Pass -gcodeview-ghash when using clang-cl and lld-link

[rnk]

Summary

This precomputes some hashes that LLD uses for type merging to speed up
linking when PDBs are enabled. Only do this if any kind of /DEBUG flag
is passed to the linker. -gcodeview-ghash is orthogonal to /Z7, -g, -g1,
or -gmlt, so it is safe to set it independently from those flags. It
will not increase debug info emission.

[aganea]

llvm/cmake/modules/HandleLLVMOptions.cmake

490

Do we need prior testing of the Clang version? I believe this won't work if using Clang 10 or before as a bootstrap compiler?

[rnk]

• do the feature check

[aganea]

Great, LGTM! Thanks Reid!

llvm/cmake/modules/HandleLLVMOptions.cmake

if( MSVC )
   if (LLVM_ENABLE_PDB AND uppercase_CMAKE_BUILD_TYPE STREQUAL "RELEASE")
     append("/Zi" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     # /DEBUG disables linker GC and ICF, but we want those in Release mode.
     append("/DEBUG /OPT:REF /OPT:ICF"
           CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS
           CMAKE_SHARED_LINKER_FLAGS)
   endif()
 
+  # Get all linker flags in upper case form so we can search them.
+  set(all_linker_flags_uppercase
+    "${CMAKE_EXE_LINKER_FLAGS} ${CMAKE_MODULE_LINKER_FLAGS} ${CMAKE_SHARED_LINKER_FLAGS}")
+  string(TOUPPER "${all_linker_flags_uppercase}" all_linker_flags_uppercase)
+
+  if (CLANG_CL AND LINKER_IS_LLD)
+    # If we are using clang-cl with lld-link and /debug is present in any of the
+    # linker flag variables, pass -gcodeview-ghash to the compiler to speed up
+    # linking. This flag is orthogonal from /Zi, /Z7, and other flags that
+    # enable debug info emission, and only has an effect if those are also in
+    # use.
+    string(FIND "${all_linker_flags_uppercase}" "/DEBUG" linker_flag_idx)
+    if (${linker_flag_idx} GREATER -1)
+      add_flag_if_supported("-gcodeview-ghash" GCODEVIEW_GHASH)
+    endif()
+  endif()
+
   # Disable string literal const->non-const type conversion.
   # "When specified, the compiler requires strict const-qualification
   # conformance for pointers initialized by using string literals."
   append("/Zc:strictStrings" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
   # "Generate Intrinsic Functions".
   append("/Oi" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT LLVM_ENABLE_LTO)
     # being unused, because we're not generating object files).
     # This checks CMAKE_CXX_COMPILER_ID in addition to check_cxx_compiler_flag()
     # because cl.exe does not emit an error on flags it doesn't understand,
     # letting check_cxx_compiler_flag() claim it understands all flags.
     check_cxx_compiler_flag("/Brepro" SUPPORTS_BREPRO)
     if (SUPPORTS_BREPRO)
       # Check if /INCREMENTAL is passed to the linker and complain that it
       # won't work with /Brepro.
-      string(TOUPPER "${CMAKE_EXE_LINKER_FLAGS}" upper_exe_flags)
-      string(TOUPPER "${CMAKE_MODULE_LINKER_FLAGS}" upper_module_flags)
-      string(TOUPPER "${CMAKE_SHARED_LINKER_FLAGS}" upper_shared_flags)
-
-      string(FIND "${upper_exe_flags} ${upper_module_flags} ${upper_shared_flags}"
-        "/INCREMENTAL" linker_flag_idx)
-
+      string(FIND "${all_linker_flags_uppercase}" "/INCREMENTAL" linker_flag_idx)
       if (${linker_flag_idx} GREATER -1)
         message(WARNING "/Brepro not compatible with /INCREMENTAL linking - builds will be non-deterministic")
       else()
         append("/Brepro" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
       endif()
     endif()
   endif()
   # By default MSVC has a 2^16 limit on the number of sections in an object file,