Fix SLP score for out of order contiguous loads

SLP uses the distance between pointers to optimize
the getShallowScore. However the current code misses
the case where we are trying to vectorize for VF=4, and the distance
between pointers is 2. In that case the returned score
reflects the case of contiguous loads, when it's not actually
contiguous.

The attached unit tests have 5 loads, where the program order
is not the same as the offset order in the GEPs. So, the choice
of which 4 loads to bundle together matters. If we pick the
first 4, then we can vectorize with VF=4. If we pick the
last 4, then we can only vectorize with VF=2.

This patch makes a more conservative choice, to consider
all distances>1 to not be a case of contiguous load, and
give those cases a lower score.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D123516

llvm/test/Transforms/SLPVectorizer/AArch64/tsc-s116.ll

@@ -0,0 +1,65 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -slp-vectorizer -S -mtriple=aarch64-unknown-unknown | FileCheck %s
+
+; This test is reduced from the TSVC evaluation of vectorizers:
+; https://github.com/llvm/llvm-test-suite/commits/main/MultiSource/Benchmarks/TSVC/LoopRerolling-flt/tsc.c
+
+; FIXME
+; This test is currently getting vectorized with VF=2. We should be able
+; to vectorize it with VF=4. Specifically, we should be able to have 1 load of
+; <4 x float> instead of 2 loads of <2 x float>, and there should be no need
+; for shufflevectors.
+; The current issue comes from the Left-Hand-Side fmul operands.
+; These operands are coming from 4 loads which are not
+; contiguous. The score estimation needs to be corrected, so that these 4 loads
+; are not selected for vectorization. Instead we should vectorize with
+; contiguous loads, from %a plus offsets 0 to 3, or offsets 1 to 4.
+
+define void @s116_modified(float* %a) {
+; CHECK-LABEL: @s116_modified(
+; CHECK-NEXT:    [[GEP0:%.*]] = getelementptr inbounds float, float* [[A:%.*]], i64 0
+; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr inbounds float, float* [[A]], i64 1
+; CHECK-NEXT:    [[GEP3:%.*]] = getelementptr inbounds float, float* [[A]], i64 3
+; CHECK-NEXT:    [[LD0:%.*]] = load float, float* [[GEP0]], align 4
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast float* [[GEP1]] to <2 x float>*
+; CHECK-NEXT:    [[TMP2:%.*]] = load <2 x float>, <2 x float>* [[TMP1]], align 4
+; CHECK-NEXT:    [[TMP3:%.*]] = bitcast float* [[GEP3]] to <2 x float>*
+; CHECK-NEXT:    [[TMP4:%.*]] = load <2 x float>, <2 x float>* [[TMP3]], align 4
+; CHECK-NEXT:    [[TMP5:%.*]] = insertelement <4 x float> poison, float [[LD0]], i32 0
+; CHECK-NEXT:    [[TMP6:%.*]] = extractelement <2 x float> [[TMP2]], i32 1
+; CHECK-NEXT:    [[TMP7:%.*]] = insertelement <4 x float> [[TMP5]], float [[TMP6]], i32 1
+; CHECK-NEXT:    [[TMP8:%.*]] = shufflevector <2 x float> [[TMP4]], <2 x float> poison, <4 x i32> <i32 0, i32 1, i32 undef, i32 undef>
+; CHECK-NEXT:    [[TMP9:%.*]] = shufflevector <4 x float> [[TMP7]], <4 x float> [[TMP8]], <4 x i32> <i32 0, i32 1, i32 4, i32 5>
+; CHECK-NEXT:    [[TMP10:%.*]] = extractelement <2 x float> [[TMP2]], i32 0
+; CHECK-NEXT:    [[TMP11:%.*]] = insertelement <4 x float> poison, float [[TMP10]], i32 0
+; CHECK-NEXT:    [[TMP12:%.*]] = shufflevector <2 x float> [[TMP2]], <2 x float> poison, <4 x i32> <i32 0, i32 1, i32 undef, i32 undef>
+; CHECK-NEXT:    [[TMP13:%.*]] = shufflevector <4 x float> [[TMP11]], <4 x float> [[TMP12]], <4 x i32> <i32 0, i32 4, i32 5, i32 3>
+; CHECK-NEXT:    [[TMP14:%.*]] = extractelement <2 x float> [[TMP4]], i32 0
+; CHECK-NEXT:    [[TMP15:%.*]] = insertelement <4 x float> [[TMP13]], float [[TMP14]], i32 3
+; CHECK-NEXT:    [[TMP16:%.*]] = fmul fast <4 x float> [[TMP9]], [[TMP15]]
+; CHECK-NEXT:    [[TMP17:%.*]] = bitcast float* [[GEP0]] to <4 x float>*
+; CHECK-NEXT:    store <4 x float> [[TMP16]], <4 x float>* [[TMP17]], align 4
+; CHECK-NEXT:    ret void
+;
+  %gep0 = getelementptr inbounds float, float* %a, i64 0
+  %gep1 = getelementptr inbounds float, float* %a, i64 1
+  %gep2 = getelementptr inbounds float, float* %a, i64 2
+  %gep3 = getelementptr inbounds float, float* %a, i64 3
+  %gep4 = getelementptr inbounds float, float* %a, i64 4
+  %ld0 = load float, float* %gep0
+  %ld1 = load float, float* %gep1
+  %ld2 = load float, float* %gep2
+  %ld3 = load float, float* %gep3
+  %ld4 = load float, float* %gep4
+  %mul0 = fmul fast float %ld0, %ld1
+  %mul1 = fmul fast float %ld2, %ld1
+  %mul2 = fmul fast float %ld3, %ld2
+  %mul3 = fmul fast float %ld4, %ld3
+  store float %mul0, float* %gep0
+  store float %mul1, float* %gep1
+  store float %mul2, float* %gep2
+  store float %mul3, float* %gep3
+  ret void
+}
+
+