Fix: allow DSP builds without C_PACK by falling back to the configured LAPACK provider

source/CMakeLists.txt

@@ -157,21 +157,25 @@ target_link_libraries(abacus_link_libs INTERFACE abacus::feature_libs
 set(_abacus_linalg_libs)
 set(_abacus_linalg_include_dirs)
 
+# C_PACK is optional for DSP builds. With MT_CPACK_DIR set, the serial
+# generalized eigensolver is offloaded to cpack_zhegvx_driver from libc_pack.a;
+# otherwise the fallback in source_hsolver/cpack_zhegvx_fallback.cpp forwards it
+# to the configured LAPACK provider's zhegvx_ (see issue #8247).
 if(USE_DSP)
-  if(NOT MT_CPACK_DIR)
-    message(
-      FATAL_ERROR
-        "USE_DSP requires MT_CPACK_DIR to be set to the DSP C_PACK install prefix "
-        "(the directory containing lib/libc_pack.a).")
-  endif()
-  set(_abacus_cpack_library "${MT_CPACK_DIR}/lib/libc_pack.a")
-  if(NOT EXISTS "${_abacus_cpack_library}")
-    message(
-      FATAL_ERROR
-        "MT_CPACK_DIR is set to '${MT_CPACK_DIR}' but '${_abacus_cpack_library}' does not exist."
-    )
-  endif()
-  list(APPEND _abacus_linalg_libs ${_abacus_cpack_library})
+  if(MT_CPACK_DIR)
+    set(_abacus_cpack_library "${MT_CPACK_DIR}/lib/libc_pack.a")
+    if(NOT EXISTS "${_abacus_cpack_library}")
+      message(
+        FATAL_ERROR
+          "MT_CPACK_DIR is set to '${MT_CPACK_DIR}' but '${_abacus_cpack_library}' does not exist."
+      )
+    endif()
+    list(APPEND _abacus_linalg_libs ${_abacus_cpack_library})
+  else()
+    message(STATUS
+      "USE_DSP without MT_CPACK_DIR: C_PACK disabled, the serial generalized "
+      "eigensolver falls back to the configured LAPACK provider (zhegvx).")
+  endif()
 endif()
 
 if(USE_KML)

source/source_hsolver/CMakeLists.txt

@@ -35,6 +35,14 @@ if(ENABLE_LCAO)
     )
   endif ()
 
+  # DSP build without C_PACK: provide the zhegvx_ fallback for
+  # cpack_zhegvx_driver (used by diago_lapack.cpp under __DSP).
+  if(USE_DSP AND NOT MT_CPACK_DIR)
+    list(APPEND objects
+        cpack_zhegvx_fallback.cpp
+    )
+  endif()
+
   if(USE_CUDA)
       set(cuda_objects
           ./kernels/hegvd_op.cpp

source/source_hsolver/cpack_zhegvx_fallback.cpp

@@ -0,0 +1,64 @@
+// Fallback for DSP builds without C_PACK (MT_CPACK_DIR unset).
+//
+// diago_lapack.cpp calls cpack_zhegvx_driver under __DSP. The symbol normally
+// comes from libc_pack.a; when C_PACK is not linked, this translation unit
+// provides it by forwarding to the configured LAPACK provider's zhegvx_. CMake adds this
+// file only when USE_DSP is on and MT_CPACK_DIR is unset, so it never coexists
+// with libc_pack.a. See issue #8247.
+
+#include "source_base/module_external/lapack_connector.h"
+
+#ifdef __DSP
+
+void cpack_zhegvx_driver(const int* itype,
+                         const char* jobz,
+                         const char* range,
+                         const char* uplo,
+                         const int* n,
+                         std::complex<double>* a,
+                         const int* lda,
+                         std::complex<double>* b,
+                         const int* ldb,
+                         const double* vl,
+                         const double* vu,
+                         const int* il,
+                         const int* iu,
+                         const double* abstol,
+                         int* m,
+                         double* w,
+                         std::complex<double>* z,
+                         const int* ldz,
+                         std::complex<double>* work,
+                         const int* lwork,
+                         double* rwork,
+                         int* iwork,
+                         int* ifail,
+                         int* info)
+{
+    zhegvx_(itype,
+            jobz,
+            range,
+            uplo,
+            n,
+            a,
+            lda,
+            b,
+            ldb,
+            vl,
+            vu,
+            il,
+            iu,
+            abstol,
+            m,
+            w,
+            z,
+            ldz,
+            work,
+            lwork,
+            rwork,
+            iwork,
+            ifail,
+            info);
+}
+
+#endif

source/source_hsolver/unittests/CMakeLists.txt

@@ -69,17 +69,22 @@ if (ENABLE_MPI)
   )
 
   if(ENABLE_LCAO)
+    # DSP build without C_PACK: diago_lapack.cpp then needs the zhegvx_
+    # fallback for cpack_zhegvx_driver.
+    if(USE_DSP AND NOT MT_CPACK_DIR)
+      set(_dsp_cpack_fallback_src ../cpack_zhegvx_fallback.cpp)
+    endif()
     if(TARGET ELPA::ELPA)
     AddTest(
       TARGET MODULE_HSOLVER_LCAO
       LIBS parameter  ELPA::ELPA base genelpa psi device
-      SOURCES test_diago_lcao.cpp ../diago_elpa.cpp ../diago_elpa_native.cpp ../diago_scalapack.cpp ../diago_lapack.cpp
+      SOURCES test_diago_lcao.cpp ../diago_elpa.cpp ../diago_elpa_native.cpp ../diago_scalapack.cpp ../diago_lapack.cpp ${_dsp_cpack_fallback_src}
     )
     else()
       AddTest(
         TARGET MODULE_HSOLVER_LCAO
         LIBS parameter  base psi device
-        SOURCES test_diago_lcao.cpp ../diago_scalapack.cpp ../diago_lapack.cpp
+        SOURCES test_diago_lcao.cpp ../diago_scalapack.cpp ../diago_lapack.cpp ${_dsp_cpack_fallback_src}
       )
     endif()
 

source/source_hsolver/unittests/test_diago_lcao.cpp

@@ -491,6 +491,7 @@ INSTANTIATE_TEST_SUITE_P(
         DiagoPrepare<std::complex<double>>(0, 0, 1, 0, "genelpa", "H-KPoints-Si2.dat", "S-KPoints-Si2.dat"),
 #endif
         // DiagoPrepare<std::complex<double>>(0, 0, 32, 0, "genelpa", "H-KPoints-Si64.dat", "S-KPoints-Si64.dat"),
+        DiagoPrepare<std::complex<double>>(0, 0, 1, 0, "lapack", "H-KPoints-Si2.dat", "S-KPoints-Si2.dat"),
         DiagoPrepare<std::complex<double>>(0, 0, 1, 0, "scalapack_gvx", "H-KPoints-Si2.dat", "S-KPoints-Si2.dat"),
         DiagoPrepare<std::complex<double>>(0, 0, 32, 0, "scalapack_gvx", "H-KPoints-Si64.dat", "S-KPoints-Si64.dat")));