[TableGen] Improve decoding options for non-orthogonal instructions

[oota-llvm.git] / include / llvm / Target / Target.td

diff --git a/include/llvm/Target/Target.td b/include/llvm/Target/Target.td
index e0aea181a639cbac21c5073b12979f889c6163ad..01c6e343208736b643f458e133a0d8493746d87b 100644 (file)
--- a/include/llvm/Target/Target.td
+++ b/include/llvm/Target/Target.td
@@ -441,6 +441,30 @@ class Instruction {
   string PostEncoderMethod = "";
   string DecoderMethod = "";
 
+  // Is the instruction decoder method able to completely determine if the
+  // given instruction is valid or not. If the TableGen definition of the
+  // instruction specifies bitpattern A??B where A and B are static bits, the
+  // hasCompleteDecoder flag says whether the decoder method fully handles the
+  // ?? space, i.e. if it is a final arbiter for the instruction validity.
+  // If not then the decoder attempts to continue decoding when the decoder
+  // method fails.
+  //
+  // This allows to handle situations where the encoding is not fully
+  // orthogonal. Example:
+  // * InstA with bitpattern 0b0000????,
+  // * InstB with bitpattern 0b000000?? but the associated decoder method
+  //   DecodeInstB() returns Fail when ?? is 0b00 or 0b11.
+  //
+  // The decoder tries to decode a bitpattern that matches both InstA and
+  // InstB bitpatterns first as InstB (because it is the most specific
+  // encoding). In the default case (hasCompleteDecoder = 1), when
+  // DecodeInstB() returns Fail the bitpattern gets rejected. By setting
+  // hasCompleteDecoder = 0 in InstB, the decoder is informed that
+  // DecodeInstB() is not able to determine if all possible values of ?? are
+  // valid or not. If DecodeInstB() returns Fail the decoder will attempt to
+  // decode the bitpattern as InstA too.
+  bit hasCompleteDecoder = 1;
+
   /// Target-specific flags. This becomes the TSFlags field in TargetInstrDesc.
   bits<64> TSFlags = 0;
 
@@ -595,6 +619,7 @@ class Operand<ValueType ty> : DAGOperand {
   string PrintMethod = "printOperand";
   string EncoderMethod = "";
   string DecoderMethod = "";
+  bit hasCompleteDecoder = 1;
   string OperandType = "OPERAND_UNKNOWN";
   dag MIOperandInfo = (ops);