Add a pseudo instruction REG_SEQUENCE that takes a list of registers and

sub-register indices and outputs a single super register which is formed from
a consecutive sequence of registers.

This is used as register allocation / coalescing aid and it is useful to
represent instructions that output register pairs / quads. For example,
v1024, v1025 = vload <address>
where v1024 and v1025 forms a register pair.

This really should be modelled as
v1024<3>, v1025<4> = vload <address>
but it would violate SSA property before register allocation is done.

Currently we use insert_subreg to form the super register:
v1026 = implicit_def
v1027 - insert_subreg v1026, v1024, 3
v1028 = insert_subreg v1027, v1025, 4
...
      = use v1024
      = use v1028

But this adds pseudo live interval overlap between v1024 and v1025.

We can now modeled it as
v1024, v1025 = vload <address>
v1026 = REG_SEQUENCE v1024, 3, v1025, 4
...
      = use v1024
      = use v1026

After coalescing, it will be
v1026<3>, v1025<4> = vload <address>
...
      = use v1026<3>
      = use v1026


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102815 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/Target.td

@@ -473,6 +473,15 @@ def DBG_VALUE : Instruction {
   let Namespace = "TargetOpcode";
   let isAsCheapAsAMove = 1;
 }
+
+def REG_SEQUENCE : Instruction {
+  let OutOperandList = (outs unknown:$dst);
+  let InOperandList = (ins variable_ops);
+  let AsmString = "";
+  let Namespace = "TargetOpcode";
+  let neverHasSideEffects = 1;
+  let isAsCheapAsAMove = 1;
+}
 }
 
 //===----------------------------------------------------------------------===//

include/llvm/Target/TargetOpcodes.h

@@ -62,9 +62,17 @@ namespace TargetOpcode {
     /// instructions are insufficient. The actual MachineInstrs to perform
     /// the copy are emitted with the TargetInstrInfo::copyRegToReg hook.
     COPY_TO_REGCLASS = 10,
-    
+
     /// DBG_VALUE - a mapping of the llvm.dbg.value intrinsic
-    DBG_VALUE = 11
+    DBG_VALUE = 11,
+
+    /// REG_SEQUENCE - This variadic instruction is used to form a register that
+    /// represent a consecutive sequence of sub-registers. It's used as register
+    /// coalescing / allocation aid and must be eliminated before code emission.
+    /// e.g. v1027 = REG_SEQUENCE v1024, 3, v1025, 4, v1026, 5
+    /// After register coalescing references of v1024 should be replace with
+    /// v1027:3, v1025 with v1027:4, etc.
+    REG_SEQUENCE = 12
   };
 } // end namespace TargetOpcode
 } // end namespace llvm

utils/TableGen/CodeEmitterGen.cpp

@@ -35,7 +35,8 @@ void CodeEmitterGen::reverseBits(std::vector<Record*> &Insts) {
         R->getName() == "IMPLICIT_DEF" ||
         R->getName() == "SUBREG_TO_REG" ||
         R->getName() == "COPY_TO_REGCLASS" ||
-        R->getName() == "DBG_VALUE") continue;
+        R->getName() == "DBG_VALUE" ||
+        R->getName() == "REG_SEQUENCE") continue;
 
     BitsInit *BI = R->getValueAsBitsInit("Inst");
 
@@ -113,7 +114,8 @@ void CodeEmitterGen::run(raw_ostream &o) {
         R->getName() == "IMPLICIT_DEF" ||
         R->getName() == "SUBREG_TO_REG" ||
         R->getName() == "COPY_TO_REGCLASS" ||
-        R->getName() == "DBG_VALUE") {
+        R->getName() == "DBG_VALUE" ||
+        R->getName() == "REG_SEQUENCE") {
       o << "    0U,\n";
       continue;
     }
@@ -152,7 +154,8 @@ void CodeEmitterGen::run(raw_ostream &o) {
         InstName == "IMPLICIT_DEF" ||
         InstName == "SUBREG_TO_REG" ||
         InstName == "COPY_TO_REGCLASS" ||
-        InstName == "DBG_VALUE") continue;
+        InstName == "DBG_VALUE" ||
+        InstName == "REG_SEQUENCE") continue;
 
     BitsInit *BI = R->getValueAsBitsInit("Inst");
     const std::vector<RecordVal> &Vals = R->getValues();

utils/TableGen/CodeGenTarget.cpp

@@ -326,6 +326,7 @@ void CodeGenTarget::ComputeInstrsByEnum() const {
   const CodeGenInstruction *COPY_TO_REGCLASS =
     GetInstByName("COPY_TO_REGCLASS", Insts);
   const CodeGenInstruction *DBG_VALUE = GetInstByName("DBG_VALUE", Insts);
+  const CodeGenInstruction *REG_SEQUENCE = GetInstByName("REG_SEQUENCE", Insts);
 
   // Print out the rest of the instructions now.
   InstrsByEnum.push_back(PHI);
@@ -340,6 +341,7 @@ void CodeGenTarget::ComputeInstrsByEnum() const {
   InstrsByEnum.push_back(SUBREG_TO_REG);
   InstrsByEnum.push_back(COPY_TO_REGCLASS);
   InstrsByEnum.push_back(DBG_VALUE);
+  InstrsByEnum.push_back(REG_SEQUENCE);
   
   unsigned EndOfPredefines = InstrsByEnum.size();
   
@@ -357,7 +359,8 @@ void CodeGenTarget::ComputeInstrsByEnum() const {
         CGI != IMPLICIT_DEF &&
         CGI != SUBREG_TO_REG &&
         CGI != COPY_TO_REGCLASS &&
-        CGI != DBG_VALUE)
+        CGI != DBG_VALUE &&
+        CGI != REG_SEQUENCE)
       InstrsByEnum.push_back(CGI);
   }