[libclc] [libclc] Support the generic address space (PR #137183)

[Matt Arsenault via cfe-commits]

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@@ -420,12 +420,37 @@ foreach( t ${LIBCLC_TARGETS_TO_BUILD} )
       -D${CLC_TARGET_DEFINE}
       # All libclc builtin libraries see CLC headers
       -I${CMAKE_CURRENT_SOURCE_DIR}/clc/include
+      # Error on undefined macros
+      -Werror=undef
     )
 
     if( NOT "${cpu}" STREQUAL "" )
       list( APPEND build_flags -mcpu=${cpu} )
     endif()
 
+    # Generic address space support.
+    # Note: when declaring builtins, we must consider that even if a target
+    # formally/nominally supports the generic address space, in practice that
+    # target may map it to the same target address space as another address
+    # space (often the private one). In such cases we must be careful not to
+    # multiply-define a builtin in a single target address space, as it would
+    # result in a mangling clash.
+    # For this reason we must consider the target support of the generic
+    # address space separately from the *implementation* decision about whether
+    # to declare certain builtins in that address space.
+    # FIXME: Shouldn't clang automatically enable this extension based on the
+    # target?
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arsenm wrote:

Yes, the extension should be reported as available or not by the target macros 

https://github.com/llvm/llvm-project/pull/137183