[tablegen][globalisel] Add support for nested instruction matching.

Summary:
Lift the restrictions that prevented the tree walking introduced in the
previous change and add support for patterns like:
  (G_ADD (G_MUL (G_SEXT $src1), (G_SEXT $src2)), $src3) -> SMADDWrrr $dst, $src1, $src2, $src3
Also adds support for G_SEXT and G_ZEXT to support these cases.

One particular aspect of this that I should draw attention to is that I've
tried to be overly conservative in determining the safety of matches that
involve non-adjacent instructions and multiple basic blocks. This is intended
to be used as a cheap initial check and we may add a more expensive check in
the future. The current rules are:
* Reject if any instruction may load/store (we'd need to check for intervening
  memory operations.
* Reject if any instruction has implicit operands.
* Reject if any instruction has unmodelled side-effects.
See isObviouslySafeToFold().

Reviewers: t.p.northover, javed.absar, qcolombet, aditya_nandakumar, ab, rovka

Reviewed By: ab

Subscribers: igorb, dberris, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D30539

llvm-svn: 299430

2 files changed, 5 insertions, 36 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/InstructionSelector.cpp b/llvm/lib/CodeGen/GlobalISel/InstructionSelector.cpp
index dd748613346..fb9d01ef854 100644
--- a/llvm/lib/CodeGen/GlobalISel/InstructionSelector.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/InstructionSelector.cpp
@@ -94,3 +94,8 @@ bool InstructionSelector::isOperandImmEqual(
       return *VRegVal == Value;
   return false;
 }
+
+bool InstructionSelector::isObviouslySafeToFold(MachineInstr &MI) const {
+  return !MI.mayLoadOrStore() && !MI.hasUnmodeledSideEffects() &&
+         MI.implicit_operands().begin() == MI.implicit_operands().end();
+}
diff --git a/llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp b/llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
index f238031fdec..875e8b175b9 100644
--- a/llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
@@ -840,42 +840,6 @@ bool AArch64InstructionSelector::select(MachineInstr &I) const {
     // operands to use appropriate classes.
     return constrainSelectedInstRegOperands(I, TII, TRI, RBI);
   }
-  case TargetOpcode::G_MUL: {
-    // Reject the various things we don't support yet.
-    if (unsupportedBinOp(I, RBI, MRI, TRI))
-      return false;
-
-    const unsigned DefReg = I.getOperand(0).getReg();
-    const RegisterBank &RB = *RBI.getRegBank(DefReg, MRI, TRI);
-
-    if (RB.getID() != AArch64::GPRRegBankID) {
-      DEBUG(dbgs() << "G_MUL on bank: " << RB << ", expected: GPR\n");
-      return false;
-    }
-
-    unsigned ZeroReg;
-    unsigned NewOpc;
-    if (Ty.isScalar() && Ty.getSizeInBits() <= 32) {
-      NewOpc = AArch64::MADDWrrr;
-      ZeroReg = AArch64::WZR;
-    } else if (Ty == LLT::scalar(64)) {
-      NewOpc = AArch64::MADDXrrr;
-      ZeroReg = AArch64::XZR;
-    } else {
-      DEBUG(dbgs() << "G_MUL has type: " << Ty << ", expected: "
-                   << LLT::scalar(32) << " or " << LLT::scalar(64) << '\n');
-      return false;
-    }
-
-    I.setDesc(TII.get(NewOpc));
-
-    I.addOperand(MachineOperand::CreateReg(ZeroReg, /*isDef=*/false));
-
-    // Now that we selected an opcode, we need to constrain the register
-    // operands to use appropriate classes.
-    return constrainSelectedInstRegOperands(I, TII, TRI, RBI);
-  }
-
   case TargetOpcode::G_FADD:
   case TargetOpcode::G_FSUB:
   case TargetOpcode::G_FMUL: