[llvm] [VectorCombine] isExtractExtractCheap - specify the extract/insert shuffle mask to improve shuffle costs (PR #114780)

[David Green via llvm-commits]

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@@ -688,9 +688,9 @@ define i32 @load_multiple_extracts_with_constant_idx(ptr %x) {
 define i32 @load_multiple_extracts_with_constant_idx_profitable(ptr %x) {
 ; CHECK-LABEL: @load_multiple_extracts_with_constant_idx_profitable(
 ; CHECK-NEXT:    [[LV:%.*]] = load <8 x i32>, ptr [[X:%.*]], align 16
-; CHECK-NEXT:    [[E_0:%.*]] = extractelement <8 x i32> [[LV]], i32 0
-; CHECK-NEXT:    [[E_1:%.*]] = extractelement <8 x i32> [[LV]], i32 6
-; CHECK-NEXT:    [[RES:%.*]] = add i32 [[E_0]], [[E_1]]
+; CHECK-NEXT:    [[SHIFT:%.*]] = shufflevector <8 x i32> [[LV]], <8 x i32> poison, <8 x i32> <i32 6, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
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davemgreen wrote:

I don't believe there is a way to specify demanded elts to the cost functions, but would it be possible to manually specify the expected length of the add (maybe using the known register vector length if the length of the vector is longer)? It doesn't help with the cost/scalarization of the load, but might make the cost overall a little more accurate.

https://github.com/llvm/llvm-project/pull/114780