[llvm] [VPlan] Don't use the legacy cost model for loop conditions (PR #156864)

[David Sherwood via llvm-commits]

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@@ -1312,42 +1334,42 @@ define void @pred_udiv_select_cost(ptr %A, ptr %B, ptr %C, i64 %n, i8 %y) #1 {
 ; PRED-NEXT:    br i1 [[CONFLICT_RDX]], label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
 ; PRED:       [[VECTOR_PH]]:
 ; PRED-NEXT:    [[TMP5:%.*]] = call i64 @llvm.vscale.i64()
-; PRED-NEXT:    [[TMP6:%.*]] = mul nuw i64 [[TMP5]], 16
+; PRED-NEXT:    [[TMP6:%.*]] = mul nuw i64 [[TMP5]], 4
 ; PRED-NEXT:    [[TMP7:%.*]] = call i64 @llvm.vscale.i64()
-; PRED-NEXT:    [[TMP8:%.*]] = shl nuw i64 [[TMP7]], 4
+; PRED-NEXT:    [[TMP8:%.*]] = shl nuw i64 [[TMP7]], 2
 ; PRED-NEXT:    [[TMP9:%.*]] = sub i64 [[TMP0]], [[TMP8]]
 ; PRED-NEXT:    [[TMP10:%.*]] = icmp ugt i64 [[TMP0]], [[TMP8]]
 ; PRED-NEXT:    [[TMP11:%.*]] = select i1 [[TMP10]], i64 [[TMP9]], i64 0
-; PRED-NEXT:    [[ACTIVE_LANE_MASK_ENTRY:%.*]] = call <vscale x 16 x i1> @llvm.get.active.lane.mask.nxv16i1.i64(i64 0, i64 [[TMP0]])
-; PRED-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <vscale x 16 x i8> poison, i8 [[Y]], i64 0
-; PRED-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <vscale x 16 x i8> [[BROADCAST_SPLATINSERT]], <vscale x 16 x i8> poison, <vscale x 16 x i32> zeroinitializer
+; PRED-NEXT:    [[ACTIVE_LANE_MASK_ENTRY:%.*]] = call <vscale x 4 x i1> @llvm.get.active.lane.mask.nxv4i1.i64(i64 0, i64 [[TMP0]])
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david-arm wrote:

Looks like the reason why this test has changed behaviour is due to us previously adding on the cost of the original scalar exit condition (an icmp) when in reality the vplan doesn't have one. Nice!

I think it's the same conclusion you came to with the induction-costs-sve.ll test below.

https://github.com/llvm/llvm-project/pull/156864