MemoryArena rewindability/Improved IRInst construction (#837)

* Make MemoryArena rewindable.

* Add test for rewinding MemoryArena

* Use memory rewinding in IRInst lookup instead of malloc/free.

* Small tidy.

* Don't bother recreating instruction if after lookup it's found it's new.

* Fix 32 bit signed compare issue.

1 files changed, 22 insertions, 1 deletions

diff --git a/tools/slang-test/unit-test-memory-arena.cpp b/tools/slang-test/unit-test-memory-arena.cpp
index 1b09ae9e7..e993d4f7a 100644
--- a/tools/slang-test/unit-test-memory-arena.cpp
+++ b/tools/slang-test/unit-test-memory-arena.cpp
@@ -169,6 +169,21 @@ static void memoryArenaUnitTest()
                         arena.reset();
                         blocks.Clear();
                     }
+                    else if (var == 3)
+                    {
+                        arena.rewindToCursor(nullptr);
+                        blocks.Clear();
+                    }
+                    else if (var == 4)
+                    {
+                        // Rewind to a random position
+                        int rewindIndex = randGen.nextInt32UpTo(int32_t(blocks.Count()));
+                        // rewind to this block
+                        arena.rewindToCursor(blocks[rewindIndex].m_data);
+                        // All the blocks (includign this one) and now deallocated
+                        blocks.SetSize(rewindIndex);
+
+                    }
                     else
                     {
                         size_t usedMemory = arena.calcTotalMemoryUsed();
@@ -248,6 +263,12 @@ static void memoryArenaUnitTest()
             }
         }
     }
+    {
+        // Do lots of allocations and test out rewind
+        
+
+
+    }
 }
 
-SLANG_UNIT_TEST("MemoryArena", memoryArenaUnitTest);
\ No newline at end of file
+SLANG_UNIT_TEST("MemoryArena", memoryArenaUnitTest);