[mips] Add comment which explains why we need to change the assembler options before and after inline asm blocks. NFC.

llvm-svn: 225521

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Target/Mips/MipsAsmPrinter.cpp b/llvm/lib/Target/Mips/MipsAsmPrinter.cpp
index 53b79eeacea..50c0441e0dd 100644
--- a/llvm/lib/Target/Mips/MipsAsmPrinter.cpp
+++ b/llvm/lib/Target/Mips/MipsAsmPrinter.cpp
@@ -747,6 +747,12 @@ void MipsAsmPrinter::emitInlineAsmStart(
     const MCSubtargetInfo &StartInfo) const {
   MipsTargetStreamer &TS = getTargetStreamer();
 
+  // GCC's choice of assembler options for inline assembly code ('at', 'macro'
+  // and 'reorder') is different from LLVM's choice for generated code ('noat',
+  // 'nomacro' and 'noreorder').
+  // In order to maintain compatibility with inline assembly code which depends
+  // on GCC's assembler options being used, we have to switch to those options
+  // for the duration of the inline assembly block and then switch back.
   TS.emitDirectiveSetPush();
   TS.emitDirectiveSetAt();
   TS.emitDirectiveSetMacro();