Automerge: [ARM][MVE] Invalid tail predication in LowOverheadLoop pass (#163941)

statham-arm · github-actions[bot] · commit 0cda0f9a45b0 · 2025-10-22T08:51:45.000Z
When a loop is converted into a low-overhead loop using tail predication
via FPSCR.LTPSIZE, the MQPRCopy pseudo-instruction is expanded into
either two VMOVD or a single MVE_VORR, depending on whether the values
written to the lanes with a 'false' predicate matter. (MVE_VORR uses the
ambient LTPSIZE predicate, so it won't write those lanes at all; the
double VMOVD is slower but gets them right.)

This check was done based on whether the output of the MQPRCopy is live
coming out of the loop. But it missed a case where the live-out value is
not _itself_ an MQPRCopy, but is a predicated operation taking its false
lanes from an MQPRCopy.

Fixes #162644, and adds a new MIR test case derived from the reproducer
in that bug.
diff --git a/llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp b/llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp
@@ -1036,6 +1036,7 @@ bool LowOverheadLoop::ValidateLiveOuts() {
   while (!Worklist.empty()) {
     MachineInstr *MI = Worklist.pop_back_val();
     if (MI->getOpcode() == ARM::MQPRCopy) {
+      LLVM_DEBUG(dbgs() << " Must generate copy as VMOV: " << *MI);
       VMOVCopies.insert(MI);
       MachineInstr *CopySrc =
           RDI.getUniqueReachingMIDef(MI, MI->getOperand(1).getReg());
@@ -1045,6 +1046,20 @@ bool LowOverheadLoop::ValidateLiveOuts() {
       LLVM_DEBUG(dbgs() << " Unable to handle live out: " << *MI);
       VMOVCopies.clear();
       return false;
+    } else if (isVectorPredicated(MI)) {
+      // If this is a predicated instruction with merging semantics,
+      // check where it gets its false lanes from, if any.
+      int InactiveIdx = findVPTInactiveOperandIdx(*MI);
+      if (InactiveIdx != -1) {
+        SmallPtrSet<MachineInstr *, 2> Defs;
+        MachineInstr *FalseSrc = RDI.getUniqueReachingMIDef(
+            MI, MI->getOperand(InactiveIdx).getReg());
+        if (FalseSrc) {
+          LLVM_DEBUG(dbgs()
+                     << " Must check source of false lanes for: " << *MI);
+          Worklist.push_back(FalseSrc);
+        }
+      }
     }
   }
 
diff --git a/llvm/lib/Target/ARM/Thumb2InstrInfo.cpp b/llvm/lib/Target/ARM/Thumb2InstrInfo.cpp
@@ -805,6 +805,16 @@ int llvm::findFirstVPTPredOperandIdx(const MachineInstr &MI) {
   return -1;
 }
 
+int llvm::findVPTInactiveOperandIdx(const MachineInstr &MI) {
+  const MCInstrDesc &MCID = MI.getDesc();
+
+  for (unsigned i = 0, e = MCID.getNumOperands(); i != e; ++i)
+    if (MCID.operands()[i].OperandType == ARM::OPERAND_VPRED_R)
+      return i + ARM::SUBOP_vpred_r_inactive;
+
+  return -1;
+}
+
 ARMVCC::VPTCodes llvm::getVPTInstrPredicate(const MachineInstr &MI,
                                             Register &PredReg) {
   int PIdx = findFirstVPTPredOperandIdx(MI);
diff --git a/llvm/lib/Target/ARM/Thumb2InstrInfo.h b/llvm/lib/Target/ARM/Thumb2InstrInfo.h
@@ -90,6 +90,9 @@ inline ARMVCC::VPTCodes getVPTInstrPredicate(const MachineInstr &MI) {
   Register PredReg;
   return getVPTInstrPredicate(MI, PredReg);
 }
+// Identify the input operand in an MVE predicated instruction which
+// contributes the values of any inactive vector lanes.
+int findVPTInactiveOperandIdx(const MachineInstr &MI);
 
 // Recomputes the Block Mask of Instr, a VPT or VPST instruction.
 // This rebuilds the block mask of the instruction depending on the predicates
diff --git a/llvm/test/CodeGen/Thumb2/LowOverheadLoops/vctp-vs-unpredicated-copy.mir b/llvm/test/CodeGen/Thumb2/LowOverheadLoops/vctp-vs-unpredicated-copy.mir
@@ -0,0 +1,146 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple=thumbv8.1m.main -mattr=+mve -run-pass=arm-low-overhead-loops %s -verify-machineinstrs -o - | FileCheck %s
+
+# From bug #162644. The _wrong_ output of this test is to generate the
+# body of the tail-predicated loop like this:
+#
+#     $q2 = MVE_VORR killed $q0, killed $q0, 0, $noreg, $noreg, undef $q2
+#     renamable $r0, renamable $q3 = MVE_VLDRWU32_post killed renamable $r0, 16, 0, $noreg, renamable $lr :: (load unknown-size from %ir.13, align 4)
+#     $q0 = MVE_VORR $q1, $q1, 0, $noreg, $noreg, undef $q0
+#     renamable $q0 = MVE_VADDf32 killed renamable $q2, killed renamable $q3, 0, killed $noreg, renamable $lr, killed renamable $q0
+#     $lr = MVE_LETP killed renamable $lr, %bb.1
+#
+# in which the second MVE_VORR, copying q1 into q0, is an invalid conversion of
+# the input MQPRCopy, because it won't copy the vector lanes disabled by
+# FPSCR.LTPSIZE, and those are needed in the output value of the loop.
+#
+# In the right output, that MQPRCopy is expanded into a pair of VMOVD copying
+# d2,d3 into d0,d1 respectively, which are unaffected by LTPSIZE.
+
+--- |
+  target datalayout = "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64"
+  target triple = "thumbv8.1m.main-unknown-none-eabihf"
+
+  @inactive = dso_local local_unnamed_addr global <4 x float> zeroinitializer, align 16
+
+  define <4 x float> @test_func(ptr %0, i32 %1) {
+    %3 = load <4 x float>, ptr @inactive, align 16
+    %4 = add i32 %1, 3
+    %5 = call i32 @llvm.smin.i32(i32 %1, i32 4)
+    %6 = sub i32 %4, %5
+    %7 = lshr i32 %6, 2
+    %8 = add nuw nsw i32 %7, 1
+    %9 = call i32 @llvm.start.loop.iterations.i32(i32 %8)
+    br label %10
+
+  10:                                               ; preds = %10, %2
+    %11 = phi <4 x float> [ splat (float 0x3FB99999A0000000), %2 ], [ %17, %10 ]
+    %12 = phi i32 [ %1, %2 ], [ %19, %10 ]
+    %13 = phi ptr [ %0, %2 ], [ %18, %10 ]
+    %14 = phi i32 [ %9, %2 ], [ %20, %10 ]
+    %15 = tail call <4 x i1> @llvm.arm.mve.vctp32(i32 %12)
+    %16 = tail call <4 x float> @llvm.masked.load.v4f32.p0(ptr %13, i32 4, <4 x i1> %15, <4 x float> zeroinitializer)
+    %17 = tail call <4 x float> @llvm.arm.mve.add.predicated.v4f32.v4i1(<4 x float> %11, <4 x float> %16, <4 x i1> %15, <4 x float> %3)
+    %18 = getelementptr inbounds nuw i8, ptr %13, i32 16
+    %19 = add i32 %12, -4
+    %20 = call i32 @llvm.loop.decrement.reg.i32(i32 %14, i32 1)
+    %21 = icmp ne i32 %20, 0
+    br i1 %21, label %10, label %22
+
+  22:                                               ; preds = %10
+    ret <4 x float> %17
+  }
+...
+---
+name:            test_func
+alignment:       4
+legalized:       false
+tracksRegLiveness: true
+registers:       []
+liveins:
+  - { reg: '$r0', virtual-reg: '' }
+  - { reg: '$r1', virtual-reg: '' }
+stack:
+  - { id: 0, name: '', type: spill-slot, offset: -4, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '$lr', callee-saved-restored: false,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+  - { id: 1, name: '', type: spill-slot, offset: -8, size: 4, alignment: 4,
+      stack-id: default, callee-saved-register: '$r7', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+body:             |
+  ; CHECK-LABEL: name: test_func
+  ; CHECK: bb.0 (%ir-block.2):
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $lr, $r0, $r1, $r7
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   frame-setup tPUSH 14 /* CC::al */, $noreg, killed $r7, killed $lr, implicit-def $sp, implicit $sp
+  ; CHECK-NEXT:   frame-setup CFI_INSTRUCTION def_cfa_offset 8
+  ; CHECK-NEXT:   frame-setup CFI_INSTRUCTION offset $lr, -4
+  ; CHECK-NEXT:   frame-setup CFI_INSTRUCTION offset $r7, -8
+  ; CHECK-NEXT:   $r2 = t2MOVi16 target-flags(arm-lo16) @inactive, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   $r2 = t2MOVTi16 killed $r2, target-flags(arm-hi16) @inactive, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   renamable $q1 = MVE_VLDRWU32 killed renamable $r2, 0, 0, $noreg, $noreg :: (dereferenceable load (s128) from @inactive)
+  ; CHECK-NEXT:   $r3 = t2MOVi16 52429, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   $r3 = t2MOVTi16 killed $r3, 15820, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   renamable $q0 = MVE_VDUP32 killed renamable $r3, 0, $noreg, $noreg, undef renamable $q0
+  ; CHECK-NEXT:   $lr = MVE_DLSTP_32 killed renamable $r1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1 (%ir-block.10, align 4):
+  ; CHECK-NEXT:   successors: %bb.1(0x7c000000), %bb.2(0x04000000)
+  ; CHECK-NEXT:   liveins: $lr, $d2, $d3, $q0, $r0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   $q2 = MVE_VORR killed $q0, killed $q0, 0, $noreg, $noreg, undef $q2
+  ; CHECK-NEXT:   renamable $r0, renamable $q3 = MVE_VLDRWU32_post killed renamable $r0, 16, 0, $noreg, renamable $lr :: (load unknown-size from %ir.13, align 4)
+  ; CHECK-NEXT:   $d0 = VMOVD $d2, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   $d1 = VMOVD $d3, 14 /* CC::al */, $noreg
+  ; CHECK-NEXT:   renamable $q0 = MVE_VADDf32 killed renamable $q2, killed renamable $q3, 0, killed $noreg, renamable $lr, killed renamable $q0
+  ; CHECK-NEXT:   $lr = MVE_LETP killed renamable $lr, %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2 (%ir-block.22):
+  ; CHECK-NEXT:   liveins: $q0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   frame-destroy tPOP_RET 14 /* CC::al */, $noreg, def $r7, def $pc, implicit killed $q0
+  bb.0 (%ir-block.2):
+    successors: %bb.1(0x80000000)
+    liveins: $r0, $r1, $r7, $lr
+
+    frame-setup tPUSH 14 /* CC::al */, $noreg, killed $r7, killed $lr, implicit-def $sp, implicit $sp
+    frame-setup CFI_INSTRUCTION def_cfa_offset 8
+    frame-setup CFI_INSTRUCTION offset $lr, -4
+    frame-setup CFI_INSTRUCTION offset $r7, -8
+    $r2 = t2MOVi16 target-flags(arm-lo16) @inactive, 14 /* CC::al */, $noreg
+    tCMPi8 renamable $r1, 4, 14 /* CC::al */, $noreg, implicit-def $cpsr
+    $r2 = t2MOVTi16 killed $r2, target-flags(arm-hi16) @inactive, 14 /* CC::al */, $noreg
+    renamable $r3 = t2MOVi 1, 14 /* CC::al */, $noreg, $noreg
+    renamable $q1 = MVE_VLDRWU32 killed renamable $r2, 0, 0, $noreg, $noreg :: (dereferenceable load (s128) from @inactive)
+    $r2 = tMOVr $r1, 14 /* CC::al */, $noreg
+    t2IT 10, 8, implicit-def $itstate
+    renamable $r2 = tMOVi8 $noreg, 4, 10 /* CC::ge */, killed $cpsr, implicit killed renamable $r2, implicit killed $itstate
+    renamable $r2, dead $cpsr = tSUBrr renamable $r1, killed renamable $r2, 14 /* CC::al */, $noreg
+    renamable $r2, dead $cpsr = tADDi8 killed renamable $r2, 3, 14 /* CC::al */, $noreg
+    renamable $r2 = nuw nsw t2ADDrs killed renamable $r3, killed renamable $r2, 19, 14 /* CC::al */, $noreg, $noreg
+    $r3 = t2MOVi16 52429, 14 /* CC::al */, $noreg
+    $r3 = t2MOVTi16 killed $r3, 15820, 14 /* CC::al */, $noreg
+    renamable $q0 = MVE_VDUP32 killed renamable $r3, 0, $noreg, $noreg, undef renamable $q0
+    renamable $lr = t2DoLoopStartTP killed renamable $r2, renamable $r1
+
+  bb.1 (%ir-block.10, align 4):
+    successors: %bb.1(0x7c000000), %bb.2(0x04000000)
+    liveins: $lr, $q0, $q1, $r0, $r1
+
+    renamable $vpr = MVE_VCTP32 renamable $r1, 0, $noreg, $noreg
+    $q2 = MQPRCopy killed $q0
+    MVE_VPST 8, implicit $vpr
+    renamable $r0, renamable $q3 = MVE_VLDRWU32_post killed renamable $r0, 16, 1, renamable $vpr, renamable $lr :: (load unknown-size from %ir.13, align 4)
+    $q0 = MQPRCopy $q1
+    MVE_VPST 8, implicit $vpr
+    renamable $q0 = MVE_VADDf32 killed renamable $q2, killed renamable $q3, 1, killed renamable $vpr, renamable $lr, killed renamable $q0
+    renamable $r1, dead $cpsr = tSUBi8 killed renamable $r1, 4, 14 /* CC::al */, $noreg
+    renamable $lr = t2LoopEndDec killed renamable $lr, %bb.1, implicit-def dead $cpsr
+    tB %bb.2, 14 /* CC::al */, $noreg
+
+  bb.2 (%ir-block.22):
+    liveins: $q0
+
+    frame-destroy tPOP_RET 14 /* CC::al */, $noreg, def $r7, def $pc, implicit killed $q0
+...

Original file line number	Diff line number	Diff line change
`@@ -90,6 +90,9 @@ inline ARMVCC::VPTCodes getVPTInstrPredicate(const MachineInstr &MI) {`
`90`	`90`	`Register PredReg;`
`91`	`91`	`return getVPTInstrPredicate(MI, PredReg);`
`92`	`92`	`}`
	`93`	`+// Identify the input operand in an MVE predicated instruction which`
	`94`	`+// contributes the values of any inactive vector lanes.`
	`95`	`+int findVPTInactiveOperandIdx(const MachineInstr &MI);`
`93`	`96`
`94`	`97`	`// Recomputes the Block Mask of Instr, a VPT or VPST instruction.`
`95`	`98`	`// This rebuilds the block mask of the instruction depending on the predicates`