[RISCV] Use x0 in vsetvli when avl is equal to vlmax.

We could use x0 form in vsetvli when we already know the vlmax and avl is equal to it.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D156404

Author: jacquesguan

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -552,6 +552,12 @@ void RISCVDAGToDAGISel::selectVSETVLI(SDNode *Node) {
 
   SDValue VLOperand;
   unsigned Opcode = RISCV::PseudoVSETVLI;
+  if (auto *C = dyn_cast<ConstantSDNode>(Node->getOperand(1))) {
+    const unsigned VLEN = Subtarget->getRealMinVLen();
+    if (VLEN == Subtarget->getRealMaxVLen())
+      if (VLEN / RISCVVType::getSEWLMULRatio(SEW, VLMul) == C->getZExtValue())
+        VLMax = true;
+  }
   if (VLMax || isAllOnesConstant(Node->getOperand(1))) {
     VLOperand = CurDAG->getRegister(RISCV::X0, XLenVT);
     Opcode = RISCV::PseudoVSETVLIX0;

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

@@ -1,8 +1,14 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: sed 's/iXLen/i32/g' %s | llc -mtriple=riscv32 -mattr=+v \
-; RUN:   -verify-machineinstrs | FileCheck %s
+; RUN:   -verify-machineinstrs | FileCheck %s --check-prefixes=CHECK,VLENUNKNOWN
 ; RUN: sed 's/iXLen/i64/g' %s | llc -mtriple=riscv64 -mattr=+v \
-; RUN:   -verify-machineinstrs | FileCheck %s
+; RUN:   -verify-machineinstrs | FileCheck %s --check-prefixes=CHECK,VLENUNKNOWN
+; RUN: sed 's/iXLen/i32/g' %s | llc -mtriple=riscv32 -mattr=+v \
+; RUN:   -riscv-v-vector-bits-max=128 -verify-machineinstrs \
+; RUN:   | FileCheck %s --check-prefixes=CHECK,VLEN128
+; RUN: sed 's/iXLen/i64/g' %s | llc -mtriple=riscv64 -mattr=+v \
+; RUN:   -riscv-v-vector-bits-max=128 -verify-machineinstrs \
+; RUN:   | FileCheck %s --check-prefixes=CHECK,VLEN128
 
 declare iXLen @llvm.riscv.vsetvli.iXLen(iXLen, iXLen, iXLen)
 declare iXLen @llvm.riscv.vsetvlimax.iXLen(iXLen, iXLen)
@@ -135,3 +141,18 @@ define iXLen @test_vsetvli_negone_e8m1(iXLen %avl) nounwind {
   %vl = call iXLen @llvm.riscv.vsetvli.iXLen(iXLen -1, iXLen 0, iXLen 0)
   ret iXLen %vl
 }
+
+define iXLen @test_vsetvli_eqvlmax_e8m8(iXLen %avl) nounwind {
+; VLENUNKNOWN-LABEL: test_vsetvli_eqvlmax_e8m8:
+; VLENUNKNOWN:       # %bb.0:
+; VLENUNKNOWN-NEXT:    li a0, 128
+; VLENUNKNOWN-NEXT:    vsetvli a0, a0, e8, m8, ta, ma
+; VLENUNKNOWN-NEXT:    ret
+;
+; VLEN128-LABEL: test_vsetvli_eqvlmax_e8m8:
+; VLEN128:       # %bb.0:
+; VLEN128-NEXT:    vsetvli a0, zero, e8, m8, ta, ma
+; VLEN128-NEXT:    ret
+  %vl = call iXLen @llvm.riscv.vsetvli.iXLen(iXLen 128, iXLen 0, iXLen 3)
+  ret iXLen %vl
+}