[mlir][llvm] Improve access group import handling.

[gysit]

Summary

In particular, do not drop all access groups of a loop annotation
if one of them is unused. This change makes the access group import
robust against stale access group references in loop metadata.

[Dinistro]

LGTM! Thanks for fixing that

mlir/lib/Target/LLVMIR/LoopAnnotationImporter.cpp

LoopMetadataConversion::convertParallelAccesses() {
   FailureOr<SmallVector<llvm::MDNode *>> nodes =
       lookupMDNodes("llvm.loop.parallel_accesses");
   if (failed(nodes))
     return failure();
   SmallVector<SymbolRefAttr> refs;
   for (llvm::MDNode *node : *nodes) {
     FailureOr<SmallVector<SymbolRefAttr>> accessGroups =
         loopAnnotationImporter.lookupAccessGroupAttrs(node);
-    if (failed(accessGroups))
-      return emitWarning(loc) << "could not lookup access group";
+    if (failed(accessGroups)) {
+      emitWarning(loc) << "could not lookup access group";
+      continue;
+    }
     llvm::append_range(refs, *accessGroups);
   }
   return refs;
 }
 
 LoopAnnotationAttr LoopMetadataConversion::convert() {
   if (failed(initPropertyMap()))
     return {};

mlir/test/Target/LLVMIR/Import/import-failure.ll

 
 !0 = distinct !{!0, !1, !2}
 !1 = !{!"llvm.loop.disable_nonforced"}
 !2 = !{!"llvm.loop.typo"}
 
 ; // -----
 
 ; CHECK:      import-failure.ll
+; CHECK-SAME: warning: could not lookup access group
+define void @unused_access_group(ptr %arg) {
+entry:
+  %0 = load i32, ptr %arg, !llvm.access.group !0
+  br label %end, !llvm.loop !1
+end:
+  ret void
+}
+
+!0 = distinct !{}
+!1 = distinct !{!1, !2}
+!2 = !{!"llvm.loop.parallel_accesses", !0, !3}
+!3 = distinct !{}
+
+; // -----
+
+; CHECK:      import-failure.ll
 ; CHECK-SAME: warning: expected function_entry_count to be attached to a function
 ; CHECK:      warning: unhandled metadata: !0 = !{!"function_entry_count", i64 42}
 define void @cond_br(i1 %arg) {
 entry:
   br i1 %arg, label %bb1, label %bb2, !prof !0
 bb1:
   ret void
 bb2:

mlir/test/Target/LLVMIR/Import/metadata-loop.ll

 end:
   ret void
 }
 
 !0 = distinct !{}
 !1 = distinct !{!1, !2}
 !2 = !{!"llvm.loop.parallel_accesses", !0, !3}
 !3 = distinct !{}
+
+; // -----
+
+; Verify the unused access group is not imported.
+; CHECK:   llvm.metadata @__llvm_global_metadata {
+; CHECK-COUNT1: llvm.access_group
+
+; CHECK-LABEL: @unused_parallel_access
+define void @unused_parallel_access(ptr %arg) {
+entry:
+  %0 = load i32, ptr %arg, !llvm.access.group !0
+  br label %end, !llvm.loop !1
+end:
+  ret void
+}
+
+!0 = distinct !{}
+!1 = distinct !{!1, !2}
+!2 = !{!"llvm.loop.parallel_accesses", !0, !3}
+!3 = distinct !{}