[mips] Add error message when trying to use $at in '.set noat' mode.

Summary:
Patch by David Chisnall
His work was sponsored by: DARPA, AFRL

Differential Revision: http://llvm-reviews.chandlerc.com/D3158

llvm-svn: 204621

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp b/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
index 8c0c181feca..f54a62b666c 100644
--- a/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
+++ b/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
@@ -1174,7 +1174,12 @@ bool MipsAssemblerOptions::setATReg(unsigned Reg) {
   return true;
 }
 
-int MipsAsmParser::getATReg() { return Options.getATRegNum(); }
+int MipsAsmParser::getATReg() {
+  int AT = Options.getATRegNum();
+  if (AT == 0)
+    TokError("Pseudo instruction requires $at, which is not available");
+  return AT;
+}
 
 unsigned MipsAsmParser::getReg(int RC, int RegNo) {
   return *(getContext().getRegisterInfo()->getRegClass(RC).begin() + RegNo);