PowerPC: Simplify FADD in round-to-zero mode.

As part of the the sequence generated to implement long double -> int
conversions, we need to perform an FADD in round-to-zero mode.  This is
problematical since the FPSCR is not at all modeled at the SelectionDAG
level, and thus there is a risk of getting floating point instructions
generated out of sequence with the instructions to modify FPSCR.

The current code handles this by somewhat "special" patterns that in part
have dummy operands, and/or duplicate existing instructions, making them
awkward to handle in the asm parser.

This commit changes this by leaving the "FADD in round-to-zero mode"
as an atomic operation on the SelectionDAG level, and only split it up into
real instructions at the MI level (via custom inserter).  Since at *this*
level the FPSCR *is* modeled (via the "RM" hard register), much of the
"special" stuff can just go away, and the resulting patterns can be used by
the asm parser.

No significant change in generated code expected.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178006 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 26 insertions, 47 deletions

diff --git a/lib/Target/PowerPC/PPCISelLowering.cpp b/lib/Target/PowerPC/PPCISelLowering.cpp
index 6631e4ab8f..5ff23d1cef 100644
--- a/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -584,10 +584,7 @@ const char *PPCTargetLowering::getTargetNodeName(unsigned Opcode) const {
   case PPCISD::STCX:            return "PPCISD::STCX";
   case PPCISD::COND_BRANCH:     return "PPCISD::COND_BRANCH";
   case PPCISD::MFFS:            return "PPCISD::MFFS";
-  case PPCISD::MTFSB0:          return "PPCISD::MTFSB0";
-  case PPCISD::MTFSB1:          return "PPCISD::MTFSB1";
   case PPCISD::FADDRTZ:         return "PPCISD::FADDRTZ";
-  case PPCISD::MTFSF:           return "PPCISD::MTFSF";
   case PPCISD::TC_RETURN:       return "PPCISD::TC_RETURN";
   case PPCISD::CR6SET:          return "PPCISD::CR6SET";
   case PPCISD::CR6UNSET:        return "PPCISD::CR6UNSET";
@@ -5667,50 +5664,8 @@ void PPCTargetLowering::ReplaceNodeResults(SDNode *N,
                              MVT::f64, N->getOperand(0),
                              DAG.getIntPtrConstant(1));
 
-    // This sequence changes FPSCR to do round-to-zero, adds the two halves
-    // of the long double, and puts FPSCR back the way it was.  We do not
-    // actually model FPSCR.
-    std::vector<EVT> NodeTys;
-    SDValue Ops[4], Result, MFFSreg, InFlag, FPreg;
-
-    NodeTys.push_back(MVT::f64);   // Return register
-    NodeTys.push_back(MVT::Glue);    // Returns a flag for later insns
-    Result = DAG.getNode(PPCISD::MFFS, dl, NodeTys, &InFlag, 0);
-    MFFSreg = Result.getValue(0);
-    InFlag = Result.getValue(1);
-
-    NodeTys.clear();
-    NodeTys.push_back(MVT::Glue);   // Returns a flag
-    Ops[0] = DAG.getConstant(31, MVT::i32);
-    Ops[1] = InFlag;
-    Result = DAG.getNode(PPCISD::MTFSB1, dl, NodeTys, Ops, 2);
-    InFlag = Result.getValue(0);
-
-    NodeTys.clear();
-    NodeTys.push_back(MVT::Glue);   // Returns a flag
-    Ops[0] = DAG.getConstant(30, MVT::i32);
-    Ops[1] = InFlag;
-    Result = DAG.getNode(PPCISD::MTFSB0, dl, NodeTys, Ops, 2);
-    InFlag = Result.getValue(0);
-
-    NodeTys.clear();
-    NodeTys.push_back(MVT::f64);    // result of add
-    NodeTys.push_back(MVT::Glue);   // Returns a flag
-    Ops[0] = Lo;
-    Ops[1] = Hi;
-    Ops[2] = InFlag;
-    Result = DAG.getNode(PPCISD::FADDRTZ, dl, NodeTys, Ops, 3);
-    FPreg = Result.getValue(0);
-    InFlag = Result.getValue(1);
-
-    NodeTys.clear();
-    NodeTys.push_back(MVT::f64);
-    Ops[0] = DAG.getConstant(1, MVT::i32);
-    Ops[1] = MFFSreg;
-    Ops[2] = FPreg;
-    Ops[3] = InFlag;
-    Result = DAG.getNode(PPCISD::MTFSF, dl, NodeTys, Ops, 4);
-    FPreg = Result.getValue(0);
+    // Add the two halves of the long double in round-to-zero mode.
+    SDValue FPreg = DAG.getNode(PPCISD::FADDRTZ, dl, MVT::f64, Lo, Hi);
 
     // We know the low half is about to be thrown away, so just use something
     // convenient.
@@ -6523,6 +6478,30 @@ PPCTargetLowering::EmitInstrWithCustomInserter(MachineInstr *MI,
     BB = exitMBB;
     BuildMI(*BB, BB->begin(), dl, TII->get(PPC::SRW),dest).addReg(TmpReg)
       .addReg(ShiftReg);
+  } else if (MI->getOpcode() == PPC::FADDrtz) {
+    // This pseudo performs an FADD with rounding mode temporarily forced
+    // to round-to-zero.  We emit this via custom inserter since the FPSCR
+    // is not modeled at the SelectionDAG level.
+    unsigned Dest = MI->getOperand(0).getReg();
+    unsigned Src1 = MI->getOperand(1).getReg();
+    unsigned Src2 = MI->getOperand(2).getReg();
+    DebugLoc dl   = MI->getDebugLoc();
+
+    MachineRegisterInfo &RegInfo = F->getRegInfo();
+    unsigned MFFSReg = RegInfo.createVirtualRegister(&PPC::F8RCRegClass);
+
+    // Save FPSCR value.
+    BuildMI(*BB, MI, dl, TII->get(PPC::MFFS), MFFSReg);
+
+    // Set rounding mode to round-to-zero.
+    BuildMI(*BB, MI, dl, TII->get(PPC::MTFSB1)).addImm(31);
+    BuildMI(*BB, MI, dl, TII->get(PPC::MTFSB0)).addImm(30);
+
+    // Perform addition.
+    BuildMI(*BB, MI, dl, TII->get(PPC::FADD), Dest).addReg(Src1).addReg(Src2);
+
+    // Restore FPSCR value.
+    BuildMI(*BB, MI, dl, TII->get(PPC::MTFSF)).addImm(1).addReg(MFFSReg);
   } else {
     llvm_unreachable("Unexpected instr type to insert");
   }