diff --git a/tslang/lib/TypeScript/GCPass.cpp b/tslang/lib/TypeScript/GCPass.cpp
index 11db0c0d3..80debfc78 100644
--- a/tslang/lib/TypeScript/GCPass.cpp
+++ b/tslang/lib/TypeScript/GCPass.cpp
@@ -206,9 +206,14 @@ class GCPass : public mlir::PassWrapper<GCPass, ModulePass>
             return;
         }
 
+        // The allocator's own state is inaccessible memory it reads and writes, as LLVM declares
+        // malloc: `memory(inaccessiblemem: readwrite)`. `allockind` alone no longer stops the
+        // merge - with this LLVM two GC_malloc(8) calls, the storage of two empty arrays, became
+        // one call, both arrays grew through GC_realloc of the same block, and the heap was
+        // corrupted (`s += x` produced an empty string further on, only with optimization).
         auto *context = funcOp->getContext();
         auto memoryEffects = LLVM::MemoryEffectsAttr::get(context, LLVM::ModRefInfo::Mod, LLVM::ModRefInfo::NoModRef,
-                                                            LLVM::ModRefInfo::NoModRef, LLVM::ModRefInfo::NoModRef,
+                                                            LLVM::ModRefInfo::ModRef, LLVM::ModRefInfo::NoModRef,
                                                             LLVM::ModRefInfo::NoModRef, LLVM::ModRefInfo::NoModRef);
         funcOp.setMemoryEffectsAttr(memoryEffects);
 
diff --git a/tslang/lib/TypeScript/LowerToLLVM.cpp b/tslang/lib/TypeScript/LowerToLLVM.cpp
index b3311de30..89bd928e7 100644
--- a/tslang/lib/TypeScript/LowerToLLVM.cpp
+++ b/tslang/lib/TypeScript/LowerToLLVM.cpp
@@ -6638,9 +6638,10 @@ class GCNewExplicitlyTypedOpLowering : public TsLlvmPattern<mlir_ts::GCNewExplic
         auto gcMallocExplicitlyTypedFunc = ch.getOrInsertFunction("GC_malloc_explicitly_typed", th.getFunctionType(i8PtrTy, {th.getSizeType(), th.getSizeType()}));
         // Without this, two `new` sites with identical (size, typeDescr) args - e.g. two
         // instances of the same class - look like redundant calls to GVN/EarlyCSE at -O3
-        // and get merged into one shared allocation. See GCPass.cpp markAsAllocatorIfNeeded.
+        // and get merged into one shared allocation. See GCPass.cpp markAsAllocatorIfNeeded, and
+        // the allocator's state as inaccessible memory it reads and writes, which is what stops it.
         gcMallocExplicitlyTypedFunc.setMemoryEffectsAttr(LLVM::MemoryEffectsAttr::get(
-            rewriter.getContext(), LLVM::ModRefInfo::Mod, LLVM::ModRefInfo::NoModRef, LLVM::ModRefInfo::NoModRef,
+            rewriter.getContext(), LLVM::ModRefInfo::Mod, LLVM::ModRefInfo::NoModRef, LLVM::ModRefInfo::ModRef,
             LLVM::ModRefInfo::NoModRef, LLVM::ModRefInfo::NoModRef, LLVM::ModRefInfo::NoModRef));
         markGCMallocExplicitlyTypedAsAllocator(gcMallocExplicitlyTypedFunc);
         auto value = rewriter.create<LLVM::CallOp>(loc, gcMallocExplicitlyTypedFunc, ValueRange{sizeOfTypeValue, transformed.getTypeDescr()});
diff --git a/tslang/test/tester/CMakeLists.txt b/tslang/test/tester/CMakeLists.txt
index c23b97214..f13890fe9 100644
--- a/tslang/test/tester/CMakeLists.txt
+++ b/tslang/test/tester/CMakeLists.txt
@@ -276,6 +276,7 @@ tslang_add_test(NAME test-compile-00-funcs-capture COMMAND test-runner "${PROJEC
 tslang_add_test(NAME test-compile-00-funcs-vararg COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00funcs_vararg.ts")
 tslang_add_test(NAME test-compile-00-declare-function-twice COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00declare_function_twice.ts")
 tslang_add_test(NAME test-compile-00-function-overloads COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00function_overloads.ts")
+tslang_add_test(NAME test-compile-00-gc-allocations-distinct COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00gc_allocations_distinct.ts")
 tslang_add_test(NAME test-compile-01-funcs-vararg COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/01funcs_vararg.ts")
 # printf is avaliable in JIT only, C has snprintf
 #tslang_add_test(NAME test-compile-02-funcs-vararg COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/02funcs_vararg.ts")
@@ -726,6 +727,7 @@ tslang_add_test(NAME test-jit-00-funcs-capture COMMAND test-runner -jit "${PROJE
 tslang_add_test(NAME test-jit-00-funcs-vararg COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00funcs_vararg.ts")
 tslang_add_test(NAME test-jit-00-declare-function-twice COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00declare_function_twice.ts")
 tslang_add_test(NAME test-jit-00-function-overloads COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00function_overloads.ts")
+tslang_add_test(NAME test-jit-00-gc-allocations-distinct COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00gc_allocations_distinct.ts")
 tslang_add_test(NAME test-jit-01-funcs-vararg COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/01funcs_vararg.ts")
 tslang_add_test(NAME test-jit-02-funcs-vararg COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/02funcs_vararg.ts")
 tslang_add_test(NAME test-jit-00-funcs-bindings COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00funcs_bindings.ts")
@@ -1651,6 +1653,7 @@ set(TSLANG_CORPUS
     00funcs_typeless_function_as_generic.ts
     00funcs_vararg.ts
     00function_overloads.ts
+    00gc_allocations_distinct.ts
     00funcs.ts
     00generator_manual_next.ts
     00generator_manual_next2.ts
diff --git a/tslang/test/tester/tests/00gc_allocations_distinct.ts b/tslang/test/tester/tests/00gc_allocations_distinct.ts
new file mode 100644
index 000000000..5b5332dbd
--- /dev/null
+++ b/tslang/test/tester/tests/00gc_allocations_distinct.ts
@@ -0,0 +1,36 @@
+// Two allocations of the same size are two blocks. The collector's allocators were marked so that
+// the optimizer could merge two identical calls: the storage of two empty arrays became one block,
+// both arrays grew through GC_realloc of it, and the heap was corrupted - here, `s += a` came out
+// empty. Only with optimization, which the compile and jit tiers use.
+class Point {
+    x = 0;
+}
+
+function main() {
+    const src: number[] = [1, 2];
+    let nums: number[] = [];
+    const t: number[] = [...src];
+    for (const v of t) nums.push(v);
+    assert(nums.length == 2, "nums");
+
+    const a = nums[0] + ",";
+    let s = "";
+    s += a;
+    assert(s == "1,", "append");
+
+    let e1: number[] = [];
+    let e2: number[] = [];
+    e1.push(1);
+    e2.push(2);
+    e2.push(3);
+    assert(e1.length == 1 && e1[0] == 1, "first empty array");
+    assert(e2.length == 2 && e2[0] == 2, "second empty array");
+
+    const p = new Point();
+    const q = new Point();
+    p.x = 1;
+    q.x = 2;
+    assert(p.x == 1 && q.x == 2, "two instances");
+
+    print("done.");
+}