[LegalizeTypes] Teach DAGTypeLegalizer::GenWidenVectorLoads to pad with undef if needed when concatenating small or loads to match a larger load

In the included test case the align 16 allowed the v23f32 load to handled as load v16f32, load v4f32, and load v4f32(one element not used). These loads all need to be concatenated together into a final vector. In this case we tried to concatenate the two v4f32 loads to match the type of the v16f32 load so we could do a second concat_vectors, but those loads alone only add up to v8f32. So we need to two v4f32 undefs to pad it.

It appears we've tried to hack around a similar issue in this code before by adding undef padding to loads in one of the earlier loops in this function. Originally in r147964 by padding all loads narrower than previous loads to the same size. Later modifed to only the last load in r293088. This patch removes that earlier code and just handles it on demand where we know we need it.

Fixes PR46820

Differential Revision: https://reviews.llvm.org/D84463

Author: topperc

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

@@ -4912,7 +4912,8 @@ SDValue DAGTypeLegalizer::GenWidenVectorLoads(SmallVectorImpl<SDValue> &LdChain,
 
   int LdWidth = LdVT.getSizeInBits();
   int WidthDiff = WidenWidth - LdWidth;
-  // Allow wider loads.
+  // Allow wider loads if they are sufficiently aligned to avoid memory faults
+  // and if the original load is simple.
   unsigned LdAlign = (!LD->isSimple()) ? 0 : LD->getAlignment();
 
   // Find the vector type that can load from.
@@ -4964,19 +4965,6 @@ SDValue DAGTypeLegalizer::GenWidenVectorLoads(SmallVectorImpl<SDValue> &LdChain,
                       LD->getPointerInfo().getWithOffset(Offset),
                       LD->getOriginalAlign(), MMOFlags, AAInfo);
       LdChain.push_back(L.getValue(1));
-      if (L->getValueType(0).isVector() && NewVTWidth >= LdWidth) {
-        // Later code assumes the vector loads produced will be mergeable, so we
-        // must pad the final entry up to the previous width. Scalars are
-        // combined separately.
-        SmallVector<SDValue, 16> Loads;
-        Loads.push_back(L);
-        unsigned size = L->getValueSizeInBits(0);
-        while (size < LdOp->getValueSizeInBits(0)) {
-          Loads.push_back(DAG.getUNDEF(L->getValueType(0)));
-          size += L->getValueSizeInBits(0);
-        }
-        L = DAG.getNode(ISD::CONCAT_VECTORS, dl, LdOp->getValueType(0), Loads);
-      }
     } else {
       L = DAG.getLoad(NewVT, dl, Chain, BasePtr,
                       LD->getPointerInfo().getWithOffset(Offset),
@@ -5017,8 +5005,17 @@ SDValue DAGTypeLegalizer::GenWidenVectorLoads(SmallVectorImpl<SDValue> &LdChain,
     EVT NewLdTy = LdOps[i].getValueType();
     if (NewLdTy != LdTy) {
       // Create a larger vector.
+      unsigned NumOps = NewLdTy.getSizeInBits() / LdTy.getSizeInBits();
+      assert(NewLdTy.getSizeInBits() % LdTy.getSizeInBits() == 0);
+      SmallVector<SDValue, 16> WidenOps(NumOps);
+      unsigned j = 0;
+      for (; j != End-Idx; ++j)
+        WidenOps[j] = ConcatOps[Idx+j];
+      for (; j != NumOps; ++j)
+        WidenOps[j] = DAG.getUNDEF(LdTy);
+
       ConcatOps[End-1] = DAG.getNode(ISD::CONCAT_VECTORS, dl, NewLdTy,
-                                     makeArrayRef(&ConcatOps[Idx], End - Idx));
+                                     WidenOps);
       Idx = End - 1;
       LdTy = NewLdTy;
     }

llvm/test/CodeGen/X86/pr46820.ll

@@ -0,0 +1,47 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -mattr=avx512f | FileCheck %s
+
+; The alignment of 16 causes type legalization to split this as 3 loads,
+; v16f32, v4f32, and v4f32. This loads 24 elements, but the load is aligned
+; to 16 bytes so this i safe. There was an issue with type legalization building
+; the proper concat_vectors for this because the two v4f32s don't add up to
+; v16f32 and require padding.
+
+define <23 x float> @load23(<23 x float>* %p) {
+; CHECK-LABEL: load23:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movq %rdi, %rax
+; CHECK-NEXT:    vmovups 64(%rsi), %ymm0
+; CHECK-NEXT:    vmovups (%rsi), %zmm1
+; CHECK-NEXT:    vmovaps 64(%rsi), %xmm2
+; CHECK-NEXT:    vmovss {{.*#+}} xmm3 = mem[0],zero,zero,zero
+; CHECK-NEXT:    vmovss %xmm3, 88(%rdi)
+; CHECK-NEXT:    vmovaps %xmm2, 64(%rdi)
+; CHECK-NEXT:    vmovaps %zmm1, (%rdi)
+; CHECK-NEXT:    vextractf128 $1, %ymm0, %xmm0
+; CHECK-NEXT:    vmovlps %xmm0, 80(%rdi)
+; CHECK-NEXT:    vzeroupper
+; CHECK-NEXT:    retq
+  %t0 = load <23 x float>, <23 x float>* %p, align 16
+  ret <23 x float> %t0
+}
+
+; Same test as above with minimal alignment just to demonstrate the different
+; codegen.
+define <23 x float> @load23_align_1(<23 x float>* %p) {
+; CHECK-LABEL: load23_align_1:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movq %rdi, %rax
+; CHECK-NEXT:    vmovups (%rsi), %zmm0
+; CHECK-NEXT:    vmovups 64(%rsi), %xmm1
+; CHECK-NEXT:    movq 80(%rsi), %rcx
+; CHECK-NEXT:    vmovss {{.*#+}} xmm2 = mem[0],zero,zero,zero
+; CHECK-NEXT:    vmovss %xmm2, 88(%rdi)
+; CHECK-NEXT:    movq %rcx, 80(%rdi)
+; CHECK-NEXT:    vmovaps %xmm1, 64(%rdi)
+; CHECK-NEXT:    vmovaps %zmm0, (%rdi)
+; CHECK-NEXT:    vzeroupper
+; CHECK-NEXT:    retq
+  %t0 = load <23 x float>, <23 x float>* %p, align 1
+  ret <23 x float> %t0
+}