Fix nvfortran two-pass IPO causing full rebuild on every build (#1345)

Author: sbryngelson

CMakeLists.txt

@@ -491,7 +491,32 @@ function(MFC_SETUP_TARGET)
     # Here we need to split into "library" and "executable" to perform IPO on the NVIDIA compiler.
     # A little hacky, but it *is* an edge-case for *one* compiler.
     if (NVHPC_USE_TWO_PASS_IPO AND NOT(MFC_OpenMP AND ARGS_OpenMP))
+        # nvfortran -Mextract does not produce .o files, only inline library
+        # data. An OBJECT library with -Mextract causes CMake to rebuild
+        # everything on every build because the expected .o outputs never
+        # exist. We use a wrapper script as RULE_LAUNCH_COMPILE that runs
+        # the compiler and then touches the expected .o output file.
+        set(_ipo_wrapper "${CMAKE_BINARY_DIR}/${ARGS_TARGET}_extract_wrapper.sh")
+        file(WRITE "${_ipo_wrapper}" [=[#!/bin/sh
+# Find the -o argument (the object file CMake expects)
+out=
+prev=
+for arg do
+  if [ "$prev" = "-o" ]; then out="$arg"; break; fi
+  prev="$arg"
+done
+# Run the compiler; propagate its exit status on failure
+"$@"
+status=$?
+[ "$status" -eq 0 ] || exit "$status"
+# Touch the .o so CMake's dependency tracking sees it
+[ -n "$out" ] && touch "$out"
+exit 0
+]=])
+        file(CHMOD "${_ipo_wrapper}" PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE)
         add_library(${ARGS_TARGET}_lib OBJECT ${ARGS_SOURCES})
+        set_target_properties(${ARGS_TARGET}_lib PROPERTIES
+            RULE_LAUNCH_COMPILE "${_ipo_wrapper}")
         target_compile_options(${ARGS_TARGET}_lib PRIVATE
 		    $<$<COMPILE_LANGUAGE:Fortran>:-Mextract=lib:${ARGS_TARGET}_lib>
 	        $<$<COMPILE_LANGUAGE:Fortran>:-Minline>