[flang] Inline array size call when dim is compile time constant

[razvanlupusoru]

Summary

Instead of calling _FortranASizeDim, we can instead load extent
directly from descriptor. Add this support for cases where dim
is a known constant at compile time.

[clementval]

Looks good. Can you just add a lit test for that in flang/test/Lower/Intrinsics/size.f90?

[razvanlupusoru]

Added lit test. Previously this test was calling _FortranASizeDim but now loads from descriptor directly.

[clementval]

LGTM

flang/lib/Optimizer/Builder/IntrinsicCall.cpp

 //===-- IntrinsicCall.cpp -------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
IntrinsicLibrary::genSize(mlir::Type resultType,
   // an array of SIZE with DIM in most cases, but it may not be
   // possible in some cases like when in SIZE(function_call()).
   if (isStaticallyAbsent(args, 1))
     return builder.createConvert(loc, resultType,
                                  fir::runtime::genSize(builder, loc, array));
 
   // Get the DIM argument.
   mlir::Value dim = fir::getBase(args[1]);
+  if (std::optional<std::int64_t> cstDim = fir::getIntIfConstant(dim)) {
+    // If it is a compile time constant, skip the runtime call.
+    return builder.createConvert(loc, resultType,
+                                 fir::factory::readExtent(builder, loc,
+                                                          fir::BoxValue{array},
+                                                          cstDim.value() - 1));
+  }
   if (!fir::isa_ref_type(dim.getType()))
     return builder.createConvert(
         loc, resultType, fir::runtime::genSizeDim(builder, loc, array, dim));
 
   mlir::Value isDynamicallyAbsent = builder.genIsNullAddr(loc, dim);
   return builder
       .genIfOp(loc, {resultType}, isDynamicallyAbsent,
                /*withElseRegion=*/true)