[RISCV] Handle LHS == 0 in isVLKnownLE (llvm#148860)

If a VL is zero then it's known to be less than or equal to every other
VL.

This looks weird on its own since a VL of zero isn't that common. The
test diffs come from a type being split resulting in a VP intrinsic's
EVL being zero.

The motivation for this is to split off part of an upcoming patch I plan
on submitting for RISCVVLOptimizer, which generalizes it to handle
recurrences, and needs to reason about an initial state of demanded VLs
set to zero.

Author: lukel97

llvm/lib/Target/RISCV/RISCVInstrInfo.cpp

@@ -4812,6 +4812,8 @@ bool RISCV::isVLKnownLE(const MachineOperand &LHS, const MachineOperand &RHS) {
     return true;
   if (RHS.isImm() && RHS.getImm() == RISCV::VLMaxSentinel)
     return true;
+  if (LHS.isImm() && LHS.getImm() == 0)
+    return true;
   if (LHS.isImm() && LHS.getImm() == RISCV::VLMaxSentinel)
     return false;
   if (!LHS.isImm() || !RHS.isImm())

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-vsadd-vp.ll

@@ -439,9 +439,10 @@ define <256 x i8> @vsadd_vi_v258i8_evl129(<256 x i8> %va, <256 x i1> %m) {
 define <256 x i8> @vsadd_vi_v258i8_evl128(<256 x i8> %va, <256 x i1> %m) {
 ; CHECK-LABEL: vsadd_vi_v258i8_evl128:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    li a1, 128
-; CHECK-NEXT:    vsetvli zero, a1, e8, m8, ta, ma
+; CHECK-NEXT:    vsetivli zero, 0, e8, m8, ta, ma
 ; CHECK-NEXT:    vlm.v v24, (a0)
+; CHECK-NEXT:    li a0, 128
+; CHECK-NEXT:    vsetvli zero, a0, e8, m8, ta, ma
 ; CHECK-NEXT:    vsadd.vi v8, v8, -1, v0.t
 ; CHECK-NEXT:    vmv1r.v v0, v24
 ; CHECK-NEXT:    vsetivli zero, 0, e8, m8, ta, ma

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-vsaddu-vp.ll

@@ -435,9 +435,10 @@ define <256 x i8> @vsaddu_vi_v258i8_evl129(<256 x i8> %va, <256 x i1> %m) {
 define <256 x i8> @vsaddu_vi_v258i8_evl128(<256 x i8> %va, <256 x i1> %m) {
 ; CHECK-LABEL: vsaddu_vi_v258i8_evl128:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    li a1, 128
-; CHECK-NEXT:    vsetvli zero, a1, e8, m8, ta, ma
+; CHECK-NEXT:    vsetivli zero, 0, e8, m8, ta, ma
 ; CHECK-NEXT:    vlm.v v24, (a0)
+; CHECK-NEXT:    li a0, 128
+; CHECK-NEXT:    vsetvli zero, a0, e8, m8, ta, ma
 ; CHECK-NEXT:    vsaddu.vi v8, v8, -1, v0.t
 ; CHECK-NEXT:    vmv1r.v v0, v24
 ; CHECK-NEXT:    vsetivli zero, 0, e8, m8, ta, ma

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-vssub-vp.ll

@@ -454,14 +454,15 @@ define <256 x i8> @vssub_vi_v258i8_evl129(<256 x i8> %va, <256 x i1> %m) {
 define <256 x i8> @vssub_vi_v258i8_evl128(<256 x i8> %va, <256 x i1> %m) {
 ; CHECK-LABEL: vssub_vi_v258i8_evl128:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    li a1, 128
-; CHECK-NEXT:    vsetvli zero, a1, e8, m8, ta, ma
+; CHECK-NEXT:    vsetivli zero, 0, e8, m8, ta, ma
 ; CHECK-NEXT:    vlm.v v24, (a0)
-; CHECK-NEXT:    li a0, -1
-; CHECK-NEXT:    vssub.vx v8, v8, a0, v0.t
+; CHECK-NEXT:    li a0, 128
+; CHECK-NEXT:    li a1, -1
+; CHECK-NEXT:    vsetvli zero, a0, e8, m8, ta, ma
+; CHECK-NEXT:    vssub.vx v8, v8, a1, v0.t
 ; CHECK-NEXT:    vmv1r.v v0, v24
 ; CHECK-NEXT:    vsetivli zero, 0, e8, m8, ta, ma
-; CHECK-NEXT:    vssub.vx v16, v16, a0, v0.t
+; CHECK-NEXT:    vssub.vx v16, v16, a1, v0.t
 ; CHECK-NEXT:    ret
   %v = call <256 x i8> @llvm.vp.ssub.sat.v258i8(<256 x i8> %va, <256 x i8> splat (i8 -1), <256 x i1> %m, i32 128)
   ret <256 x i8> %v

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-vssubu-vp.ll

@@ -449,14 +449,15 @@ define <256 x i8> @vssubu_vi_v258i8_evl129(<256 x i8> %va, <256 x i1> %m) {
 define <256 x i8> @vssubu_vi_v258i8_evl128(<256 x i8> %va, <256 x i1> %m) {
 ; CHECK-LABEL: vssubu_vi_v258i8_evl128:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    li a1, 128
-; CHECK-NEXT:    vsetvli zero, a1, e8, m8, ta, ma
+; CHECK-NEXT:    vsetivli zero, 0, e8, m8, ta, ma
 ; CHECK-NEXT:    vlm.v v24, (a0)
-; CHECK-NEXT:    li a0, -1
-; CHECK-NEXT:    vssubu.vx v8, v8, a0, v0.t
+; CHECK-NEXT:    li a0, 128
+; CHECK-NEXT:    li a1, -1
+; CHECK-NEXT:    vsetvli zero, a0, e8, m8, ta, ma
+; CHECK-NEXT:    vssubu.vx v8, v8, a1, v0.t
 ; CHECK-NEXT:    vmv1r.v v0, v24
 ; CHECK-NEXT:    vsetivli zero, 0, e8, m8, ta, ma
-; CHECK-NEXT:    vssubu.vx v16, v16, a0, v0.t
+; CHECK-NEXT:    vssubu.vx v16, v16, a1, v0.t
 ; CHECK-NEXT:    ret
   %v = call <256 x i8> @llvm.vp.usub.sat.v258i8(<256 x i8> %va, <256 x i8> splat (i8 -1), <256 x i1> %m, i32 128)
   ret <256 x i8> %v