Index: lib/Target/Mips/AsmParser/MipsAsmParser.cpp
===================================================================
--- lib/Target/Mips/AsmParser/MipsAsmParser.cpp
+++ lib/Target/Mips/AsmParser/MipsAsmParser.cpp
@@ -5106,7 +5106,7 @@
 
   CC = StringSwitch<unsigned>(Name)
            .Case("zero", 0)
-           .Case("at", 1)
+           .Cases("at", "AT", 1)
            .Case("a0", 4)
            .Case("a1", 5)
            .Case("a2", 6)
Index: test/MC/Mips/mips-register-names-o32.s
===================================================================
--- test/MC/Mips/mips-register-names-o32.s
+++ test/MC/Mips/mips-register-names-o32.s
@@ -7,6 +7,7 @@
 .set noat
 addiu	$zero, $zero, 0 # CHECK: encoding: [0x24,0x00,0x00,0x00]
 addiu	$at, $zero, 0   # CHECK: encoding: [0x24,0x01,0x00,0x00]
+addiu	$AT, $zero, 0   # CHECK: encoding: [0x24,0x01,0x00,0x00]
 addiu	$v0, $zero, 0   # CHECK: encoding: [0x24,0x02,0x00,0x00]
 addiu	$v1, $zero, 0   # CHECK: encoding: [0x24,0x03,0x00,0x00]
 addiu	$a0, $zero, 0   # CHECK: encoding: [0x24,0x04,0x00,0x00]
Index: test/MC/Mips/mips64-register-names-o32.s
===================================================================
--- test/MC/Mips/mips64-register-names-o32.s
+++ test/MC/Mips/mips64-register-names-o32.s
@@ -8,6 +8,7 @@
 .set noat
 addiu	$zero, $zero, 0 # CHECK: encoding: [0x24,0x00,0x00,0x00]
 addiu	$at, $zero, 0   # CHECK: encoding: [0x24,0x01,0x00,0x00]
+addiu	$AT, $zero, 0   # CHECK: encoding: [0x24,0x01,0x00,0x00]
 addiu	$v0, $zero, 0   # CHECK: encoding: [0x24,0x02,0x00,0x00]
 addiu	$v1, $zero, 0   # CHECK: encoding: [0x24,0x03,0x00,0x00]
 addiu	$a0, $zero, 0   # CHECK: encoding: [0x24,0x04,0x00,0x00]