Index: include/llvm/MC/MCParser/MCAsmLexer.h
===================================================================
--- include/llvm/MC/MCParser/MCAsmLexer.h
+++ include/llvm/MC/MCParser/MCAsmLexer.h
@@ -74,6 +74,7 @@
 
   SMLoc getLoc() const;
   SMLoc getEndLoc() const;
+  SMRange getLocRange() const;
 
   /// getStringContents - Get the contents of a string token (without quotes).
   StringRef getStringContents() const {
Index: lib/MC/MCParser/MCAsmLexer.cpp
===================================================================
--- lib/MC/MCParser/MCAsmLexer.cpp
+++ lib/MC/MCParser/MCAsmLexer.cpp
@@ -30,3 +30,7 @@
 SMLoc AsmToken::getEndLoc() const {
   return SMLoc::getFromPointer(Str.data() + Str.size());
 }
+
+SMRange AsmToken::getLocRange() const {
+  return SMRange(getLoc(), getEndLoc());
+}
Index: lib/Target/Mips/AsmParser/MipsAsmParser.cpp
===================================================================
--- lib/Target/Mips/AsmParser/MipsAsmParser.cpp
+++ lib/Target/Mips/AsmParser/MipsAsmParser.cpp
@@ -27,6 +27,7 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/TargetRegistry.h"
+#include "llvm/Support/SourceMgr.h"
 #include <memory>
 
 using namespace llvm;
@@ -101,6 +102,10 @@
                        // selected. This usually happens after an '.end func'
                        // directive.
 
+  // Print a warning along with its fix-it message at the given range.
+  void printWarningWithFixIt(const Twine &Msg, const Twine &FixMsg,
+                             SMRange Range, bool ShowColors = true);
+
 #define GET_ASSEMBLER_HEADER
 #include "MipsGenAsmMatcher.inc"
 
@@ -1602,6 +1607,14 @@
   }
 }
 
+void
+MipsAsmParser::printWarningWithFixIt(const Twine &Msg, const Twine &FixMsg,
+                                     SMRange Range, bool ShowColors) {
+  getSourceManager().PrintMessage(Range.Start, SourceMgr::DK_Warning, Msg,
+                                  { Range }, { SMFixIt(Range, FixMsg) },
+                                  ShowColors);
+}
+
 int MipsAsmParser::matchCPURegisterName(StringRef Name) {
   int CC;
 
@@ -1644,6 +1657,23 @@
   if (!(isABI_N32() || isABI_N64()))
     return CC;
 
+  if (12 <= CC && CC <= 15) {
+    // Name is one of t4-t7
+    AsmToken RegTok = getLexer().peekTok();
+    SMRange RegRange = RegTok.getLocRange();
+
+    StringRef FixedName = StringSwitch<StringRef>(Name)
+                              .Case("t4", "t0")
+                              .Case("t5", "t1")
+                              .Case("t6", "t2")
+                              .Case("t7", "t3")
+                              .Default("");
+    assert(FixedName != "" &&  "Register name is not one of t4-t7.");
+
+    printWarningWithFixIt("register names $t4-$t7 are only available in O32.",
+                          "Did you mean $" + FixedName + "?", RegRange);
+  }
+
   // Although SGI documentation just cuts out t0-t3 for n32/n64,
   // GNU pushes the values of t0-t3 to override the o32/o64 values for t4-t7
   // We are supporting both cases, so for t0-t3 we'll just push them to t4-t7.
Index: test/MC/Mips/mips64-register-names-n32-n64.s
===================================================================
--- test/MC/Mips/mips64-register-names-n32-n64.s
+++ test/MC/Mips/mips64-register-names-n32-n64.s
@@ -1,7 +1,11 @@
-# RUN: llvm-mc %s -triple=mips64-unknown-freebsd -show-encoding | FileCheck %s
+# RUN: llvm-mc %s -triple=mips64-unknown-freebsd -show-encoding 2>%t0 \
+# RUN:     | FileCheck %s
+# RUN: FileCheck -check-prefix=WARNING %s < %t0
+#
 # RUN: llvm-mc %s -triple=mips64-unknown-freebsd -show-encoding \
-# RUN:     -mattr=-n64,+n32 | FileCheck %s
-
+# RUN:     -mattr=-n64,+n32 2>%t1 | FileCheck %s
+# RUN: FileCheck -check-prefix=WARNING %s < %t1
+#
 # Check that the register names are mapped to their correct numbers for n32/n64
 # Second byte of addiu with $zero at rt contains the number of the source
 # register.
@@ -23,9 +27,25 @@
 daddiu	$t1, $zero, 0 # [*] # CHECK: encoding: [0x64,0x0d,0x00,0x00]
 daddiu	$t2, $zero, 0 # [*] # CHECK: encoding: [0x64,0x0e,0x00,0x00]
 daddiu	$t3, $zero, 0 # [*] # CHECK: encoding: [0x64,0x0f,0x00,0x00]
+# WARNING: mips64-register-names-n32-n64.s:[[@LINE+4]]:9: warning: register names $t4-$t7 are only available in O32.
+# WARNING-NEXT: daddiu  $t4, $zero, 0       # {{CHECK}}: encoding: [0x64,0x0c,0x00,0x00]
+# WARNING-NEXT:          ^~
+# WARNING-NEXT:          Did you mean $t0?
 daddiu	$t4, $zero, 0       # CHECK: encoding: [0x64,0x0c,0x00,0x00]
+# WARNING: mips64-register-names-n32-n64.s:[[@LINE+4]]:9: warning: register names $t4-$t7 are only available in O32.
+# WARNING-NEXT: daddiu  $t5, $zero, 0       # {{CHECK}}: encoding: [0x64,0x0d,0x00,0x00]
+# WARNING-NEXT:          ^~
+# WARNING-NEXT:          Did you mean $t1?
 daddiu	$t5, $zero, 0       # CHECK: encoding: [0x64,0x0d,0x00,0x00]
+# WARNING: mips64-register-names-n32-n64.s:[[@LINE+4]]:9: warning: register names $t4-$t7 are only available in O32.
+# WARNING-NEXT: daddiu  $t6, $zero, 0       # {{CHECK}}: encoding: [0x64,0x0e,0x00,0x00]
+# WARNING-NEXT:          ^~
+# WARNING-NEXT:          Did you mean $t2?
 daddiu	$t6, $zero, 0       # CHECK: encoding: [0x64,0x0e,0x00,0x00]
+# WARNING: mips64-register-names-n32-n64.s:[[@LINE+4]]:9: warning: register names $t4-$t7 are only available in O32.
+# WARNING-NEXT: daddiu  $t7, $zero, 0       # {{CHECK}}: encoding: [0x64,0x0f,0x00,0x00]
+# WARNING-NEXT:          ^~
+# WARNING-NEXT:          Did you mean $t3?
 daddiu	$t7, $zero, 0       # CHECK: encoding: [0x64,0x0f,0x00,0x00]
 daddiu	$s0, $zero, 0       # CHECK: encoding: [0x64,0x10,0x00,0x00]
 daddiu	$s1, $zero, 0       # CHECK: encoding: [0x64,0x11,0x00,0x00]