[llvm] [SLP]Fix graph traversal in getSpillCost (PR #124984)

[Alexey Bataev via llvm-commits]

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@@ -149,37 +149,27 @@ define <4 x float> @exp_4x(ptr %a) {
 ; CHECK-SAME: (ptr [[A:%.*]]) #[[ATTR1]] {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = load <4 x float>, ptr [[A]], align 16
-; CHECK-NEXT:    [[VECEXT:%.*]] = extractelement <4 x float> [[TMP0]], i32 0
-; CHECK-NEXT:    [[TMP1:%.*]] = tail call fast float @expf(float [[VECEXT]])
-; CHECK-NEXT:    [[VECINS:%.*]] = insertelement <4 x float> undef, float [[TMP1]], i32 0
-; CHECK-NEXT:    [[VECEXT_1:%.*]] = extractelement <4 x float> [[TMP0]], i32 1
-; CHECK-NEXT:    [[TMP2:%.*]] = tail call fast float @expf(float [[VECEXT_1]])
-; CHECK-NEXT:    [[VECINS_1:%.*]] = insertelement <4 x float> [[VECINS]], float [[TMP2]], i32 1
-; CHECK-NEXT:    [[VECEXT_2:%.*]] = extractelement <4 x float> [[TMP0]], i32 2
-; CHECK-NEXT:    [[TMP3:%.*]] = tail call fast float @expf(float [[VECEXT_2]])
-; CHECK-NEXT:    [[VECINS_2:%.*]] = insertelement <4 x float> [[VECINS_1]], float [[TMP3]], i32 2
-; CHECK-NEXT:    [[VECEXT_3:%.*]] = extractelement <4 x float> [[TMP0]], i32 3
-; CHECK-NEXT:    [[TMP4:%.*]] = tail call fast float @expf(float [[VECEXT_3]])
-; CHECK-NEXT:    [[VECINS_3:%.*]] = insertelement <4 x float> [[VECINS_2]], float [[TMP4]], i32 3
-; CHECK-NEXT:    ret <4 x float> [[VECINS_3]]
+; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <4 x float> [[TMP0]], <4 x float> poison, <2 x i32> <i32 0, i32 1>
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alexey-bataev wrote:

> I did some digging into the cost for this routine, and noticed something interesting. A VF=4, the spill cost is computed as 8. But at VF=2, the spill cost is 0. I don't understand why that is true. Might be something worth digging into here?

It happens, because the vectorizer assumes that function calls are vectorized using vector intrinsics. And because of that it does not include spill costs, because it does not  know that these functions are going to be scalarized and actually spills will be generated. Patch https://github.com/llvm/llvm-project/pull/125070 should fix this

https://github.com/llvm/llvm-project/pull/124984