[AMDGPU] Fix edge case of buffer OOB handling

llvm/lib/Target/AMDGPU/AMDGPU.td

@@ -119,6 +119,12 @@ def FeatureUnalignedDSAccess : SubtargetFeature<"unaligned-ds-access",
   "Hardware supports unaligned local and region loads and stores"
 >;
 
+def FeatureRequireNaturallyAlignedBufferAccess : SubtargetFeature<"require-naturally-aligned-buffer-access",
+  "RequireNaturallyAlignedBufferAccess",
+  "true",
+  "Requires natural alignment of buffer accesses"
+>;
+
 def FeatureApertureRegs : SubtargetFeature<"aperture-regs",
   "HasApertureRegs",
   "true",

llvm/lib/Target/AMDGPU/GCNSubtarget.h

@@ -76,6 +76,7 @@ class GCNSubtarget final : public AMDGPUGenSubtargetInfo,
   bool BackOffBarrier = false;
   bool UnalignedScratchAccess = false;
   bool UnalignedAccessMode = false;
+  bool RequireNaturallyAlignedBufferAccess = false;
   bool HasApertureRegs = false;
   bool SupportsXNACK = false;
   bool KernargPreload = false;
@@ -600,6 +601,10 @@ class GCNSubtarget final : public AMDGPUGenSubtargetInfo,
     return UnalignedAccessMode;
   }
 
+  bool hasRequireNaturallyAlignedBufferAccess() const {
+    return RequireNaturallyAlignedBufferAccess;
+  }
+
   bool hasApertureRegs() const {
     return HasApertureRegs;
   }

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -1840,6 +1840,20 @@ bool SITargetLowering::allowsMisalignedMemoryAccessesImpl(
            Subtarget->hasUnalignedBufferAccessEnabled();
   }
 
+  // Check natural alignment of buffer if the target requires it. This is needed
+  // only if robust out-of-bounds guarantees are needed. Normally hardware will
+  // ensure proper out-of-bounds behavior, but in the edge case where an access
+  // starts out-of-bounds and then enter in-bounds, the entire access would be
+  // treated as out-of-bounds. Requiring the natural alignment avoids the
+  // problem.
+  if (AddrSpace == AMDGPUAS::BUFFER_FAT_POINTER ||
+      AddrSpace == AMDGPUAS::BUFFER_RESOURCE ||
+      AddrSpace == AMDGPUAS::BUFFER_STRIDED_POINTER) {
+    if (Subtarget->hasRequireNaturallyAlignedBufferAccess() &&
+        Alignment < Align(PowerOf2Ceil(divideCeil(Size, 8))))
+      return false;
+  }
+
   // Smaller than dword value must be aligned.
   if (Size < 32)
     return false;

llvm/test/Transforms/LoadStoreVectorizer/AMDGPU/unaligned-buffer.ll

@@ -0,0 +1,80 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -mtriple=amdgcn--amdpal -passes=load-store-vectorizer -mattr=+require-naturally-aligned-buffer-access -S -o - %s | FileCheck --check-prefix=ALIGNED %s
+; RUN: opt -mtriple=amdgcn--amdpal -passes=load-store-vectorizer -S -o - %s | FileCheck --check-prefixes=UNALIGNED %s
+
+; The test checks that require-naturally-aligned-buffer-access target feature prevents merging loads if the target load would not be naturally aligned.
+
+define amdgpu_kernel void @merge_align_4(ptr addrspace(7) nocapture %p, ptr addrspace(7) nocapture %p2) #0 {
+;
+; ALIGNED-LABEL: define amdgpu_kernel void @merge_align_4(
+; ALIGNED-SAME: ptr addrspace(7) nocapture [[P:%.*]], ptr addrspace(7) nocapture [[P2:%.*]]) #[[ATTR0:[0-9]+]] {
+; ALIGNED-NEXT:  [[ENTRY:.*:]]
+; ALIGNED-NEXT:    [[GEP_M8:%.*]] = getelementptr i8, ptr addrspace(7) [[P]], i32 -8
+; ALIGNED-NEXT:    [[LD_M8:%.*]] = load i32, ptr addrspace(7) [[GEP_M8]], align 4
+; ALIGNED-NEXT:    [[GEP_M4:%.*]] = getelementptr i8, ptr addrspace(7) [[P]], i32 -4
+; ALIGNED-NEXT:    [[LD_M4:%.*]] = load i32, ptr addrspace(7) [[GEP_M4]], align 4
+; ALIGNED-NEXT:    [[GEP_0:%.*]] = getelementptr i8, ptr addrspace(7) [[P]], i32 0
+; ALIGNED-NEXT:    [[LD_0:%.*]] = load i32, ptr addrspace(7) [[GEP_0]], align 4
+; ALIGNED-NEXT:    [[GEP_4:%.*]] = getelementptr i8, ptr addrspace(7) [[P]], i64 4
+; ALIGNED-NEXT:    [[LD_4:%.*]] = load i32, ptr addrspace(7) [[GEP_4]], align 4
+; ALIGNED-NEXT:    ret void
+;
+; UNALIGNED-LABEL: define amdgpu_kernel void @merge_align_4(
+; UNALIGNED-SAME: ptr addrspace(7) nocapture [[P:%.*]], ptr addrspace(7) nocapture [[P2:%.*]]) {
+; UNALIGNED-NEXT:  [[ENTRY:.*:]]
+; UNALIGNED-NEXT:    [[GEP_M8:%.*]] = getelementptr i8, ptr addrspace(7) [[P]], i32 -8
+; UNALIGNED-NEXT:    [[TMP0:%.*]] = load <4 x i32>, ptr addrspace(7) [[GEP_M8]], align 4
+; UNALIGNED-NEXT:    [[LD_M81:%.*]] = extractelement <4 x i32> [[TMP0]], i32 0
+; UNALIGNED-NEXT:    [[LD_M42:%.*]] = extractelement <4 x i32> [[TMP0]], i32 1
+; UNALIGNED-NEXT:    [[LD_03:%.*]] = extractelement <4 x i32> [[TMP0]], i32 2
+; UNALIGNED-NEXT:    [[LD_44:%.*]] = extractelement <4 x i32> [[TMP0]], i32 3
+; UNALIGNED-NEXT:    ret void
+;
+entry:
+  %gep_m8 = getelementptr i8, ptr addrspace(7) %p, i32 -8
+  %ld_m8 = load i32, ptr addrspace(7) %gep_m8, align 4
+  %gep_m4 = getelementptr i8, ptr addrspace(7) %p, i32 -4
+  %ld_m4 = load i32, ptr addrspace(7) %gep_m4, align 4
+  %gep_0 = getelementptr i8, ptr addrspace(7) %p, i32 0
+  %ld_0 = load i32, ptr addrspace(7) %gep_0, align 4
+  %gep_4 = getelementptr i8, ptr addrspace(7) %p, i64 4
+  %ld_4 = load i32, ptr addrspace(7) %gep_4, align 4
+  ret void
+}
+
+; The test checks that require-naturally-aligned-buffer-access target feature does not prevent merging loads if the target load would be naturally aligned.
+
+define amdgpu_kernel void @merge_align_16(ptr addrspace(7) nocapture %p, ptr addrspace(7) nocapture %p2) #0 {
+; ALIGNED-LABEL: define amdgpu_kernel void @merge_align_16(
+; ALIGNED-SAME: ptr addrspace(7) nocapture [[P:%.*]], ptr addrspace(7) nocapture [[P2:%.*]]) #[[ATTR0]] {
+; ALIGNED-NEXT:  [[ENTRY:.*:]]
+; ALIGNED-NEXT:    [[GEP_M8:%.*]] = getelementptr i8, ptr addrspace(7) [[P]], i32 -8
+; ALIGNED-NEXT:    [[TMP0:%.*]] = load <4 x i32>, ptr addrspace(7) [[GEP_M8]], align 16
+; ALIGNED-NEXT:    [[LD_M81:%.*]] = extractelement <4 x i32> [[TMP0]], i32 0
+; ALIGNED-NEXT:    [[LD_M42:%.*]] = extractelement <4 x i32> [[TMP0]], i32 1
+; ALIGNED-NEXT:    [[LD_03:%.*]] = extractelement <4 x i32> [[TMP0]], i32 2
+; ALIGNED-NEXT:    [[LD_44:%.*]] = extractelement <4 x i32> [[TMP0]], i32 3
+; ALIGNED-NEXT:    ret void
+;
+; UNALIGNED-LABEL: define amdgpu_kernel void @merge_align_16(
+; UNALIGNED-SAME: ptr addrspace(7) nocapture [[P:%.*]], ptr addrspace(7) nocapture [[P2:%.*]]) {
+; UNALIGNED-NEXT:  [[ENTRY:.*:]]
+; UNALIGNED-NEXT:    [[GEP_M8:%.*]] = getelementptr i8, ptr addrspace(7) [[P]], i32 -8
+; UNALIGNED-NEXT:    [[TMP0:%.*]] = load <4 x i32>, ptr addrspace(7) [[GEP_M8]], align 16
+; UNALIGNED-NEXT:    [[LD_M81:%.*]] = extractelement <4 x i32> [[TMP0]], i32 0
+; UNALIGNED-NEXT:    [[LD_M42:%.*]] = extractelement <4 x i32> [[TMP0]], i32 1
+; UNALIGNED-NEXT:    [[LD_03:%.*]] = extractelement <4 x i32> [[TMP0]], i32 2
+; UNALIGNED-NEXT:    [[LD_44:%.*]] = extractelement <4 x i32> [[TMP0]], i32 3
+; UNALIGNED-NEXT:    ret void
+;
+entry:
+  %gep_m8 = getelementptr i8, ptr addrspace(7) %p, i32 -8
+  %ld_m8 = load i32, ptr addrspace(7) %gep_m8, align 16
+  %gep_m4 = getelementptr i8, ptr addrspace(7) %p, i32 -4
+  %ld_m4 = load i32, ptr addrspace(7) %gep_m4, align 4
+  %gep_0 = getelementptr i8, ptr addrspace(7) %p, i32 0
+  %ld_0 = load i32, ptr addrspace(7) %gep_0, align 8
+  %gep_4 = getelementptr i8, ptr addrspace(7) %p, i64 4
+  %ld_4 = load i32, ptr addrspace(7) %gep_4, align 4
+  ret void
+}