[SLP] Lit test that cannot get vectorized due to lack of look-ahead operand reordering heuristic.

[vporpo]

Summary

The code in this test is not vectorized by SLP because its operand reordering cannot look beyond the immediate predecessors.
This will get fixed in a follow-up patch that introduces the look-ahead operand reordering heuristic.

[vporpo]

@ABataev requested this to be checked in before the look-ahead patch. Please commit this if it looks good, I don't have commit access.

test/Transforms/SLPVectorizer/X86/lookahead.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -slp-vectorizer -S < %s -mtriple=x86_64-unknown-linux -mcpu=corei7-avx | FileCheck %s
+;
+; This checks the look-ahead operand reordering heuristic
+;
+; A[0] B[0] C[0] D[0]  C[1] D[1] A[1] B[1]
+;     \  /   \  /          \  /   \  /
+;       -     -              -     -
+;        \   /                \   /
+;          +                    +
+;          |                    |
+;         S[0]                 S[1]
+;
+define void @test(double* %array) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[IDX0:%.*]] = getelementptr inbounds double, double* [[ARRAY:%.*]], i64 0
+; CHECK-NEXT:    [[IDX1:%.*]] = getelementptr inbounds double, double* [[ARRAY]], i64 1
+; CHECK-NEXT:    [[IDX2:%.*]] = getelementptr inbounds double, double* [[ARRAY]], i64 2
+; CHECK-NEXT:    [[IDX3:%.*]] = getelementptr inbounds double, double* [[ARRAY]], i64 3
+; CHECK-NEXT:    [[IDX4:%.*]] = getelementptr inbounds double, double* [[ARRAY]], i64 4
+; CHECK-NEXT:    [[IDX5:%.*]] = getelementptr inbounds double, double* [[ARRAY]], i64 5
+; CHECK-NEXT:    [[IDX6:%.*]] = getelementptr inbounds double, double* [[ARRAY]], i64 6
+; CHECK-NEXT:    [[IDX7:%.*]] = getelementptr inbounds double, double* [[ARRAY]], i64 7
+; CHECK-NEXT:    [[A_0:%.*]] = load double, double* [[IDX0]], align 8
+; CHECK-NEXT:    [[A_1:%.*]] = load double, double* [[IDX1]], align 8
+; CHECK-NEXT:    [[B_0:%.*]] = load double, double* [[IDX2]], align 8
+; CHECK-NEXT:    [[B_1:%.*]] = load double, double* [[IDX3]], align 8
+; CHECK-NEXT:    [[C_0:%.*]] = load double, double* [[IDX4]], align 8
+; CHECK-NEXT:    [[C_1:%.*]] = load double, double* [[IDX5]], align 8
+; CHECK-NEXT:    [[D_0:%.*]] = load double, double* [[IDX6]], align 8
+; CHECK-NEXT:    [[D_1:%.*]] = load double, double* [[IDX7]], align 8
+; CHECK-NEXT:    [[SUBAB_0:%.*]] = fsub fast double [[A_0]], [[B_0]]
+; CHECK-NEXT:    [[SUBCD_0:%.*]] = fsub fast double [[C_0]], [[D_0]]
+; CHECK-NEXT:    [[SUBAB_1:%.*]] = fsub fast double [[A_1]], [[B_1]]
+; CHECK-NEXT:    [[SUBCD_1:%.*]] = fsub fast double [[C_1]], [[D_1]]
+; CHECK-NEXT:    [[ADDABCD_0:%.*]] = fadd fast double [[SUBAB_0]], [[SUBCD_0]]
+; CHECK-NEXT:    [[ADDCDAB_1:%.*]] = fadd fast double [[SUBCD_1]], [[SUBAB_1]]
+; CHECK-NEXT:    store double [[ADDABCD_0]], double* [[IDX0]], align 8
+; CHECK-NEXT:    store double [[ADDCDAB_1]], double* [[IDX1]], align 8
+; CHECK-NEXT:    ret void
+;
+entry:
+  %idx0 = getelementptr inbounds double, double* %array, i64 0
+  %idx1 = getelementptr inbounds double, double* %array, i64 1
+  %idx2 = getelementptr inbounds double, double* %array, i64 2
+  %idx3 = getelementptr inbounds double, double* %array, i64 3
+  %idx4 = getelementptr inbounds double, double* %array, i64 4
+  %idx5 = getelementptr inbounds double, double* %array, i64 5
+  %idx6 = getelementptr inbounds double, double* %array, i64 6
+  %idx7 = getelementptr inbounds double, double* %array, i64 7
+
+
+  %A_0 = load double, double *%idx0, align 8
+  %A_1 = load double, double *%idx1, align 8
+  %B_0 = load double, double *%idx2, align 8
+  %B_1 = load double, double *%idx3, align 8
+  %C_0 = load double, double *%idx4, align 8
+  %C_1 = load double, double *%idx5, align 8
+  %D_0 = load double, double *%idx6, align 8
+  %D_1 = load double, double *%idx7, align 8
+
+  %subAB_0 = fsub fast double %A_0, %B_0
+  %subCD_0 = fsub fast double %C_0, %D_0
+
+  %subAB_1 = fsub fast double %A_1, %B_1
+  %subCD_1 = fsub fast double %C_1, %D_1
+
+  %addABCD_0 = fadd fast double %subAB_0, %subCD_0
+  %addCDAB_1 = fadd fast double %subCD_1, %subAB_1
+
+  store double %addABCD_0, double *%idx0, align 8
+  store double %addCDAB_1, double *%idx1, align 8
+  ret void
+}