[LV] Don't add fixed-order recurrence phis to forced scalars.

Fixed-order recurrence phis cannot be forced to be scalar, they will
always be widened at the moment.

Make sure we don't add them to ForcedScalars, otherwise the legacy cost
model will compute incorrect costs.

This fixes an assertion reported with
llvm/llvm-project#129645.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -6464,10 +6464,15 @@ void LoopVectorizationCostModel::setCostBasedWideningDecision(ElementCount VF) {
                  getMemoryInstructionCost(Member, ElementCount::getFixed(1))));
         }
       }
-    } else
+    } else {
+      // Cannot scalarize fixed-order recurrence phis at the moment.
+      if (isa<PHINode>(I) && Legal->isFixedOrderRecurrence(cast<PHINode>(I)))
+        continue;
+
       // Make sure I gets scalarized and a cost estimate without
       // scalarization overhead.
       ForcedScalars[VF].insert(I);
+    }
   }
 }
 

llvm/test/Transforms/LoopVectorize/X86/fixed-order-recurrence.ll

@@ -91,7 +91,7 @@ for.body:                                         ; preds = %for.body.preheader,
   br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
 }
 
-define void @thirdorderrec(ptr nocapture noundef readonly %x, ptr noalias nocapture noundef writeonly %y, i32 noundef %n) #0 {
+define void @thirdorderrec(ptr nocapture noundef readonly %x, ptr noalias nocapture noundef writeonly %y, i32 noundef %n) {
 ; CHECK-LABEL: @thirdorderrec(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[CMP38:%.*]] = icmp sgt i32 [[N:%.*]], 3
@@ -352,3 +352,101 @@ loop:
 exit:
   ret void
 }
+
+define void @test_for_tried_to_force_scalar(ptr noalias %A, ptr noalias %B, ptr noalias %C, i64 %n) #0 {
+; CHECK-LABEL: @test_for_tried_to_force_scalar(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = add i64 [[N:%.*]], 1
+; CHECK-NEXT:    [[CONFLICT_RDX20:%.*]] = icmp ule i64 [[TMP0]], 8
+; CHECK-NEXT:    br i1 [[CONFLICT_RDX20]], label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
+; CHECK:       vector.ph:
+; CHECK-NEXT:    [[N_MOD_VF:%.*]] = urem i64 [[TMP0]], 8
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq i64 [[N_MOD_VF]], 0
+; CHECK-NEXT:    [[TMP4:%.*]] = select i1 [[TMP3]], i64 8, i64 [[N_MOD_VF]]
+; CHECK-NEXT:    [[N_VEC:%.*]] = sub i64 [[TMP0]], [[TMP4]]
+; CHECK-NEXT:    [[VECTOR_RECUR_INIT:%.*]] = insertelement <4 x ptr> poison, ptr [[A:%.*]], i32 3
+; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
+; CHECK:       vector.body:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VECTOR_RECUR:%.*]] = phi <4 x ptr> [ [[VECTOR_RECUR_INIT]], [[VECTOR_PH]] ], [ [[TMP28:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[TMP5:%.*]] = add i64 [[INDEX]], 0
+; CHECK-NEXT:    [[TMP6:%.*]] = add i64 [[INDEX]], 1
+; CHECK-NEXT:    [[TMP7:%.*]] = add i64 [[INDEX]], 2
+; CHECK-NEXT:    [[TMP8:%.*]] = add i64 [[INDEX]], 3
+; CHECK-NEXT:    [[TMP9:%.*]] = add i64 [[INDEX]], 4
+; CHECK-NEXT:    [[TMP10:%.*]] = add i64 [[INDEX]], 5
+; CHECK-NEXT:    [[TMP11:%.*]] = add i64 [[INDEX]], 6
+; CHECK-NEXT:    [[TMP12:%.*]] = add i64 [[INDEX]], 7
+; CHECK-NEXT:    [[TMP13:%.*]] = getelementptr nusw [3 x float], ptr [[A]], i64 [[TMP5]]
+; CHECK-NEXT:    [[TMP14:%.*]] = getelementptr nusw [3 x float], ptr [[A]], i64 [[TMP6]]
+; CHECK-NEXT:    [[TMP15:%.*]] = getelementptr nusw [3 x float], ptr [[A]], i64 [[TMP7]]
+; CHECK-NEXT:    [[TMP16:%.*]] = getelementptr nusw [3 x float], ptr [[A]], i64 [[TMP8]]
+; CHECK-NEXT:    [[TMP17:%.*]] = insertelement <4 x ptr> poison, ptr [[TMP13]], i32 0
+; CHECK-NEXT:    [[TMP18:%.*]] = insertelement <4 x ptr> [[TMP17]], ptr [[TMP14]], i32 1
+; CHECK-NEXT:    [[TMP19:%.*]] = insertelement <4 x ptr> [[TMP18]], ptr [[TMP15]], i32 2
+; CHECK-NEXT:    [[TMP20:%.*]] = insertelement <4 x ptr> [[TMP19]], ptr [[TMP16]], i32 3
+; CHECK-NEXT:    [[TMP21:%.*]] = getelementptr nusw [3 x float], ptr [[A]], i64 [[TMP9]]
+; CHECK-NEXT:    [[TMP22:%.*]] = getelementptr nusw [3 x float], ptr [[A]], i64 [[TMP10]]
+; CHECK-NEXT:    [[TMP23:%.*]] = getelementptr nusw [3 x float], ptr [[A]], i64 [[TMP11]]
+; CHECK-NEXT:    [[TMP24:%.*]] = getelementptr nusw [3 x float], ptr [[A]], i64 [[TMP12]]
+; CHECK-NEXT:    [[TMP25:%.*]] = insertelement <4 x ptr> poison, ptr [[TMP21]], i32 0
+; CHECK-NEXT:    [[TMP26:%.*]] = insertelement <4 x ptr> [[TMP25]], ptr [[TMP22]], i32 1
+; CHECK-NEXT:    [[TMP27:%.*]] = insertelement <4 x ptr> [[TMP26]], ptr [[TMP23]], i32 2
+; CHECK-NEXT:    [[TMP28]] = insertelement <4 x ptr> [[TMP27]], ptr [[TMP24]], i32 3
+; CHECK-NEXT:    [[TMP29:%.*]] = shufflevector <4 x ptr> [[TMP20]], <4 x ptr> [[TMP28]], <4 x i32> <i32 3, i32 4, i32 5, i32 6>
+; CHECK-NEXT:    [[WIDE_VEC:%.*]] = load <12 x float>, ptr [[TMP21]], align 4
+; CHECK-NEXT:    [[STRIDED_VEC:%.*]] = shufflevector <12 x float> [[WIDE_VEC]], <12 x float> poison, <4 x i32> <i32 0, i32 3, i32 6, i32 9>
+; CHECK-NEXT:    [[TMP30:%.*]] = extractelement <4 x float> [[STRIDED_VEC]], i32 3
+; CHECK-NEXT:    store float [[TMP30]], ptr [[C:%.*]], align 4
+; CHECK-NEXT:    [[TMP31:%.*]] = extractelement <4 x ptr> [[TMP29]], i32 0
+; CHECK-NEXT:    [[TMP38:%.*]] = load float, ptr [[TMP31]], align 4
+; CHECK-NEXT:    [[TMP33:%.*]] = extractelement <4 x ptr> [[TMP29]], i32 1
+; CHECK-NEXT:    [[TMP32:%.*]] = load float, ptr [[TMP33]], align 4
+; CHECK-NEXT:    [[TMP35:%.*]] = extractelement <4 x ptr> [[TMP29]], i32 2
+; CHECK-NEXT:    [[TMP34:%.*]] = load float, ptr [[TMP35]], align 4
+; CHECK-NEXT:    [[TMP37:%.*]] = extractelement <4 x ptr> [[TMP29]], i32 3
+; CHECK-NEXT:    [[TMP36:%.*]] = load float, ptr [[TMP37]], align 4
+; CHECK-NEXT:    store float [[TMP36]], ptr [[B:%.*]], align 4
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 8
+; CHECK-NEXT:    [[TMP39:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
+; CHECK-NEXT:    br i1 [[TMP39]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP10:![0-9]+]]
+; CHECK:       middle.block:
+; CHECK-NEXT:    br label [[SCALAR_PH]]
+; CHECK:       scalar.ph:
+; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[SCALAR_RECUR_INIT:%.*]] = phi ptr [ [[TMP24]], [[MIDDLE_BLOCK]] ], [ [[A]], [[ENTRY]] ]
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[PREV:%.*]] = phi ptr [ [[SCALAR_RECUR_INIT]], [[SCALAR_PH]] ], [ [[NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[NEXT]] = getelementptr nusw [3 x float], ptr [[A]], i64 [[IV]]
+; CHECK-NEXT:    [[TMP40:%.*]] = load float, ptr [[NEXT]], align 4
+; CHECK-NEXT:    store float [[TMP40]], ptr [[C]], align 4
+; CHECK-NEXT:    [[TMP41:%.*]] = load float, ptr [[PREV]], align 4
+; CHECK-NEXT:    store float [[TMP41]], ptr [[B]], align 4
+; CHECK-NEXT:    [[IV_NEXT]] = add nsw i64 [[IV]], 1
+; CHECK-NEXT:    [[EXITCOND_NOT:%.*]] = icmp eq i64 [[IV]], [[N]]
+; CHECK-NEXT:    br i1 [[EXITCOND_NOT]], label [[EXIT:%.*]], label [[LOOP]], !llvm.loop [[LOOP11:![0-9]+]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
+  %prev = phi ptr [ %A, %entry ], [ %next, %loop ]
+  %next = getelementptr nusw [3 x float], ptr %A, i64 %iv
+  %0 = load float, ptr %next, align 4
+  store float %0, ptr %C, align 4
+  %1 = load float, ptr %prev, align 4
+  store float %1, ptr %B, align 4
+  %iv.next = add nsw i64 %iv, 1
+  %exitcond.not = icmp eq i64 %iv, %n
+  br i1 %exitcond.not, label %exit, label %loop
+
+exit:
+  ret void
+}
+
+attributes #0 = { "target-cpu"="znver3" }