diff --git a/src/librustc_codegen_llvm/back/write.rs b/src/librustc_codegen_llvm/back/write.rs
index 81619c219757b..dabfa4c540b19 100644
--- a/src/librustc_codegen_llvm/back/write.rs
+++ b/src/librustc_codegen_llvm/back/write.rs
@@ -10,7 +10,7 @@
 
 use attributes;
 use back::bytecode::{self, RLIB_BYTECODE_EXTENSION};
-use back::lto::{self, ModuleBuffer, ThinBuffer, SerializedModule};
+use back::lto::{self, ThinBuffer, SerializedModule};
 use back::link::{self, get_linker, remove};
 use back::command::Command;
 use back::linker::LinkerInfo;
@@ -564,8 +564,8 @@ unsafe fn optimize(cgcx: &CodegenContext,
             // Some options cause LLVM bitcode to be emitted, which uses ThinLTOBuffers, so we need
             // to make sure we run LLVM's NameAnonGlobals pass when emitting bitcode; otherwise
             // we'll get errors in LLVM.
-            let using_thin_buffers = llvm::LLVMRustThinLTOAvailable() && (config.emit_bc
-                || config.obj_is_bitcode || config.emit_bc_compressed || config.embed_bitcode);
+            let using_thin_buffers = config.emit_bc || config.obj_is_bitcode
+                || config.emit_bc_compressed || config.embed_bitcode;
             let mut have_name_anon_globals_pass = false;
             if !config.no_prepopulate_passes {
                 llvm::LLVMRustAddAnalysisPasses(tm, fpm, llmod);
@@ -729,15 +729,8 @@ unsafe fn codegen(cgcx: &CodegenContext,
 
 
         if write_bc || config.emit_bc_compressed || config.embed_bitcode {
-            let thin;
-            let old;
-            let data = if llvm::LLVMRustThinLTOAvailable() {
-                thin = ThinBuffer::new(llmod);
-                thin.data()
-            } else {
-                old = ModuleBuffer::new(llmod);
-                old.data()
-            };
+            let thin = ThinBuffer::new(llmod);
+            let data = thin.data();
             timeline.record("make-bc");
 
             if write_bc {
@@ -1385,12 +1378,8 @@ fn execute_optimize_work_item(cgcx: &CodegenContext,
         // builds we don't actually want to LTO the allocator modules if
         // it shows up. This is due to various linker shenanigans that
         // we'll encounter later.
-        //
-        // Additionally here's where we also factor in the current LLVM
-        // version. If it doesn't support ThinLTO we skip this.
         Lto::ThinLocal => {
-            module.kind != ModuleKind::Allocator &&
-                unsafe { llvm::LLVMRustThinLTOAvailable() }
+            module.kind != ModuleKind::Allocator
         }
     };
 
diff --git a/src/librustc_codegen_llvm/base.rs b/src/librustc_codegen_llvm/base.rs
index a4c7a7123b964..ecdff946bf385 100644
--- a/src/librustc_codegen_llvm/base.rs
+++ b/src/librustc_codegen_llvm/base.rs
@@ -738,19 +738,6 @@ pub fn codegen_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 {
     check_for_rustc_errors_attr(tcx);
 
-    if let Some(true) = tcx.sess.opts.debugging_opts.thinlto {
-        if unsafe { !llvm::LLVMRustThinLTOAvailable() } {
-            tcx.sess.fatal("this compiler's LLVM does not support ThinLTO");
-        }
-    }
-
-    if (tcx.sess.opts.debugging_opts.pgo_gen.is_some() ||
-        !tcx.sess.opts.debugging_opts.pgo_use.is_empty()) &&
-        unsafe { !llvm::LLVMRustPGOAvailable() }
-    {
-        tcx.sess.fatal("this compiler's LLVM does not support PGO");
-    }
-
     let cgu_name_builder = &mut CodegenUnitNameBuilder::new(tcx);
 
     // Codegen the metadata.
diff --git a/src/librustc_codegen_llvm/llvm/ffi.rs b/src/librustc_codegen_llvm/llvm/ffi.rs
index f046ea030272a..12d4670e4be4b 100644
--- a/src/librustc_codegen_llvm/llvm/ffi.rs
+++ b/src/librustc_codegen_llvm/llvm/ffi.rs
@@ -1626,8 +1626,6 @@ extern "C" {
     pub fn LLVMRustModuleBufferFree(p: &'static mut ModuleBuffer);
     pub fn LLVMRustModuleCost(M: &Module) -> u64;
 
-    pub fn LLVMRustThinLTOAvailable() -> bool;
-    pub fn LLVMRustPGOAvailable() -> bool;
     pub fn LLVMRustThinLTOBufferCreate(M: &Module) -> &'static mut ThinLTOBuffer;
     pub fn LLVMRustThinLTOBufferFree(M: &'static mut ThinLTOBuffer);
     pub fn LLVMRustThinLTOBufferPtr(M: &ThinLTOBuffer) -> *const c_char;
diff --git a/src/rustllvm/ArchiveWrapper.cpp b/src/rustllvm/ArchiveWrapper.cpp
index 49a4962858ca7..390450a520526 100644
--- a/src/rustllvm/ArchiveWrapper.cpp
+++ b/src/rustllvm/ArchiveWrapper.cpp
@@ -145,7 +145,6 @@ extern "C" void LLVMRustArchiveIteratorFree(LLVMRustArchiveIteratorRef RAI) {
 
 extern "C" const char *
 LLVMRustArchiveChildName(LLVMRustArchiveChildConstRef Child, size_t *Size) {
-#if LLVM_VERSION_GE(4, 0)
   Expected<StringRef> NameOrErr = Child->getName();
   if (!NameOrErr) {
     // rustc_codegen_llvm currently doesn't use this error string, but it might be
@@ -154,11 +153,6 @@ LLVMRustArchiveChildName(LLVMRustArchiveChildConstRef Child, size_t *Size) {
     LLVMRustSetLastError(toString(NameOrErr.takeError()).c_str());
     return nullptr;
   }
-#else
-  ErrorOr<StringRef> NameOrErr = Child->getName();
-  if (NameOrErr.getError())
-    return nullptr;
-#endif
   StringRef Name = NameOrErr.get();
   *Size = Name.size();
   return Name.data();
@@ -167,19 +161,11 @@ LLVMRustArchiveChildName(LLVMRustArchiveChildConstRef Child, size_t *Size) {
 extern "C" const char *LLVMRustArchiveChildData(LLVMRustArchiveChildRef Child,
                                                 size_t *Size) {
   StringRef Buf;
-#if LLVM_VERSION_GE(4, 0)
   Expected<StringRef> BufOrErr = Child->getBuffer();
   if (!BufOrErr) {
     LLVMRustSetLastError(toString(BufOrErr.takeError()).c_str());
     return nullptr;
   }
-#else
-  ErrorOr<StringRef> BufOrErr = Child->getBuffer();
-  if (BufOrErr.getError()) {
-    LLVMRustSetLastError(BufOrErr.getError().message().c_str());
-    return nullptr;
-  }
-#endif
   Buf = BufOrErr.get();
   *Size = Buf.size();
   return Buf.data();
diff --git a/src/rustllvm/Linker.cpp b/src/rustllvm/Linker.cpp
index 534e4b910902e..59f0b23361e54 100644
--- a/src/rustllvm/Linker.cpp
+++ b/src/rustllvm/Linker.cpp
@@ -42,7 +42,6 @@ LLVMRustLinkerAdd(RustLinker *L, char *BC, size_t Len) {
   std::unique_ptr<MemoryBuffer> Buf =
       MemoryBuffer::getMemBufferCopy(StringRef(BC, Len));
 
-#if LLVM_VERSION_GE(4, 0)
   Expected<std::unique_ptr<Module>> SrcOrError =
       llvm::getLazyBitcodeModule(Buf->getMemBufferRef(), L->Ctx);
   if (!SrcOrError) {
@@ -51,20 +50,8 @@ LLVMRustLinkerAdd(RustLinker *L, char *BC, size_t Len) {
   }
 
   auto Src = std::move(*SrcOrError);
-#else
-  ErrorOr<std::unique_ptr<Module>> Src =
-      llvm::getLazyBitcodeModule(std::move(Buf), L->Ctx);
-  if (!Src) {
-    LLVMRustSetLastError(Src.getError().message().c_str());
-    return false;
-  }
-#endif
 
-#if LLVM_VERSION_GE(4, 0)
   if (L->L.linkInModule(std::move(Src))) {
-#else
-  if (L->L.linkInModule(std::move(Src.get()))) {
-#endif
     LLVMRustSetLastError("");
     return false;
   }
diff --git a/src/rustllvm/PassWrapper.cpp b/src/rustllvm/PassWrapper.cpp
index 06f75d981e3d6..200175da1bf2a 100644
--- a/src/rustllvm/PassWrapper.cpp
+++ b/src/rustllvm/PassWrapper.cpp
@@ -32,7 +32,6 @@
 #include "llvm/Target/TargetSubtargetInfo.h"
 #endif
 
-#if LLVM_VERSION_GE(4, 0)
 #include "llvm/Transforms/IPO/AlwaysInliner.h"
 #include "llvm/Transforms/IPO/FunctionImport.h"
 #include "llvm/Transforms/Utils/FunctionImportUtils.h"
@@ -40,14 +39,9 @@
 #if LLVM_VERSION_LE(4, 0)
 #include "llvm/Object/ModuleSummaryIndexObjectFile.h"
 #endif
-#endif
 
 #include "llvm-c/Transforms/PassManagerBuilder.h"
 
-#if LLVM_VERSION_GE(4, 0)
-#define PGO_AVAILABLE
-#endif
-
 using namespace llvm;
 using namespace llvm::legacy;
 
@@ -121,12 +115,8 @@ bool LLVMRustPassManagerBuilderPopulateThinLTOPassManager(
   LLVMPassManagerBuilderRef PMBR,
   LLVMPassManagerRef PMR
 ) {
-#if LLVM_VERSION_GE(4, 0)
   unwrap(PMBR)->populateThinLTOPassManager(*unwrap(PMR));
   return true;
-#else
-  return false;
-#endif
 }
 
 #ifdef LLVM_COMPONENT_X86
@@ -288,17 +278,12 @@ static Optional<Reloc::Model> fromRust(LLVMRustRelocMode RustReloc) {
     return Reloc::PIC_;
   case LLVMRustRelocMode::DynamicNoPic:
     return Reloc::DynamicNoPIC;
-#if LLVM_VERSION_GE(4, 0)
   case LLVMRustRelocMode::ROPI:
     return Reloc::ROPI;
   case LLVMRustRelocMode::RWPI:
     return Reloc::RWPI;
   case LLVMRustRelocMode::ROPIRWPI:
     return Reloc::ROPI_RWPI;
-#else
-  default:
-    break;
-#endif
   }
   report_fatal_error("Bad RelocModel.");
 }
@@ -450,11 +435,8 @@ extern "C" void LLVMRustConfigurePassManagerBuilder(
   unwrap(PMBR)->SLPVectorize = SLPVectorize;
   unwrap(PMBR)->OptLevel = fromRust(OptLevel);
   unwrap(PMBR)->LoopVectorize = LoopVectorize;
-#if LLVM_VERSION_GE(4, 0)
   unwrap(PMBR)->PrepareForThinLTO = PrepareForThinLTO;
-#endif
 
-#ifdef PGO_AVAILABLE
   if (PGOGenPath) {
     assert(!PGOUsePath);
     unwrap(PMBR)->EnablePGOInstrGen = true;
@@ -464,9 +446,6 @@ extern "C" void LLVMRustConfigurePassManagerBuilder(
     assert(!PGOGenPath);
     unwrap(PMBR)->PGOInstrUse = PGOUsePath;
   }
-#else
-  assert(!PGOGenPath && !PGOUsePath && "Should've caught earlier");
-#endif
 }
 
 // Unfortunately, the LLVM C API doesn't provide a way to set the `LibraryInfo`
@@ -716,7 +695,6 @@ extern "C" void LLVMRustPrintPasses() {
   LLVMInitializePasses();
   struct MyListener : PassRegistrationListener {
     void passEnumerate(const PassInfo *Info) {
-#if LLVM_VERSION_GE(4, 0)
       StringRef PassArg = Info->getPassArgument();
       StringRef PassName = Info->getPassName();
       if (!PassArg.empty()) {
@@ -726,11 +704,6 @@ extern "C" void LLVMRustPrintPasses() {
         printf("%15.*s - %.*s\n", (int)PassArg.size(), PassArg.data(),
                (int)PassName.size(), PassName.data());
       }
-#else
-      if (Info->getPassArgument() && *Info->getPassArgument()) {
-        printf("%15s - %s\n", Info->getPassArgument(), Info->getPassName());
-      }
-#endif
     }
   } Listener;
 
@@ -740,11 +713,7 @@ extern "C" void LLVMRustPrintPasses() {
 
 extern "C" void LLVMRustAddAlwaysInlinePass(LLVMPassManagerBuilderRef PMBR,
                                             bool AddLifetimes) {
-#if LLVM_VERSION_GE(4, 0)
   unwrap(PMBR)->Inliner = llvm::createAlwaysInlinerLegacyPass(AddLifetimes);
-#else
-  unwrap(PMBR)->Inliner = createAlwaysInlinerPass(AddLifetimes);
-#endif
 }
 
 extern "C" void LLVMRustRunRestrictionPass(LLVMModuleRef M, char **Symbols,
@@ -795,26 +764,6 @@ extern "C" void LLVMRustSetModulePIELevel(LLVMModuleRef M) {
   unwrap(M)->setPIELevel(PIELevel::Level::Large);
 }
 
-extern "C" bool
-LLVMRustThinLTOAvailable() {
-#if LLVM_VERSION_GE(4, 0)
-  return true;
-#else
-  return false;
-#endif
-}
-
-extern "C" bool
-LLVMRustPGOAvailable() {
-#ifdef PGO_AVAILABLE
-  return true;
-#else
-  return false;
-#endif
-}
-
-#if LLVM_VERSION_GE(4, 0)
-
 // Here you'll find an implementation of ThinLTO as used by the Rust compiler
 // right now. This ThinLTO support is only enabled on "recent ish" versions of
 // LLVM, and otherwise it's just blanket rejected from other compilers.
@@ -1276,94 +1225,3 @@ LLVMRustThinLTOPatchDICompileUnit(LLVMModuleRef Mod, DICompileUnit *Unit) {
   MD->clearOperands();
   MD->addOperand(Unit);
 }
-
-#else
-
-struct LLVMRustThinLTOData {
-};
-
-struct LLVMRustThinLTOModule {
-};
-
-extern "C" LLVMRustThinLTOData*
-LLVMRustCreateThinLTOData(LLVMRustThinLTOModule *modules,
-                          int num_modules,
-                          const char **preserved_symbols,
-                          int num_symbols) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" bool
-LLVMRustPrepareThinLTORename(const LLVMRustThinLTOData *Data, LLVMModuleRef M) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" bool
-LLVMRustPrepareThinLTOResolveWeak(const LLVMRustThinLTOData *Data, LLVMModuleRef M) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" bool
-LLVMRustPrepareThinLTOInternalize(const LLVMRustThinLTOData *Data, LLVMModuleRef M) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" bool
-LLVMRustPrepareThinLTOImport(const LLVMRustThinLTOData *Data, LLVMModuleRef M) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" LLVMRustThinLTOModuleImports
-LLVMRustGetLLVMRustThinLTOModuleImports(const LLVMRustThinLTOData *Data) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" void
-LLVMRustFreeThinLTOData(LLVMRustThinLTOData *Data) {
-  report_fatal_error("ThinLTO not available");
-}
-
-struct LLVMRustThinLTOBuffer {
-};
-
-extern "C" LLVMRustThinLTOBuffer*
-LLVMRustThinLTOBufferCreate(LLVMModuleRef M) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" void
-LLVMRustThinLTOBufferFree(LLVMRustThinLTOBuffer *Buffer) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" const void*
-LLVMRustThinLTOBufferPtr(const LLVMRustThinLTOBuffer *Buffer) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" size_t
-LLVMRustThinLTOBufferLen(const LLVMRustThinLTOBuffer *Buffer) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" LLVMModuleRef
-LLVMRustParseBitcodeForThinLTO(LLVMContextRef Context,
-                               const char *data,
-                               size_t len,
-                               const char *identifier) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" void
-LLVMRustThinLTOGetDICompileUnit(LLVMModuleRef Mod,
-                                DICompileUnit **A,
-                                DICompileUnit **B) {
-  report_fatal_error("ThinLTO not available");
-}
-
-extern "C" void
-LLVMRustThinLTOPatchDICompileUnit(LLVMModuleRef Mod) {
-  report_fatal_error("ThinLTO not available");
-}
-
-#endif // LLVM_VERSION_GE(4, 0)
diff --git a/src/rustllvm/RustWrapper.cpp b/src/rustllvm/RustWrapper.cpp
index affec73e3ac62..3dbde46f76241 100644
--- a/src/rustllvm/RustWrapper.cpp
+++ b/src/rustllvm/RustWrapper.cpp
@@ -907,10 +907,8 @@ LLVMRustDIBuilderCreateNameSpace(LLVMRustDIBuilderRef Builder,
       ,
       unwrapDI<DIFile>(File), LineNo
 #endif
-#if LLVM_VERSION_GE(4, 0)
       ,
       false // ExportSymbols (only relevant for C++ anonymous namespaces)
-#endif
       ));
 }
 
@@ -1547,14 +1545,6 @@ LLVMRustBuildVectorReduceFMax(LLVMBuilderRef, LLVMValueRef, bool) {
 }
 #endif
 
-#if LLVM_VERSION_LT(4, 0)
-extern "C" LLVMValueRef
-LLVMBuildExactUDiv(LLVMBuilderRef B, LLVMValueRef LHS,
-                   LLVMValueRef RHS, const char *Name) {
-  return wrap(unwrap(B)->CreateExactUDiv(unwrap(LHS), unwrap(RHS), Name));
-}
-#endif
-
 #if LLVM_VERSION_GE(6, 0)
 extern "C" LLVMValueRef
 LLVMRustBuildMinNum(LLVMBuilderRef B, LLVMValueRef LHS, LLVMValueRef RHS) {
diff --git a/src/rustllvm/rustllvm.h b/src/rustllvm/rustllvm.h
index b6fa9a2fa9508..2fd72fa749a84 100644
--- a/src/rustllvm/rustllvm.h
+++ b/src/rustllvm/rustllvm.h
@@ -57,12 +57,8 @@
 
 #include "llvm/IR/LegacyPassManager.h"
 
-#if LLVM_VERSION_GE(4, 0)
 #include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/Bitcode/BitcodeWriter.h"
-#else
-#include "llvm/Bitcode/ReaderWriter.h"
-#endif
 
 #include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/DebugInfo.h"