[RISCV] Move AVL coalescing logic upwards into computeInfoForInstr. NFC (#73909)

There is an optimisation in transferBefore where if a VSETVLIInfo uses
the AVL
of a defining vsetvli, it uses that vsetvli's AVL provided VLMAX is the
same.

This patch moves it out of transferBefore and up into
computeInfoForInstr to
show how it isn't affected by the other optimisations in transferBefore,
and to
simplify the control flow by removing an early return.

This should make #72352 easier to reason about.

Author: lukel97

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -781,6 +781,25 @@ char RISCVInsertVSETVLI::ID = 0;
 INITIALIZE_PASS(RISCVInsertVSETVLI, DEBUG_TYPE, RISCV_INSERT_VSETVLI_NAME,
                 false, false)
 
+// Return a VSETVLIInfo representing the changes made by this VSETVLI or
+// VSETIVLI instruction.
+static VSETVLIInfo getInfoForVSETVLI(const MachineInstr &MI) {
+  VSETVLIInfo NewInfo;
+  if (MI.getOpcode() == RISCV::PseudoVSETIVLI) {
+    NewInfo.setAVLImm(MI.getOperand(1).getImm());
+  } else {
+    assert(MI.getOpcode() == RISCV::PseudoVSETVLI ||
+           MI.getOpcode() == RISCV::PseudoVSETVLIX0);
+    Register AVLReg = MI.getOperand(1).getReg();
+    assert((AVLReg != RISCV::X0 || MI.getOperand(0).getReg() != RISCV::X0) &&
+           "Can't handle X0, X0 vsetvli yet");
+    NewInfo.setAVLReg(AVLReg);
+  }
+  NewInfo.setVTYPE(MI.getOperand(2).getImm());
+
+  return NewInfo;
+}
+
 static VSETVLIInfo computeInfoForInstr(const MachineInstr &MI, uint64_t TSFlags,
                                        const MachineRegisterInfo *MRI) {
   VSETVLIInfo InstrInfo;
@@ -841,6 +860,21 @@ static VSETVLIInfo computeInfoForInstr(const MachineInstr &MI, uint64_t TSFlags,
 #endif
   InstrInfo.setVTYPE(VLMul, SEW, TailAgnostic, MaskAgnostic);
 
+  // If AVL is defined by a vsetvli with the same VLMAX, we can replace the
+  // AVL operand with the AVL of the defining vsetvli.  We avoid general
+  // register AVLs to avoid extending live ranges without being sure we can
+  // kill the original source reg entirely.
+  if (InstrInfo.hasAVLReg() && InstrInfo.getAVLReg().isVirtual()) {
+    MachineInstr *DefMI = MRI->getVRegDef(InstrInfo.getAVLReg());
+    if (DefMI && isVectorConfigInstr(*DefMI)) {
+      VSETVLIInfo DefInstrInfo = getInfoForVSETVLI(*DefMI);
+      if (DefInstrInfo.hasSameVLMAX(InstrInfo) &&
+          (DefInstrInfo.hasAVLImm() || DefInstrInfo.getAVLReg() == RISCV::X0)) {
+        InstrInfo.setAVL(DefInstrInfo);
+      }
+    }
+  }
+
   return InstrInfo;
 }
 
@@ -851,25 +885,6 @@ void RISCVInsertVSETVLI::insertVSETVLI(MachineBasicBlock &MBB, MachineInstr &MI,
   insertVSETVLI(MBB, MachineBasicBlock::iterator(&MI), DL, Info, PrevInfo);
 }
 
-// Return a VSETVLIInfo representing the changes made by this VSETVLI or
-// VSETIVLI instruction.
-static VSETVLIInfo getInfoForVSETVLI(const MachineInstr &MI) {
-  VSETVLIInfo NewInfo;
-  if (MI.getOpcode() == RISCV::PseudoVSETIVLI) {
-    NewInfo.setAVLImm(MI.getOperand(1).getImm());
-  } else {
-    assert(MI.getOpcode() == RISCV::PseudoVSETVLI ||
-           MI.getOpcode() == RISCV::PseudoVSETVLIX0);
-    Register AVLReg = MI.getOperand(1).getReg();
-    assert((AVLReg != RISCV::X0 || MI.getOperand(0).getReg() != RISCV::X0) &&
-           "Can't handle X0, X0 vsetvli yet");
-    NewInfo.setAVLReg(AVLReg);
-  }
-  NewInfo.setVTYPE(MI.getOperand(2).getImm());
-
-  return NewInfo;
-}
-
 void RISCVInsertVSETVLI::insertVSETVLI(MachineBasicBlock &MBB,
                      MachineBasicBlock::iterator InsertPt, DebugLoc DL,
                      const VSETVLIInfo &Info, const VSETVLIInfo &PrevInfo) {
@@ -1065,27 +1080,8 @@ void RISCVInsertVSETVLI::transferBefore(VSETVLIInfo &Info,
   // to prevent extending live range of an avl register operand.
   // TODO: We can probably relax this for immediates.
   if (Demanded.VLZeroness && !Demanded.VLAny && PrevInfo.isValid() &&
-      PrevInfo.hasEquallyZeroAVL(Info, *MRI) && Info.hasSameVLMAX(PrevInfo)) {
+      PrevInfo.hasEquallyZeroAVL(Info, *MRI) && Info.hasSameVLMAX(PrevInfo))
     Info.setAVL(PrevInfo);
-    return;
-  }
-
-  // If AVL is defined by a vsetvli with the same VLMAX, we can
-  // replace the AVL operand with the AVL of the defining vsetvli.
-  // We avoid general register AVLs to avoid extending live ranges
-  // without being sure we can kill the original source reg entirely.
-  if (!Info.hasAVLReg() || !Info.getAVLReg().isVirtual())
-    return;
-  MachineInstr *DefMI = MRI->getVRegDef(Info.getAVLReg());
-  if (!DefMI || !isVectorConfigInstr(*DefMI))
-    return;
-
-  VSETVLIInfo DefInfo = getInfoForVSETVLI(*DefMI);
-  if (DefInfo.hasSameVLMAX(Info) &&
-      (DefInfo.hasAVLImm() || DefInfo.getAVLReg() == RISCV::X0)) {
-    Info.setAVL(DefInfo);
-    return;
-  }
 }
 
 // Given a state with which we evaluated MI (see transferBefore above for why