Index: clang/lib/CodeGen/BackendUtil.cpp
===================================================================
--- clang/lib/CodeGen/BackendUtil.cpp
+++ clang/lib/CodeGen/BackendUtil.cpp
@@ -973,9 +973,7 @@
             MPM.addPass(InstrProfiling(*Options, false));
           });
 
-    if (CodeGenOpts.OptimizationLevel == 0) {
-      MPM = PB.buildO0DefaultPipeline(Level, IsLTO || IsThinLTO);
-    } else if (IsThinLTO) {
+    if (IsThinLTO) {
       MPM = PB.buildThinLTOPreLinkDefaultPipeline(Level);
     } else if (IsLTO) {
       MPM = PB.buildLTOPreLinkDefaultPipeline(Level);
Index: clang/tools/clang-fuzzer/handle-llvm/handle_llvm.cpp
===================================================================
--- clang/tools/clang-fuzzer/handle-llvm/handle_llvm.cpp
+++ clang/tools/clang-fuzzer/handle-llvm/handle_llvm.cpp
@@ -103,11 +103,7 @@
   PB.registerLoopAnalyses(LAM);
   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
 
-  ModulePassManager MPM;
-  if (OL == OptimizationLevel::O0)
-    MPM = PB.buildO0DefaultPipeline(OL);
-  else
-    MPM = PB.buildPerModuleDefaultPipeline(OL);
+  ModulePassManager MPM = PB.buildPerModuleDefaultPipeline(OL);
   MPM.addPass(PrintModulePass(OS));
 
   MPM.run(M, MAM);
Index: flang/lib/Frontend/FrontendActions.cpp
===================================================================
--- flang/lib/Frontend/FrontendActions.cpp
+++ flang/lib/Frontend/FrontendActions.cpp
@@ -726,10 +726,7 @@
 
   // Create the pass manager.
   llvm::ModulePassManager mpm;
-  if (opts.OptimizationLevel == 0)
-    mpm = pb.buildO0DefaultPipeline(level, opts.PrepareForFullLTO ||
-                                               opts.PrepareForThinLTO);
-  else if (opts.PrepareForFullLTO)
+  if (opts.PrepareForFullLTO)
     mpm = pb.buildLTOPreLinkDefaultPipeline(level);
   else if (opts.PrepareForThinLTO)
     mpm = pb.buildThinLTOPreLinkDefaultPipeline(level);
Index: llvm/include/llvm/Passes/PassBuilder.h
===================================================================
--- llvm/include/llvm/Passes/PassBuilder.h
+++ llvm/include/llvm/Passes/PassBuilder.h
@@ -259,11 +259,6 @@
   /// optimization and code generation. It is particularly tuned to fit well
   /// when IR coming into the LTO phase was first run through \c
   /// addPreLinkLTODefaultPipeline, and the two coordinate closely.
-  ///
-  /// Note that \p Level cannot be `O0` here. The pipelines produced are
-  /// only intended for use when attempting to optimize code. If frontends
-  /// require some transformations for semantic reasons, they should explicitly
-  /// build them.
   ModulePassManager
   buildThinLTODefaultPipeline(OptimizationLevel Level,
                               const ModuleSummaryIndex *ImportSummary);
@@ -275,11 +270,6 @@
   /// run. It works to minimize the IR which needs to be analyzed without
   /// making irreversible decisions which could be made better during the LTO
   /// run.
-  ///
-  /// Note that \p Level cannot be `O0` here. The pipelines produced are
-  /// only intended for use when attempting to optimize code. If frontends
-  /// require some transformations for semantic reasons, they should explicitly
-  /// build them.
   ModulePassManager buildLTOPreLinkDefaultPipeline(OptimizationLevel Level);
 
   /// Build an LTO default optimization pipeline to a pass manager.
@@ -288,11 +278,6 @@
   /// optimization and code generation. It is particularly tuned to fit well
   /// when IR coming into the LTO phase was first run through \c
   /// addPreLinkLTODefaultPipeline, and the two coordinate closely.
-  ///
-  /// Note that \p Level cannot be `O0` here. The pipelines produced are
-  /// only intended for use when attempting to optimize code. If frontends
-  /// require some transformations for semantic reasons, they should explicitly
-  /// build them.
   ModulePassManager buildLTODefaultPipeline(OptimizationLevel Level,
                                             ModuleSummaryIndex *ExportSummary);
 
Index: llvm/lib/Passes/PassBuilder.cpp
===================================================================
--- llvm/lib/Passes/PassBuilder.cpp
+++ llvm/lib/Passes/PassBuilder.cpp
@@ -1270,12 +1270,6 @@
                               .Case("O3", OptimizationLevel::O3)
                               .Case("Os", OptimizationLevel::Os)
                               .Case("Oz", OptimizationLevel::Oz);
-    if (L == OptimizationLevel::O0 && Matches[1] != "thinlto" &&
-        Matches[1] != "lto") {
-      MPM.addPass(buildO0DefaultPipeline(L, Matches[1] == "thinlto-pre-link" ||
-                                                Matches[1] == "lto-pre-link"));
-      return Error::success();
-    }
 
     // This is consistent with old pass manager invoked via opt, but
     // inconsistent with clang. Clang doesn't enable loop vectorization
Index: llvm/lib/Passes/PassBuilderPipelines.cpp
===================================================================
--- llvm/lib/Passes/PassBuilderPipelines.cpp
+++ llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -1398,8 +1398,8 @@
 ModulePassManager
 PassBuilder::buildPerModuleDefaultPipeline(OptimizationLevel Level,
                                            bool LTOPreLink) {
-  assert(Level != OptimizationLevel::O0 &&
-         "Must request optimizations for the default pipeline!");
+  if (Level == OptimizationLevel::O0)
+    return buildO0DefaultPipeline(Level, LTOPreLink);
 
   ModulePassManager MPM;
 
@@ -1440,8 +1440,8 @@
 
 ModulePassManager
 PassBuilder::buildThinLTOPreLinkDefaultPipeline(OptimizationLevel Level) {
-  assert(Level != OptimizationLevel::O0 &&
-         "Must request optimizations for the default pipeline!");
+  if (Level == OptimizationLevel::O0)
+    return buildO0DefaultPipeline(Level, /*LTOPreLink*/true);
 
   ModulePassManager MPM;
 
@@ -1552,8 +1552,6 @@
 
 ModulePassManager
 PassBuilder::buildLTOPreLinkDefaultPipeline(OptimizationLevel Level) {
-  assert(Level != OptimizationLevel::O0 &&
-         "Must request optimizations for the default pipeline!");
   // FIXME: We should use a customized pre-link pipeline!
   return buildPerModuleDefaultPipeline(Level,
                                        /* LTOPreLink */ true);
Index: mlir/lib/ExecutionEngine/OptUtils.cpp
===================================================================
--- mlir/lib/ExecutionEngine/OptUtils.cpp
+++ mlir/lib/ExecutionEngine/OptUtils.cpp
@@ -84,11 +84,7 @@
     pb.crossRegisterProxies(lam, fam, cgam, mam);
 
     ModulePassManager mpm;
-    if (*ol == OptimizationLevel::O0)
-      mpm.addPass(pb.buildO0DefaultPipeline(*ol));
-    else
-      mpm.addPass(pb.buildPerModuleDefaultPipeline(*ol));
-
+    mpm.addPass(pb.buildPerModuleDefaultPipeline(*ol));
     mpm.run(*m, mam);
     return Error::success();
   };
Index: openmp/libomptarget/plugins-nextgen/common/PluginInterface/JIT.cpp
===================================================================
--- openmp/libomptarget/plugins-nextgen/common/PluginInterface/JIT.cpp
+++ openmp/libomptarget/plugins-nextgen/common/PluginInterface/JIT.cpp
@@ -214,11 +214,7 @@
   PB.registerLoopAnalyses(LAM);
   PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
 
-  if (OptLevel)
-    MPM.addPass(PB.buildPerModuleDefaultPipeline(getOptLevel(OptLevel)));
-  else
-    MPM.addPass(PB.buildO0DefaultPipeline(getOptLevel(OptLevel)));
-
+  MPM.addPass(PB.buildPerModuleDefaultPipeline(getOptLevel(OptLevel)));
   MPM.run(M, MAM);
 }