Index: lib/Target/Mips/AsmParser/MipsAsmParser.cpp
===================================================================
--- lib/Target/Mips/AsmParser/MipsAsmParser.cpp
+++ lib/Target/Mips/AsmParser/MipsAsmParser.cpp
@@ -3009,67 +3009,84 @@
 bool MipsAsmParser::parseSetNoAtDirective() {
   MCAsmParser &Parser = getParser();
   // Line should look like: ".set noat".
-  // set at reg to 0.
+
+  // Set the $at register to $0.
   AssemblerOptions.back()->setATReg(0);
-  // eat noat
-  Parser.Lex();
+
+  Parser.Lex(); // Eat "noat".
+
   // If this is not the end of the statement, report an error.
   if (getLexer().isNot(AsmToken::EndOfStatement)) {
     reportParseError("unexpected token, expected end of statement");
     return false;
   }
+
+  getTargetStreamer().emitDirectiveSetNoAt();
   Parser.Lex(); // Consume the EndOfStatement.
   return false;
 }
 
 bool MipsAsmParser::parseSetAtDirective() {
+  // Line can be: ".set at", which sets $at to $1
+  //          or  ".set at=$reg", which sets $at to $reg.
   MCAsmParser &Parser = getParser();
-  // Line can be .set at - defaults to $1
-  // or .set at=$reg
-  int AtRegNo;
-  getParser().Lex();
+  Parser.Lex(); // Eat "at".
+
   if (getLexer().is(AsmToken::EndOfStatement)) {
+    // No register was specified, so we set $at to $1.
     AssemblerOptions.back()->setATReg(1);
+
+    getTargetStreamer().emitDirectiveSetAt();
     Parser.Lex(); // Consume the EndOfStatement.
     return false;
-  } else if (getLexer().is(AsmToken::Equal)) {
-    getParser().Lex(); // Eat the '='.
-    if (getLexer().isNot(AsmToken::Dollar)) {
-      reportParseError("unexpected token, expected dollar sign '$'");
+  }
+
+  if (getLexer().isNot(AsmToken::Equal)) {
+    reportParseError("unexpected token, expected equals sign");
+    return false;
+  }
+  Parser.Lex(); // Eat "=".
+
+  if (getLexer().isNot(AsmToken::Dollar)) {
+    if (getLexer().is(AsmToken::EndOfStatement)) {
+      reportParseError("no register specified");
       return false;
-    }
-    Parser.Lex(); // Eat the '$'.
-    const AsmToken &Reg = Parser.getTok();
-    if (Reg.is(AsmToken::Identifier)) {
-      AtRegNo = matchCPURegisterName(Reg.getIdentifier());
-    } else if (Reg.is(AsmToken::Integer)) {
-      AtRegNo = Reg.getIntVal();
     } else {
-      reportParseError("unexpected token, expected identifier or integer");
-      return false;
-    }
-
-    if (AtRegNo < 0 || AtRegNo > 31) {
-      reportParseError("unexpected token in statement");
+      reportParseError("unexpected token, expected dollar sign '$'");
       return false;
     }
+  }
+  Parser.Lex(); // Eat "$".
 
-    if (!AssemblerOptions.back()->setATReg(AtRegNo)) {
-      reportParseError("invalid register");
-      return false;
-    }
-    getParser().Lex(); // Eat the register.
+  // Find out what "reg" is.
+  unsigned AtRegNo;
+  const AsmToken &Reg = Parser.getTok();
+  if (Reg.is(AsmToken::Identifier)) {
+    AtRegNo = matchCPURegisterName(Reg.getIdentifier());
+  } else if (Reg.is(AsmToken::Integer)) {
+    AtRegNo = Reg.getIntVal();
+  } else {
+    reportParseError("unexpected token, expected identifier or integer");
+    return false;
+  }
 
-    if (getLexer().isNot(AsmToken::EndOfStatement)) {
-      reportParseError("unexpected token, expected end of statement");
-      return false;
-    }
-    Parser.Lex(); // Consume the EndOfStatement.
+  // Check if $reg is a valid register. If it is, set $at to $reg.
+  if (!AssemblerOptions.back()->setATReg(AtRegNo)) {
+    reportParseError("invalid register");
     return false;
-  } else {
-    reportParseError("unexpected token in statement");
+  }
+  Parser.Lex(); // Eat "reg".
+
+  // If this is not the end of the statement, report an error.
+  if (getLexer().isNot(AsmToken::EndOfStatement)) {
+    reportParseError("unexpected token, expected end of statement");
     return false;
   }
+
+  getTargetStreamer().emitDirectiveSetAtWithArg(AtRegNo);
+
+  Parser.Lex(); // Consume the EndOfStatement.
+  return false;
 }
 
 bool MipsAsmParser::parseSetReorderDirective() {
Index: lib/Target/Mips/MCTargetDesc/MipsTargetStreamer.cpp
===================================================================
--- lib/Target/Mips/MCTargetDesc/MipsTargetStreamer.cpp
+++ lib/Target/Mips/MCTargetDesc/MipsTargetStreamer.cpp
@@ -43,6 +43,9 @@
 void MipsTargetStreamer::emitDirectiveSetMsa() { forbidModuleDirective(); }
 void MipsTargetStreamer::emitDirectiveSetNoMsa() { forbidModuleDirective(); }
 void MipsTargetStreamer::emitDirectiveSetAt() { forbidModuleDirective(); }
+void MipsTargetStreamer::emitDirectiveSetAtWithArg(unsigned RegNo) {
+  forbidModuleDirective();
+}
 void MipsTargetStreamer::emitDirectiveSetNoAt() { forbidModuleDirective(); }
 void MipsTargetStreamer::emitDirectiveEnd(StringRef Name) {}
 void MipsTargetStreamer::emitDirectiveEnt(const MCSymbol &Symbol) {}
@@ -144,6 +147,11 @@
   MipsTargetStreamer::emitDirectiveSetAt();
 }
 
+void MipsTargetAsmStreamer::emitDirectiveSetAtWithArg(unsigned RegNo) {
+  OS << "\t.set\tat=$" << Twine(RegNo) << "\n";
+  MipsTargetStreamer::emitDirectiveSetAtWithArg(RegNo);
+}
+
 void MipsTargetAsmStreamer::emitDirectiveSetNoAt() {
   OS << "\t.set\tnoat\n";
   MipsTargetStreamer::emitDirectiveSetNoAt();
Index: lib/Target/Mips/MipsTargetStreamer.h
===================================================================
--- lib/Target/Mips/MipsTargetStreamer.h
+++ lib/Target/Mips/MipsTargetStreamer.h
@@ -34,6 +34,7 @@
   virtual void emitDirectiveSetMsa();
   virtual void emitDirectiveSetNoMsa();
   virtual void emitDirectiveSetAt();
+  virtual void emitDirectiveSetAtWithArg(unsigned RegNo);
   virtual void emitDirectiveSetNoAt();
   virtual void emitDirectiveEnd(StringRef Name);
 
@@ -138,6 +139,7 @@
   void emitDirectiveSetMsa() override;
   void emitDirectiveSetNoMsa() override;
   void emitDirectiveSetAt() override;
+  void emitDirectiveSetAtWithArg(unsigned RegNo) override;
   void emitDirectiveSetNoAt() override;
   void emitDirectiveEnd(StringRef Name) override;
 
Index: test/MC/Mips/mips-noat.s
===================================================================
--- test/MC/Mips/mips-noat.s
+++ test/MC/Mips/mips-noat.s
@@ -7,6 +7,8 @@
 # CHECK:  lui   $1, 1
 # CHECK:  addu  $1, $1, $2
 # CHECK:  lw    $2, 0($1)
+# CHECK-LABEL: test2:
+# CHECK:  .set noat
 test1:
         lw      $2, 65536($2)
 
@@ -20,6 +22,8 @@
 # CHECK:  lui   $1, 1
 # CHECK:  addu  $1, $1, $2
 # CHECK:  lw    $2, 0($1)
+# CHECK-LABEL: test4:
+# CHECK:  .set  at=$0
 test3:
         .set at
         lw      $2, 65536($2)
Index: test/MC/Mips/set-at-directive-explicit-at.s
===================================================================
--- test/MC/Mips/set-at-directive-explicit-at.s
+++ test/MC/Mips/set-at-directive-explicit-at.s
@@ -15,6 +15,16 @@
 # WARNINGS: :[[@LINE+2]]:11: warning: used $at without ".set noat"
     .set    at=$1
     jr    $1
+
+# CHECK:   jr    $1                      # encoding: [0x08,0x00,0x20,0x00]
+# WARNINGS: :[[@LINE+2]]:11: warning: used $at without ".set noat"
+    .set    at=$at
+    jr    $at
+
+# CHECK:   jr    $1                      # encoding: [0x08,0x00,0x20,0x00]
+# WARNINGS: :[[@LINE+2]]:11: warning: used $at without ".set noat"
+    .set    at=$at
+    jr    $1
 # WARNINGS-NOT: warning: used $at without ".set noat"
 
 # CHECK:   jr    $1                      # encoding: [0x08,0x00,0x20,0x00]
Index: test/MC/Mips/set-at-directive.s
===================================================================
--- test/MC/Mips/set-at-directive.s
+++ test/MC/Mips/set-at-directive.s
@@ -4,156 +4,187 @@
 # for ".set at" and set the correct value.
     .text
 foo:
+# CHECK: .set at=$1
 # CHECK: lui  $1, 1
 # CHECK: addu $1, $1, $2
 # CHECK: lw   $2, 0($1)
     .set    at=$1
         lw      $2, 65536($2)
+# CHECK: .set at=$2
 # CHECK: lui  $2, 1
 # CHECK: addu $2, $2, $1
 # CHECK: lw   $1, 0($2)
     .set    at=$2
         lw      $1, 65536($1)
+# CHECK: .set at=$3
 # CHECK: lui  $3, 1
 # CHECK: addu $3, $3, $1
 # CHECK: lw   $1, 0($3)
     .set    at=$3
         lw      $1, 65536($1)
+# CHECK: .set at=$4
 # CHECK: lui  $4, 1
 # CHECK: addu $4, $4, $1
 # CHECK: lw   $1, 0($4)
     .set    at=$a0
         lw      $1, 65536($1)
+# CHECK: .set at=$5
 # CHECK: lui  $5, 1
 # CHECK: addu $5, $5, $1
 # CHECK: lw   $1, 0($5)
     .set    at=$a1
         lw      $1, 65536($1)
+# CHECK: .set at=$6
 # CHECK: lui  $6, 1
 # CHECK: addu $6, $6, $1
 # CHECK: lw   $1, 0($6)
     .set    at=$a2
         lw      $1, 65536($1)
+# CHECK: .set at=$7
 # CHECK: lui  $7, 1
 # CHECK: addu $7, $7, $1
 # CHECK: lw   $1, 0($7)
     .set    at=$a3
         lw      $1, 65536($1)
+# CHECK: .set at=$8
 # CHECK: lui  $8, 1
 # CHECK: addu $8, $8, $1
 # CHECK: lw   $1, 0($8)
     .set    at=$8
         lw      $1, 65536($1)
+# CHECK: .set at=$9
 # CHECK: lui  $9, 1
 # CHECK: addu $9, $9, $1
 # CHECK: lw   $1, 0($9)
     .set    at=$9
         lw      $1, 65536($1)
+# CHECK: .set at=$10
 # CHECK: lui  $10, 1
 # CHECK: addu $10, $10, $1
 # CHECK: lw   $1, 0($10)
     .set    at=$10
         lw      $1, 65536($1)
+# CHECK: .set at=$11
 # CHECK: lui  $11, 1
 # CHECK: addu $11, $11, $1
 # CHECK: lw   $1, 0($11)
     .set    at=$11
         lw      $1, 65536($1)
+# CHECK: .set at=$12
 # CHECK: lui  $12, 1
 # CHECK: addu $12, $12, $1
 # CHECK: lw   $1, 0($12)
     .set    at=$12
         lw      $1, 65536($1)
+# CHECK: .set at=$13
 # CHECK: lui  $13, 1
 # CHECK: addu $13, $13, $1
 # CHECK: lw   $1, 0($13)
     .set    at=$13
         lw      $1, 65536($1)
+# CHECK: .set at=$14
 # CHECK: lui  $14, 1
 # CHECK: addu $14, $14, $1
 # CHECK: lw   $1, 0($14)
     .set    at=$14
         lw      $1, 65536($1)
+# CHECK: .set at=$15
 # CHECK: lui  $15, 1
 # CHECK: addu $15, $15, $1
 # CHECK: lw   $1, 0($15)
     .set    at=$15
         lw      $1, 65536($1)
+# CHECK: .set at=$16
 # CHECK: lui  $16, 1
 # CHECK: addu $16, $16, $1
 # CHECK: lw   $1, 0($16)
     .set    at=$s0
         lw      $1, 65536($1)
+# CHECK: .set at=$17
 # CHECK: lui  $17, 1
 # CHECK: addu $17, $17, $1
 # CHECK: lw   $1, 0($17)
     .set    at=$s1
         lw      $1, 65536($1)
+# CHECK: .set at=$18
 # CHECK: lui  $18, 1
 # CHECK: addu $18, $18, $1
 # CHECK: lw   $1, 0($18)
     .set    at=$s2
         lw      $1, 65536($1)
+# CHECK: .set at=$19
 # CHECK: lui  $19, 1
 # CHECK: addu $19, $19, $1
 # CHECK: lw   $1, 0($19)
     .set    at=$s3
         lw      $1, 65536($1)
+# CHECK: .set at=$20
 # CHECK: lui  $20, 1
 # CHECK: addu $20, $20, $1
 # CHECK: lw   $1, 0($20)
     .set    at=$s4
         lw      $1, 65536($1)
+# CHECK: .set at=$21
 # CHECK: lui  $21, 1
 # CHECK: addu $21, $21, $1
 # CHECK: lw   $1, 0($21)
     .set    at=$s5
         lw      $1, 65536($1)
+# CHECK: .set at=$22
 # CHECK: lui  $22, 1
 # CHECK: addu $22, $22, $1
 # CHECK: lw   $1, 0($22)
     .set    at=$s6
         lw      $1, 65536($1)
+# CHECK: .set at=$23
 # CHECK: lui  $23, 1
 # CHECK: addu $23, $23, $1
 # CHECK: lw   $1, 0($23)
     .set    at=$s7
         lw      $1, 65536($1)
+# CHECK: .set at=$24
 # CHECK: lui  $24, 1
 # CHECK: addu $24, $24, $1
 # CHECK: lw   $1, 0($24)
     .set    at=$24
         lw      $1, 65536($1)
+# CHECK: .set at=$25
 # CHECK: lui  $25, 1
 # CHECK: addu $25, $25, $1
 # CHECK: lw   $1, 0($25)
     .set    at=$25
         lw      $1, 65536($1)
+# CHECK: .set at=$26
 # CHECK: lui  $26, 1
 # CHECK: addu $26, $26, $1
 # CHECK: lw   $1, 0($26)
     .set    at=$26
         lw      $1, 65536($1)
+# CHECK: .set at=$27
 # CHECK: lui  $27, 1
 # CHECK: addu $27, $27, $1
 # CHECK: lw   $1, 0($27)
     .set    at=$27
         lw      $1, 65536($1)
+# CHECK: .set at=$28
 # CHECK: lui  $gp, 1
 # CHECK: addu $gp, $gp, $1
 # CHECK: lw   $1, 0($gp)
     .set    at=$gp
         lw      $1, 65536($1)
+# CHECK: .set at=$30
 # CHECK: lui  $fp, 1
 # CHECK: addu $fp, $fp, $1
 # CHECK: lw   $1, 0($fp)
     .set    at=$fp
         lw      $1, 65536($1)
+# CHECK: .set at=$29
 # CHECK: lui  $sp, 1
 # CHECK: addu $sp, $sp, $1
 # CHECK: lw   $1, 0($sp)
     .set    at=$sp
         lw      $1, 65536($1)
+# CHECK: .set at=$31
 # CHECK: lui  $ra, 1
 # CHECK: addu $ra, $ra, $1
 # CHECK: lw   $1, 0($ra)
Index: test/MC/Mips/set-at-noat-bad-syntax.s
===================================================================
--- /dev/null
+++ test/MC/Mips/set-at-noat-bad-syntax.s
@@ -0,0 +1,29 @@
+# RUN: not llvm-mc %s -triple=mips-unknown-unknown -mcpu=mips32 2>%t1
+# RUN: FileCheck %s < %t1
+
+.set at~
+# CHECK: error: unexpected token, expected equals sign
+
+.set at=
+# CHECK: error: no register specified
+
+.set at=~
+# CHECK: error: unexpected token, expected dollar sign '$'
+
+.set at=$
+# CHECK: error: unexpected token, expected identifier or integer
+
+.set at=$-4
+# CHECK: error: unexpected token, expected identifier or integer
+
+.set at=$1000
+# CHECK: error: invalid register
+
+.set at=$foo
+# CHECK: error: invalid register
+
+.set at=$2bar
+# CHECK: error: unexpected token, expected end of statement
+
+.set noat bar
+# CHECK: error: unexpected token, expected end of statement