diff --git a/llvm/lib/Support/StringMap.cpp b/llvm/lib/Support/StringMap.cpp
--- a/llvm/lib/Support/StringMap.cpp
+++ b/llvm/lib/Support/StringMap.cpp
@@ -12,6 +12,7 @@
 
 #include "llvm/ADT/StringMap.h"
 #include "llvm/Support/MathExtras.h"
+#include "llvm/Support/ReverseIteration.h"
 #include "llvm/Support/xxhash.h"
 
 using namespace llvm;
@@ -85,6 +86,8 @@
   if (NumBuckets == 0)
     init(16);
   unsigned FullHashValue = xxHash64(Name);
+  if (shouldReverseIterate())
+    FullHashValue = ~FullHashValue;
   unsigned BucketNo = FullHashValue & (NumBuckets - 1);
   unsigned *HashTable = getHashTable(TheTable, NumBuckets);
 
@@ -140,6 +143,8 @@
   if (NumBuckets == 0)
     return -1; // Really empty table?
   unsigned FullHashValue = xxHash64(Key);
+  if (shouldReverseIterate())
+    FullHashValue = ~FullHashValue;
   unsigned BucketNo = FullHashValue & (NumBuckets - 1);
   unsigned *HashTable = getHashTable(TheTable, NumBuckets);
 
diff --git a/llvm/test/CMakeLists.txt b/llvm/test/CMakeLists.txt
--- a/llvm/test/CMakeLists.txt
+++ b/llvm/test/CMakeLists.txt
@@ -22,6 +22,7 @@
   LLVM_INLINER_MODEL_AUTOGENERATED
   LLVM_RAEVICT_MODEL_AUTOGENERATED
   LLVM_ENABLE_EXPENSIVE_CHECKS
+  LLVM_ENABLE_REVERSE_ITERATION
   LLVM_INCLUDE_DXIL_TESTS
   LLVM_TOOL_LLVM_DRIVER_BUILD
   )
diff --git a/llvm/test/CodeGen/MIR/AMDGPU/virtreg-uses-unallocatable-class.mir b/llvm/test/CodeGen/MIR/AMDGPU/virtreg-uses-unallocatable-class.mir
--- a/llvm/test/CodeGen/MIR/AMDGPU/virtreg-uses-unallocatable-class.mir
+++ b/llvm/test/CodeGen/MIR/AMDGPU/virtreg-uses-unallocatable-class.mir
@@ -1,3 +1,4 @@
+# UNSUPPORTED: reverse_iteration
 # RUN: not llc -mtriple=amdgcn-- -mcpu=gfx900 -run-pass=none -o - %s 2>&1 | FileCheck %s
 
 # Check a diagnostic is emitted if non-allocatable classes are used
diff --git a/llvm/test/ExecutionEngine/RuntimeDyld/ARM/MachO_ARM_PIC_relocations.s b/llvm/test/ExecutionEngine/RuntimeDyld/ARM/MachO_ARM_PIC_relocations.s
--- a/llvm/test/ExecutionEngine/RuntimeDyld/ARM/MachO_ARM_PIC_relocations.s
+++ b/llvm/test/ExecutionEngine/RuntimeDyld/ARM/MachO_ARM_PIC_relocations.s
@@ -1,3 +1,4 @@
+# UNSUPPORTED: reverse_iteration
 # RUN: rm -rf %t && mkdir -p %t
 # RUN: llvm-mc -triple=armv7s-apple-ios7.0.0 -filetype=obj -o %t/foo.o %s
 # RUN: llvm-rtdyld -triple=armv7s-apple-ios7.0.0 -verify -check=%s %t/foo.o
diff --git a/llvm/test/TableGen/GlobalISelCombinerMatchTableEmitter/pattern-parsing.td b/llvm/test/TableGen/GlobalISelCombinerMatchTableEmitter/pattern-parsing.td
--- a/llvm/test/TableGen/GlobalISelCombinerMatchTableEmitter/pattern-parsing.td
+++ b/llvm/test/TableGen/GlobalISelCombinerMatchTableEmitter/pattern-parsing.td
@@ -1,3 +1,4 @@
+// UNSUPPORTED: reverse_iteration
 // RUN: llvm-tblgen -I %p/../../../include -gen-global-isel-combiner-matchtable \
 // RUN:     -gicombiner-stop-after-parse -combiners=MyCombiner %s | \
 // RUN: FileCheck %s
diff --git a/llvm/test/lit.site.cfg.py.in b/llvm/test/lit.site.cfg.py.in
--- a/llvm/test/lit.site.cfg.py.in
+++ b/llvm/test/lit.site.cfg.py.in
@@ -58,6 +58,7 @@
 config.llvm_inliner_model_autogenerated = @LLVM_INLINER_MODEL_AUTOGENERATED@
 config.llvm_raevict_model_autogenerated = @LLVM_RAEVICT_MODEL_AUTOGENERATED@
 config.expensive_checks = @LLVM_ENABLE_EXPENSIVE_CHECKS@
+config.reverse_iteration = @LLVM_ENABLE_REVERSE_ITERATION@
 config.dxil_tests = @LLVM_INCLUDE_DXIL_TESTS@
 config.have_llvm_driver = @LLVM_TOOL_LLVM_DRIVER_BUILD@
 
diff --git a/llvm/utils/lit/lit/llvm/config.py b/llvm/utils/lit/lit/llvm/config.py
--- a/llvm/utils/lit/lit/llvm/config.py
+++ b/llvm/utils/lit/lit/llvm/config.py
@@ -120,6 +120,9 @@
         if have_zstd:
             features.add("zstd")
 
+        if getattr(config, "reverse_iteration", None):
+            features.add("reverse_iteration")
+
         # Check if we should run long running tests.
         long_tests = lit_config.params.get("run_long_tests", None)
         if lit.util.pythonize_bool(long_tests):