[RISCV] Handle undef AVLs in RISCVInsertVSETVLI

Before #91440 a VSETVLIInfo would have had an IMPLICIT_DEF defining
instruction, but now we look up a VNInfo which doesn't exist, which
triggers an assertion failure. Mark these undef AVLs as AVLIsIgnored.

Author: lukel97

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -48,15 +48,13 @@ static cl::opt<bool> DisableInsertVSETVLPHIOpt(
 namespace {
 
 /// Given a virtual register \p Reg, return the corresponding VNInfo for it.
-/// This should never return nullptr.
+/// This will return nullptr if the virtual register is an implicit_def.
 static VNInfo *getVNInfoFromReg(Register Reg, const MachineInstr &MI,
                                 const LiveIntervals *LIS) {
   assert(Reg.isVirtual());
   auto &LI = LIS->getInterval(Reg);
   SlotIndex SI = LIS->getSlotIndexes()->getInstructionIndex(MI);
-  VNInfo *VNI = LI.getVNInfoBefore(SI);
-  assert(VNI);
-  return VNI;
+  return LI.getVNInfoBefore(SI);
 }
 
 static unsigned getVLOpNum(const MachineInstr &MI) {
@@ -894,8 +892,12 @@ static VSETVLIInfo getInfoForVSETVLI(const MachineInstr &MI,
            "Can't handle X0, X0 vsetvli yet");
     if (AVLReg == RISCV::X0)
       NewInfo.setAVLVLMAX();
-    else
-      NewInfo.setAVLRegDef(getVNInfoFromReg(AVLReg, MI, LIS), AVLReg);
+    else if (VNInfo *VNI = getVNInfoFromReg(AVLReg, MI, LIS))
+      NewInfo.setAVLRegDef(VNI, AVLReg);
+    else {
+      assert(MI.getOperand(1).isUndef());
+      NewInfo.setAVLIgnored();
+    }
   }
   NewInfo.setVTYPE(MI.getOperand(2).getImm());
 
@@ -966,9 +968,11 @@ static VSETVLIInfo computeInfoForInstr(const MachineInstr &MI, uint64_t TSFlags,
       }
       else
         InstrInfo.setAVLImm(Imm);
+    } else if (VNInfo *VNI = getVNInfoFromReg(VLOp.getReg(), MI, LIS)) {
+      InstrInfo.setAVLRegDef(VNI, VLOp.getReg());
     } else {
-      InstrInfo.setAVLRegDef(getVNInfoFromReg(VLOp.getReg(), MI, LIS),
-                             VLOp.getReg());
+      assert(VLOp.isUndef());
+      InstrInfo.setAVLIgnored();
     }
   } else {
     assert(isScalarExtractInstr(MI));

llvm/test/CodeGen/RISCV/rvv/vsetvli-insert.ll

@@ -699,3 +699,27 @@ declare <vscale x 2 x i1> @llvm.riscv.vmsgt.nxv2i32.i32.i64(<vscale x 2 x i32>,
 declare <vscale x 2 x i1> @llvm.riscv.vmor.nxv2i1.i64(<vscale x 2 x i1>, <vscale x 2 x i1>, i64)
 declare void @llvm.riscv.vse.mask.nxv2i32.i64(<vscale x 2 x i32>, ptr nocapture, <vscale x 2 x i1>, i64)
 declare void @llvm.riscv.vse.nxv2i32.i64(<vscale x 2 x i32>, ptr nocapture, i64)
+
+define <vscale x 2 x i32> @avl_undef1(<vscale x 2 x i32>, <vscale x 2 x i32>, <vscale x 2 x i32>) {
+; CHECK-LABEL: avl_undef1:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetivli zero, 1, e32, m1, tu, ma
+; CHECK-NEXT:    vadd.vv v8, v9, v10
+; CHECK-NEXT:    ret
+  %a = call <vscale x 2 x i32> @llvm.riscv.vadd.nxv2i32.nxv2i32(
+    <vscale x 2 x i32> %0,
+    <vscale x 2 x i32> %1,
+    <vscale x 2 x i32> %2,
+    i64 undef
+  )
+  ret <vscale x 2 x i32> %a
+}
+
+define i64 @avl_undef2() {
+; CHECK-LABEL: avl_undef2:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetvli a0, a0, e32, mf2, ta, ma
+; CHECK-NEXT:    ret
+  %1 = tail call i64 @llvm.riscv.vsetvli(i64 poison, i64 2, i64 7)
+  ret i64 %1
+}