[ARM][LowOverheadLoops] Combine a VCMP and VPST into a VPT

This patch combines a VCMP followed by a VPST into a VPT, which has the
same semantics as the combination of the former two.

llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp

@@ -1298,6 +1298,12 @@ void ARMLowOverheadLoops::ConvertVPTBlocks(LowOverheadLoop &LoLoop) {
              E = ++MachineBasicBlock::iterator(Divergent->MI); I != E; ++I)
           RemovePredicate(&*I);
 
+        // Check if the instruction defining vpr is a vcmp so it can be combined
+        // with the VPST This should be the divergent instruction
+        MachineInstr *VCMP = VCMPOpcodeToVPT(Divergent->MI->getOpcode()) != 0
+                                 ? Divergent->MI
+                                 : nullptr;
+
         unsigned Size = 0;
         auto E = MachineBasicBlock::reverse_iterator(Divergent->MI);
         auto I = MachineBasicBlock::reverse_iterator(Insts.back().MI);
@@ -1307,13 +1313,32 @@ void ARMLowOverheadLoops::ConvertVPTBlocks(LowOverheadLoop &LoLoop) {
           ++Size;
           ++I;
         }
-        // Create a VPST (with a null mask for now, we'll recompute it later).
+        MachineInstrBuilder MIB;
-        MachineInstrBuilder MIB = BuildMI(*InsertAt->getParent(), InsertAt,
+        LLVM_DEBUG(dbgs() << "ARM Loops: Removing VPST: "
-                                          InsertAt->getDebugLoc(),
+                          << *Block.getPredicateThen());
-                                          TII->get(ARM::MVE_VPST));
+        if (VCMP) {
-        MIB.addImm(0);
+          // Combine the VPST and VCMP into a VPT
-        LLVM_DEBUG(dbgs() << "ARM Loops: Removing VPST: " << *Block.getPredicateThen());
+          MIB =
-        LLVM_DEBUG(dbgs() << "ARM Loops: Created VPST: " << *MIB);
+              BuildMI(*InsertAt->getParent(), InsertAt, InsertAt->getDebugLoc(),
+                      TII->get(VCMPOpcodeToVPT(VCMP->getOpcode())));
+          MIB.addImm(ARMVCC::Then);
+          // Register one
+          MIB.add(VCMP->getOperand(1));
+          // Register two
+          MIB.add(VCMP->getOperand(2));
+          // The comparison code, e.g. ge, eq, lt
+          MIB.add(VCMP->getOperand(3));
+          LLVM_DEBUG(dbgs()
+                     << "ARM Loops: Combining with VCMP to VPT: " << *MIB);
+          LoLoop.ToRemove.insert(VCMP);
+        } else {
+          // Create a VPST (with a null mask for now, we'll recompute it later)
+          // or a VPT in case there was a VCMP right before it
+          MIB = BuildMI(*InsertAt->getParent(), InsertAt,
+                        InsertAt->getDebugLoc(), TII->get(ARM::MVE_VPST));
+          MIB.addImm(0);
+          LLVM_DEBUG(dbgs() << "ARM Loops: Created VPST: " << *MIB);
+        }
         LoLoop.ToRemove.insert(Block.getPredicateThen());
         LoLoop.BlockMasksToRecompute.insert(MIB.getInstr());
       }

llvm/test/CodeGen/Thumb2/LowOverheadLoops/vcmp-vpst-combination.ll

@@ -0,0 +1,49 @@
+; RUN: llc -mtriple=thumbv8.1m.main-none-none-eabi -mattr=+mve.fp -tail-predication=force-enabled-no-reductions -o - %s | FileCheck %s
+
+define arm_aapcs_vfpcc <16 x i8> @vcmp_vpst_combination(<16 x i8>* %pSrc, i16 zeroext %blockSize, i8* nocapture %pResult, i32* nocapture %pIndex) {
+; CHECK-LABEL: vcmp_vpst_combination:
+; CHECK:       @ %bb.0: @ %entry
+; CHECK-NEXT:    .save {r7, lr}
+; CHECK-NEXT:    push {r7, lr}
+; CHECK-NEXT:    vmov.i8 q0, #0x7f
+; CHECK-NEXT:    dlstp.8 lr, r1
+; CHECK-NEXT:  .LBB0_1: @ %do.body
+; CHECK-NEXT:    @ =>This Inner Loop Header: Depth=1
+; CHECK-NEXT:    vldrb.u8 q1, [r0]
+; CHECK-NEXT:    vpt.s8 ge, q0, q1
+; CHECK-NEXT:    vmovt q0, q1
+; CHECK-NEXT:    letp lr, .LBB0_1
+; CHECK-NEXT:  @ %bb.2: @ %do.end
+; CHECK-NEXT:    pop {r7, pc}
+entry:
+  %conv = zext i16 %blockSize to i32
+  %0 = tail call { <16 x i8>, i32 } @llvm.arm.mve.vidup.v16i8(i32 0, i32 1)
+  %1 = extractvalue { <16 x i8>, i32 } %0, 0
+  br label %do.body
+
+do.body:                                          ; preds = %do.body, %entry
+  %indexVec.0 = phi <16 x i8> [ %1, %entry ], [ %add, %do.body ]
+  %curExtremIdxVec.0 = phi <16 x i8> [ zeroinitializer, %entry ], [ %6, %do.body ]
+  %curExtremValVec.0 = phi <16 x i8> [ <i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127, i8 127>, %entry ], [ %6, %do.body ]
+  %blkCnt.0 = phi i32 [ %conv, %entry ], [ %sub2, %do.body ]
+  %2 = tail call <16 x i1> @llvm.arm.mve.vctp8(i32 %blkCnt.0)
+  %3 = tail call <16 x i8> @llvm.masked.load.v16i8.p0v16i8(<16 x i8>* %pSrc, i32 1, <16 x i1> %2, <16 x i8> zeroinitializer)
+  %4 = icmp sle <16 x i8> %3, %curExtremValVec.0
+  %5 = and <16 x i1> %4, %2
+  %6 = tail call <16 x i8> @llvm.arm.mve.orr.predicated.v16i8.v16i1(<16 x i8> %3, <16 x i8> %3, <16 x i1> %5, <16 x i8> %curExtremValVec.0)
+  %add = add <16 x i8> %indexVec.0, <i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16, i8 16>
+  %sub2 = add nsw i32 %blkCnt.0, -16
+  %cmp = icmp sgt i32 %blkCnt.0, 16
+  br i1 %cmp, label %do.body, label %do.end
+
+do.end:                                           ; preds = %do.body
+  ret <16 x i8> %6
+}
+
+declare { <16 x i8>, i32 } @llvm.arm.mve.vidup.v16i8(i32, i32)
+
+declare <16 x i1> @llvm.arm.mve.vctp8(i32)
+
+declare <16 x i8> @llvm.masked.load.v16i8.p0v16i8(<16 x i8>*, i32 immarg, <16 x i1>, <16 x i8>)
+
+declare <16 x i8> @llvm.arm.mve.orr.predicated.v16i8.v16i1(<16 x i8>, <16 x i8>, <16 x i1>, <16 x i8>)