[flang][openacc] Lower acc cache directive (#65673)

The cache directive is attached directly to the acc.loop operation when
the directive appears in the loop. When it appears before a loop, the
OpenACCCacheConstruct is saved and attached when the acc.loop is
created.

Directive that cannot be attached to a loop are silently discarded. 

Depends on #65521

flang/lib/Lower/OpenACC.cpp

@@ -3096,6 +3096,41 @@ void Fortran::lower::finalizeOpenACCRoutineAttachment(
   accRoutineInfos.clear();
 }
 
+static void
+genACC(Fortran::lower::AbstractConverter &converter,
+       Fortran::semantics::SemanticsContext &semanticsContext,
+       const Fortran::parser::OpenACCCacheConstruct &cacheConstruct) {
+  fir::FirOpBuilder &builder = converter.getFirOpBuilder();
+  auto loopOp = builder.getRegion().getParentOfType<mlir::acc::LoopOp>();
+  auto crtPos = builder.saveInsertionPoint();
+  if (loopOp) {
+    builder.setInsertionPoint(loopOp);
+    Fortran::lower::StatementContext stmtCtx;
+    llvm::SmallVector<mlir::Value> cacheOperands;
+    const Fortran::parser::AccObjectListWithModifier &listWithModifier =
+        std::get<Fortran::parser::AccObjectListWithModifier>(cacheConstruct.t);
+    const auto &accObjectList =
+        std::get<Fortran::parser::AccObjectList>(listWithModifier.t);
+    const auto &modifier =
+        std::get<std::optional<Fortran::parser::AccDataModifier>>(
+            listWithModifier.t);
+
+    mlir::acc::DataClause dataClause = mlir::acc::DataClause::acc_cache;
+    if (modifier &&
+        (*modifier).v == Fortran::parser::AccDataModifier::Modifier::ReadOnly)
+      dataClause = mlir::acc::DataClause::acc_cache_readonly;
+    genDataOperandOperations<mlir::acc::CacheOp>(
+        accObjectList, converter, semanticsContext, stmtCtx, cacheOperands,
+        dataClause,
+        /*structured=*/true, /*implicit=*/false, /*setDeclareAttr*/ false);
+    loopOp.getCacheOperandsMutable().append(cacheOperands);
+  } else {
+    llvm::report_fatal_error(
+        "could not find loop to attach OpenACC cache information.");
+  }
+  builder.restoreInsertionPoint(crtPos);
+}
+
 void Fortran::lower::genOpenACCConstruct(
     Fortran::lower::AbstractConverter &converter,
     Fortran::semantics::SemanticsContext &semanticsContext,
@@ -3119,8 +3154,7 @@ void Fortran::lower::genOpenACCConstruct(
             genACC(converter, semanticsContext, standaloneConstruct);
           },
           [&](const Fortran::parser::OpenACCCacheConstruct &cacheConstruct) {
-            TODO(converter.genLocation(cacheConstruct.source),
-                 "OpenACC Cache construct not lowered yet!");
+            genACC(converter, semanticsContext, cacheConstruct);
           },
           [&](const Fortran::parser::OpenACCWaitConstruct &waitConstruct) {
             genACC(converter, waitConstruct);

flang/test/Lower/OpenACC/acc-loop.f90

@@ -305,4 +305,13 @@ program acc_loop
 !CHECK:        acc.yield
 !CHECK-NEXT: }{{$}}
 
+  !$acc loop
+  DO i = 1, n
+    !$acc cache(b)
+    a(i) = b(i)
+  END DO
+
+! CHECK: %[[CACHE:.*]] = acc.cache varPtr(%{{.*}} : !fir.ref<!fir.array<10xf32>>) bounds(%{{.*}}) -> !fir.ref<!fir.array<10xf32>> {name = "b"}
+! CHECK: acc.loop cache(%[[CACHE]] : !fir.ref<!fir.array<10xf32>>)
+
 end program