Index: llvm/docs/GlobalISel/GenericOpcode.rst
===================================================================
--- llvm/docs/GlobalISel/GenericOpcode.rst
+++ llvm/docs/GlobalISel/GenericOpcode.rst
@@ -254,6 +254,11 @@
 
 G_SBFX sign-extends the result, while G_UBFX zero-extends the result.
 
+If the least-significant bit is larger than the number of bits in the
+source, the result is undefined. If the width is zero or if
+width minus the least-significant bit is larger than the number of bits
+in the source, the result is undefined.
+
 .. code-block:: none
 
   ; Extract 5 bits starting at bit 1 from %x and store them in %a.
Index: llvm/include/llvm/Target/GenericOpcodes.td
===================================================================
--- llvm/include/llvm/Target/GenericOpcodes.td
+++ llvm/include/llvm/Target/GenericOpcodes.td
@@ -1379,14 +1379,18 @@
 // Bitfield extraction.
 //------------------------------------------------------------------------------
 
-// Generic signed bitfield extraction.
+// Generic signed bitfield extraction. If lsb is larger than the number of
+// bits in src, the result is undefined. If width is 0 or width - lsb is
+// larger than the number of bits in src, the result is undefined.
 def G_SBFX : GenericInstruction {
   let OutOperandList = (outs type0:$dst);
   let InOperandList = (ins type0:$src, type1:$lsb, type1:$width);
   let hasSideEffects = false;
 }
 
-// Generic unsigned bitfield extraction.
+// Generic unsigned bitfield extraction. If lsb is larger than the number of
+// bits in src, the result is undefined. If width is 0 or width - lsb is
+// larger than the number of bits in src, the result is undefined.
 def G_UBFX : GenericInstruction {
   let OutOperandList = (outs type0:$dst);
   let InOperandList = (ins type0:$src, type1:$lsb, type1:$width);