diff --git a/llvm/include/llvm/AsmParser/LLParser.h b/llvm/include/llvm/AsmParser/LLParser.h
--- a/llvm/include/llvm/AsmParser/LLParser.h
+++ b/llvm/include/llvm/AsmParser/LLParser.h
@@ -180,7 +180,7 @@
           BlockAddressPFS(nullptr) {}
     bool Run(
         bool UpgradeDebugInfo, DataLayoutCallbackTy DataLayoutCallback =
-                                   [](StringRef) { return std::nullopt; });
+                                   [](const Module &) { return std::nullopt; });
 
     bool parseStandaloneConstantValue(Constant *&C, const SlotMapping *Slots);
 
diff --git a/llvm/include/llvm/AsmParser/Parser.h b/llvm/include/llvm/AsmParser/Parser.h
--- a/llvm/include/llvm/AsmParser/Parser.h
+++ b/llvm/include/llvm/AsmParser/Parser.h
@@ -29,7 +29,7 @@
 class SMDiagnostic;
 class Type;
 
-typedef llvm::function_ref<std::optional<std::string>(StringRef)>
+typedef llvm::function_ref<std::optional<std::string>(const Module &)>
     DataLayoutCallbackTy;
 
 /// This function is a main interface to the LLVM Assembly Parser. It parses
@@ -86,7 +86,7 @@
 ParsedModuleAndIndex parseAssemblyFileWithIndex(
     StringRef Filename, SMDiagnostic &Err, LLVMContext &Context,
     SlotMapping *Slots = nullptr,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](const Module &) {
       return std::nullopt;
     });
 
@@ -127,7 +127,7 @@
 std::unique_ptr<Module> parseAssembly(
     MemoryBufferRef F, SMDiagnostic &Err, LLVMContext &Context,
     SlotMapping *Slots = nullptr,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](const Module &) {
       return std::nullopt;
     });
 
@@ -169,7 +169,7 @@
 bool parseAssemblyInto(
     MemoryBufferRef F, Module *M, ModuleSummaryIndex *Index, SMDiagnostic &Err,
     SlotMapping *Slots = nullptr,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](const Module &) {
       return std::nullopt;
     });
 
diff --git a/llvm/include/llvm/Bitcode/BitcodeReader.h b/llvm/include/llvm/Bitcode/BitcodeReader.h
--- a/llvm/include/llvm/Bitcode/BitcodeReader.h
+++ b/llvm/include/llvm/Bitcode/BitcodeReader.h
@@ -34,20 +34,20 @@
 class MemoryBuffer;
 class ModuleSummaryIndex;
 
-typedef llvm::function_ref<std::optional<std::string>(StringRef)>
+typedef llvm::function_ref<std::optional<std::string>(const Module &)>
     DataLayoutCallbackTy;
 
-  // These functions are for converting Expected/Error values to
-  // ErrorOr/std::error_code for compatibility with legacy clients. FIXME:
-  // Remove these functions once no longer needed by the C and libLTO APIs.
+// These functions are for converting Expected/Error values to
+// ErrorOr/std::error_code for compatibility with legacy clients. FIXME:
+// Remove these functions once no longer needed by the C and libLTO APIs.
 
-  std::error_code errorToErrorCodeAndEmitErrors(LLVMContext &Ctx, Error Err);
+std::error_code errorToErrorCodeAndEmitErrors(LLVMContext &Ctx, Error Err);
 
-  template <typename T>
-  ErrorOr<T> expectedToErrorOrAndEmitErrors(LLVMContext &Ctx, Expected<T> Val) {
-    if (!Val)
-      return errorToErrorCodeAndEmitErrors(Ctx, Val.takeError());
-    return std::move(*Val);
+template <typename T>
+ErrorOr<T> expectedToErrorOrAndEmitErrors(LLVMContext &Ctx, Expected<T> Val) {
+  if (!Val)
+    return errorToErrorCodeAndEmitErrors(Ctx, Val.takeError());
+  return std::move(*Val);
   }
 
   struct BitcodeFileContents;
@@ -108,7 +108,7 @@
     /// Read the entire bitcode module and return it.
     Expected<std::unique_ptr<Module>> parseModule(
         LLVMContext &Context, DataLayoutCallbackTy DataLayoutCallback =
-                                  [](StringRef) { return std::nullopt; });
+                                  [](const Module &) { return std::nullopt; });
 
     /// Returns information about the module to be used for LTO: whether to
     /// compile with ThinLTO, and whether it has a summary.
@@ -175,7 +175,7 @@
   /// Read the specified bitcode file, returning the module.
   Expected<std::unique_ptr<Module>> parseBitcodeFile(
       MemoryBufferRef Buffer, LLVMContext &Context,
-      DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+      DataLayoutCallbackTy DataLayoutCallback = [](const Module &) {
         return std::nullopt;
       });
 
diff --git a/llvm/include/llvm/CodeGen/MIRParser/MIRParser.h b/llvm/include/llvm/CodeGen/MIRParser/MIRParser.h
--- a/llvm/include/llvm/CodeGen/MIRParser/MIRParser.h
+++ b/llvm/include/llvm/CodeGen/MIRParser/MIRParser.h
@@ -34,7 +34,7 @@
 class SMDiagnostic;
 class StringRef;
 
-typedef llvm::function_ref<std::optional<std::string>(StringRef)>
+typedef llvm::function_ref<std::optional<std::string>(const Module &)>
     DataLayoutCallbackTy;
 
 /// This class initializes machine functions by applying the state loaded from
@@ -52,7 +52,7 @@
   /// A new, empty module is created if the LLVM IR isn't present.
   /// \returns nullptr if a parsing error occurred.
   std::unique_ptr<Module>
-  parseIRModule(DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+  parseIRModule(DataLayoutCallbackTy DataLayoutCallback = [](const Module &) {
     return std::nullopt;
   });
 
diff --git a/llvm/include/llvm/IRReader/IRReader.h b/llvm/include/llvm/IRReader/IRReader.h
--- a/llvm/include/llvm/IRReader/IRReader.h
+++ b/llvm/include/llvm/IRReader/IRReader.h
@@ -27,7 +27,7 @@
 class SMDiagnostic;
 class LLVMContext;
 
-typedef llvm::function_ref<std::optional<std::string>(StringRef)>
+typedef llvm::function_ref<std::optional<std::string>(const Module &)>
     DataLayoutCallbackTy;
 
 /// If the given MemoryBuffer holds a bitcode image, return a Module
@@ -55,7 +55,7 @@
 /// \param DataLayoutCallback Override datalayout in the llvm assembly.
 std::unique_ptr<Module> parseIR(
     MemoryBufferRef Buffer, SMDiagnostic &Err, LLVMContext &Context,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](const Module &) {
       return std::nullopt;
     });
 
@@ -65,7 +65,7 @@
 /// \param DataLayoutCallback Override datalayout in the llvm assembly.
 std::unique_ptr<Module> parseIRFile(
     StringRef Filename, SMDiagnostic &Err, LLVMContext &Context,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](const Module &) {
       return std::nullopt;
     });
 }
diff --git a/llvm/lib/AsmParser/LLParser.cpp b/llvm/lib/AsmParser/LLParser.cpp
--- a/llvm/lib/AsmParser/LLParser.cpp
+++ b/llvm/lib/AsmParser/LLParser.cpp
@@ -98,7 +98,7 @@
     if (parseTargetDefinitions())
       return true;
 
-    if (auto LayoutOverride = DataLayoutCallback(M->getTargetTriple()))
+    if (auto LayoutOverride = DataLayoutCallback(*M))
       M->setDataLayout(*LayoutOverride);
   }
 
diff --git a/llvm/lib/AsmParser/Parser.cpp b/llvm/lib/AsmParser/Parser.cpp
--- a/llvm/lib/AsmParser/Parser.cpp
+++ b/llvm/lib/AsmParser/Parser.cpp
@@ -93,7 +93,7 @@
                                                   SlotMapping *Slots) {
   return ::parseAssemblyWithIndex(F, Err, Context, Slots,
                                   /*UpgradeDebugInfo*/ true,
-                                  [](StringRef) { return std::nullopt; });
+                                  [](const Module &) { return std::nullopt; });
 }
 
 static ParsedModuleAndIndex
@@ -150,7 +150,7 @@
   // index, but we need to initialize it.
   LLVMContext unusedContext;
   return LLParser(F.getBuffer(), SM, Err, nullptr, &Index, unusedContext)
-      .Run(true, [](StringRef) { return std::nullopt; });
+      .Run(true, [](const Module &) { return std::nullopt; });
 }
 
 std::unique_ptr<ModuleSummaryIndex>
diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -821,7 +821,7 @@
   Error parseAttrKind(uint64_t Code, Attribute::AttrKind *Kind);
   Error parseModule(
       uint64_t ResumeBit, bool ShouldLazyLoadMetadata = false,
-      DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+      DataLayoutCallbackTy DataLayoutCallback = [](const Module &) {
         return std::nullopt;
       });
 
@@ -4204,8 +4204,7 @@
         TheModule->getDataLayoutStr(), TheModule->getTargetTriple());
     TheModule->setDataLayout(DL);
 
-    if (auto LayoutOverride =
-            DataLayoutCallback(TheModule->getTargetTriple()))
+    if (auto LayoutOverride = DataLayoutCallback(*TheModule))
       TheModule->setDataLayout(*LayoutOverride);
   };
 
@@ -7944,7 +7943,7 @@
 BitcodeModule::getLazyModule(LLVMContext &Context, bool ShouldLazyLoadMetadata,
                              bool IsImporting) {
   return getModuleImpl(Context, false, ShouldLazyLoadMetadata, IsImporting,
-                       [](StringRef) { return std::nullopt; });
+                       [](const Module &) { return std::nullopt; });
 }
 
 // Parse the specified bitcode buffer and merge the index into CombinedIndex.
diff --git a/llvm/lib/CodeGen/MIRParser/MIRParser.cpp b/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
--- a/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
+++ b/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
@@ -234,7 +234,7 @@
     // Create an empty module when the MIR file is empty.
     NoMIRDocuments = true;
     auto M = std::make_unique<Module>(Filename, Context);
-    if (auto LayoutOverride = DataLayoutCallback(M->getTargetTriple()))
+    if (auto LayoutOverride = DataLayoutCallback(*M))
       M->setDataLayout(*LayoutOverride);
     return M;
   }
@@ -257,7 +257,7 @@
   } else {
     // Create an new, empty module.
     M = std::make_unique<Module>(Filename, Context);
-    if (auto LayoutOverride = DataLayoutCallback(M->getTargetTriple()))
+    if (auto LayoutOverride = DataLayoutCallback(*M))
       M->setDataLayout(*LayoutOverride);
     NoLLVMIR = true;
   }
diff --git a/llvm/tools/llc/llc.cpp b/llvm/tools/llc/llc.cpp
--- a/llvm/tools/llc/llc.cpp
+++ b/llvm/tools/llc/llc.cpp
@@ -530,10 +530,12 @@
 
   // If user just wants to list available options, skip module loading
   if (!SkipModule) {
-    auto SetDataLayout =
-        [&](StringRef DataLayoutTargetTriple) -> std::optional<std::string> {
+    auto SetDataLayout = [&](const Module &M) -> std::optional<std::string> {
       // If we are supposed to override the target triple, do so now.
-      std::string IRTargetTriple = DataLayoutTargetTriple.str();
+      // TODO: it would be nice to use codegen::inferDataLayoutOverride() to
+      //  avoid code duplication, but this lambda has additional externally
+      //  visible side effects such as setting up the target and TargetOptions.
+      std::string IRTargetTriple = M.getTargetTriple();
       if (!TargetTriple.empty())
         IRTargetTriple = Triple::normalize(TargetTriple);
       TheTriple = Triple(IRTargetTriple);
diff --git a/llvm/tools/llvm-as/llvm-as.cpp b/llvm/tools/llvm-as/llvm-as.cpp
--- a/llvm/tools/llvm-as/llvm-as.cpp
+++ b/llvm/tools/llvm-as/llvm-as.cpp
@@ -121,7 +121,7 @@
 
   // Parse the file now...
   SMDiagnostic Err;
-  auto SetDataLayout = [](StringRef) -> std::optional<std::string> {
+  auto SetDataLayout = [](const Module &) -> std::optional<std::string> {
     if (ClDataLayout.empty())
       return std::nullopt;
     return ClDataLayout;
diff --git a/llvm/tools/llvm-reduce/ReducerWorkItem.cpp b/llvm/tools/llvm-reduce/ReducerWorkItem.cpp
--- a/llvm/tools/llvm-reduce/ReducerWorkItem.cpp
+++ b/llvm/tools/llvm-reduce/ReducerWorkItem.cpp
@@ -407,12 +407,11 @@
     std::unique_ptr<MIRParser> MParser =
         createMIRParser(std::move(FileOrErr.get()), Ctxt);
 
-    auto SetDataLayout =
-        [&](StringRef DataLayoutTargetTriple) -> Optional<std::string> {
+    auto SetDataLayout = [&](const Module &M) -> Optional<std::string> {
       // NB: We always call createTargetMachineForTriple() even if an explicit
       // DataLayout is already set in the module since we want to use this
       // callback to setup the TargetMachine rather than doing it later.
-      std::string IRTargetTriple = DataLayoutTargetTriple.str();
+      std::string IRTargetTriple = M.getTargetTriple();
       if (!TargetTriple.empty())
         IRTargetTriple = Triple::normalize(TargetTriple);
       TheTriple = Triple(IRTargetTriple);
@@ -420,7 +419,10 @@
         TheTriple.setTriple(sys::getDefaultTargetTriple());
       ExitOnError ExitOnErr(std::string(ToolName) + ": error: ");
       TM = ExitOnErr(codegen::createTargetMachineForTriple(TheTriple.str()));
-
+      // If an explicit data layout already defined, we do not use the one from
+      // the TargetMachine.
+      if (!M.getDataLayout().isDefault())
+        return None;
       return TM->createDataLayout().getStringRepresentation();
     };
 
diff --git a/llvm/tools/opt/opt.cpp b/llvm/tools/opt/opt.cpp
--- a/llvm/tools/opt/opt.cpp
+++ b/llvm/tools/opt/opt.cpp
@@ -509,7 +509,7 @@
   std::unique_ptr<ToolOutputFile> RemarksFile = std::move(*RemarksFileOrErr);
 
   // Load the input module...
-  auto SetDataLayout = [](StringRef) -> std::optional<std::string> {
+  auto SetDataLayout = [](const Module &) -> std::optional<std::string> {
     if (ClDataLayout.empty())
       return std::nullopt;
     return ClDataLayout;