 @@ namespace dxvk {
     result.append(m_annotations);
     result.append(m_typeConstDefs);
     result.append(m_variables);
-    result.append(m_code);
+
+    // Perform some crude dead code elimination. In some cases, our compilers
+    // may emit invalid code, such as an unreachable block branching to a loop's
+    // continue block, but those cases cannot be reasonably detected up-front.
+    std::unordered_set<uint32_t> reachableBlocks;
+    std::unordered_set<uint32_t> mergeBlocks;
+
+    classifyBlocks(reachableBlocks, mergeBlocks);
+
+    bool reachable = true;
+
+    for (auto ins : m_code) {
+      if (ins.opCode() == spv::OpFunctionEnd) {
+        reachable = true;
+        result.append(ins);
+      } else if (ins.opCode() == spv::OpLabel) {
+        uint32_t labelId = ins.arg(1);
+
+        if ((reachable = reachableBlocks.find(labelId) != reachableBlocks.end())) {
+          result.append(ins);
+        } else if (mergeBlocks.find(labelId) != mergeBlocks.end()) {
+          result.append(ins);
+          result.putIns(spv::OpUnreachable, 1);
+        }
+      } else if (reachable) {
+        result.append(ins);
+      }
+    }
+
     return result;
   }
   
@@ -3905,4 +3933,69 @@ namespace dxvk {
     }
   }
 
+
+  void SpirvModule::classifyBlocks(
+          std::unordered_set<uint32_t>& reachableBlocks,
+          std::unordered_set<uint32_t>& mergeBlocks) {
+    std::unordered_multimap<uint32_t, uint32_t> branches;
+    std::queue<uint32_t> blockQueue;
+
+    uint32_t blockId = 0;
+
+    for (auto ins : m_code) {
+      switch (ins.opCode()) {
+        case spv::OpLabel: {
+          uint32_t id = ins.arg(1);
+
+          if (!blockId)
+            branches.insert({ 0u, id });
+
+          blockId = id;
+        } break;
+
+        case spv::OpFunction: {
+          blockId = 0u;
+        } break;
+
+        case spv::OpBranch: {
+          branches.insert({ blockId, ins.arg(1) });
+        } break;
+
+        case spv::OpBranchConditional: {
+          branches.insert({ blockId, ins.arg(2) });
+          branches.insert({ blockId, ins.arg(3) });
+        } break;
+
+        case spv::OpSwitch: {
+          branches.insert({ blockId, ins.arg(2) });
+
+          for (uint32_t i = 4; i < ins.length(); i += 2)
+            branches.insert({ blockId, ins.arg(i) });
+        } break;
+
+        case spv::OpSelectionMerge:
+        case spv::OpLoopMerge: {
+          mergeBlocks.insert(ins.arg(1));
+        } break;
+
+        default:;
+      }
+    }
+
+    blockQueue.push(0);
+
+    while (!blockQueue.empty()) {
+      uint32_t id = blockQueue.front();
+
+      auto range = branches.equal_range(id);
+
+      for (auto i = range.first; i != range.second; i++) {
+        if (reachableBlocks.insert(i->second).second)
+          blockQueue.push(i->second);
+      }
+
+      blockQueue.pop();
+    }
+  }
+
 }

src/spirv/spirv_module.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <queue>
+#include <unordered_map>
 #include <unordered_set>
 
 #include "spirv_code_buffer.h"
@@ -59,7 +61,7 @@ namespace dxvk {
 
     ~SpirvModule();
     
-    SpirvCodeBuffer compile() const;
+    SpirvCodeBuffer compile();
 
     size_t getInsertionPtr() {
       return m_code.getInsertionPtr();
@@ -1326,6 +1328,10 @@ namespace dxvk {
     bool isInterfaceVar(
             spv::StorageClass       sclass) const;
 
+    void classifyBlocks(
+            std::unordered_set<uint32_t>& reachableBlocks,
+            std::unordered_set<uint32_t>& mergeBlocks);
+
   };
   
 }