Add a peephole optimization pass to the regex compiler

src/regex_impl.cc

@@ -684,6 +684,7 @@ struct RegexCompiler
         {
             m_program.forward_start_desc = compute_start_desc<RegexMode::Forward>();
             compile_node<RegexMode::Forward>(0);
+            peephole_optimize(0, m_program.instructions.size());
             push_inst(CompiledRegex::Match);
         }
 
@@ -692,6 +693,7 @@ struct RegexCompiler
             m_program.first_backward_inst = m_program.instructions.size();
             m_program.backward_start_desc = compute_start_desc<RegexMode::Backward>();
             compile_node<RegexMode::Backward>(0);
+            peephole_optimize(m_program.first_backward_inst, m_program.instructions.size());
             push_inst(CompiledRegex::Match);
         }
         else
@@ -1019,6 +1021,21 @@ private:
         return std::make_unique<CompiledRegex::StartDesc>(start_desc);
     }
 
+    void peephole_optimize(size_t begin, size_t end)
+    {
+        if (not (m_flags & RegexCompileFlags::Optimize))
+            return;
+
+        // Move saves after all assertions on the same character
+        auto is_assertion = [](CompiledRegex::Op op) { return op >= CompiledRegex::LineStart; };
+        for (auto i = begin, j = begin + 1; j < end; ++i, ++j)
+        {
+            if (m_program.instructions[i].op == CompiledRegex::Save and
+                is_assertion(m_program.instructions[j].op))
+                std::swap(m_program.instructions[i], m_program.instructions[j]);
+        }
+    }
+
     const ParsedRegex::Node& get_node(ParsedRegex::NodeIndex index) const
     {
         return m_parsed_regex.nodes[index];

src/regex_impl.hh

@@ -471,7 +471,8 @@ private:
         release_saves(m_captures);
         m_captures = -1;
         m_threads.grow_ifn();
-        const int16_t first_inst = &config.instructions[0] - &m_program.instructions[0];
+
+        const int16_t first_inst = forward ? 0 : m_program.first_backward_inst;
         m_threads.push_current({first_inst, -1});
 
         const auto& start_desc = forward ? m_program.forward_start_desc : m_program.backward_start_desc;