Fix gc error

src/compiler.jl

@@ -693,6 +693,7 @@ function shadow_alloc_rewrite(V::LLVM.API.LLVMValueRef, gutils::API.EnzymeGradie
 
     if mode == API.DEM_ForwardMode && (used || idx != 0)
         # Zero any jlvalue_t inner elements of preceeding allocation.
+
         # Specifically in forward mode, you will first run the original allocation,
         # then all shadow allocations. These allocations will thus all run before
         # any value may store into them. For example, as follows:
@@ -703,12 +704,38 @@ function shadow_alloc_rewrite(V::LLVM.API.LLVMValueRef, gutils::API.EnzymeGradie
         # As a result, by the time of the subsequent GC allocation, the memory in the preceeding
         # allocation might be undefined, and trigger a GC error. To avoid this,
         # we will explicitly zero the GC'd fields of the previous allocation.
+
+        # Reverse mode will do similarly, except doing the shadow first
         prev = LLVM.Instruction(prev)
         B = LLVM.IRBuilder()
         position!(B, LLVM.Instruction(LLVM.API.LLVMGetNextInstruction(prev)))
 
         create_recursive_stores(B, Ty, prev)
     end
+    if (mode == API.DEM_ReverseModePrimal || mode == API.DEM_ReverseModeCombined) && used
+        # Zero any jlvalue_t inner elements of preceeding allocation.
+
+        # Specifically in reverse mode, you will run the original allocation,
+        # after all shadow allocations. The shadow allocations will thus all run before any value may store into them. For example, as follows:
+        #   %"orig'" = julia.gcalloc(...)
+        #   %orig = julia.gc_alloc(...)
+        #   store "orig'"[0] = jlvaluet'
+        #   store orig[0] = jlvaluet
+        #
+        # Normally this is fine, since we will memset right after the shadow
+        # however we will do this memset non atomically and if you have a case like the following, there will be an issue
+
+        #   %"orig'" = julia.gcalloc(...)
+        #   memset("orig'")
+        #   %orig = julia.gc_alloc(...)
+        #   store "orig'"[0] = jlvaluet'
+        #   store orig[0] = jlvaluet
+        #
+        # Julia could decide to dead store eliminate the memset (not being read before the store of jlvaluet'), resulting in an error
+        B = LLVM.IRBuilder()
+        position!(B, LLVM.Instruction(LLVM.API.LLVMGetNextInstruction(V)))
+        create_recursive_stores(B, Ty, V)
+    end
 
     nothing
 end