Fix compilation with upstream clang builtin headers.

By using -nobuiltininc and adding the clang builtin headers resource dir
to the end of the compiler header search path, we can still find headers
such as immintrin.h but find the FreeBSD version of stddef.h/stdarg.h/..
first.

This is a workaround until we are able to settle on and complete a plan
to harmonize guard macros with LLVM.  We've mostly worked out this on
FreeBSD systems by removing select headers from the installed set of
devel/llvm*, but that isn't a good solution for cross build.

Submitted by:	arichardson
Obtained from:	CheriBSD
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D17002

3 files changed, 22 insertions, 1 deletions

diff --git a/Makefile.inc1 b/Makefile.inc1
index e58c21e65ff7..926b4e5fd635 100644
--- a/Makefile.inc1
+++ b/Makefile.inc1
@@ -337,6 +337,7 @@ _TOOLCHAIN_METADATA_VARS=	COMPILER_VERSION \
 				COMPILER_TYPE \
 				COMPILER_FEATURES \
 				COMPILER_FREEBSD_VERSION \
+				COMPILER_RESOURCE_DIR \
 				LINKER_VERSION \
 				LINKER_FEATURES \
 				LINKER_TYPE \
diff --git a/share/mk/bsd.compiler.mk b/share/mk/bsd.compiler.mk
index 9e43d3617768..2902e8b01f29 100644
--- a/share/mk/bsd.compiler.mk
+++ b/share/mk/bsd.compiler.mk
@@ -143,7 +143,7 @@ _cc_vars+=XCC X_
 # The value is only used/exported for the same environment that impacts
 # CC and COMPILER_* settings here.
 _exported_vars=	${X_}COMPILER_TYPE ${X_}COMPILER_VERSION \
-		${X_}COMPILER_FREEBSD_VERSION
+		${X_}COMPILER_FREEBSD_VERSION ${X_}COMPILER_RESOURCE_DIR
 ${X_}_cc_hash=	${${cc}}${MACHINE}${PATH}
 ${X_}_cc_hash:=	${${X_}_cc_hash:hash}
 # Only import if none of the vars are set somehow else.
@@ -201,6 +201,10 @@ ${X_}COMPILER_FREEBSD_VERSION=	unknown
 .endif
 .endif
 
+.if !defined(${X_}COMPILER_RESOURCE_DIR)
+${X_}COMPILER_RESOURCE_DIR!=	${${cc}:N${CCACHE_BIN}} -print-resource-dir 2>/dev/null || echo unknown
+.endif
+
 ${X_}COMPILER_FEATURES=
 .if (${${X_}COMPILER_TYPE} == "clang" && ${${X_}COMPILER_VERSION} >= 30300) || \
 	(${${X_}COMPILER_TYPE} == "gcc" && ${${X_}COMPILER_VERSION} >= 40800)
@@ -224,6 +228,7 @@ X_COMPILER_TYPE=	${COMPILER_TYPE}
 X_COMPILER_VERSION=	${COMPILER_VERSION}
 X_COMPILER_FREEBSD_VERSION=	${COMPILER_FREEBSD_VERSION}
 X_COMPILER_FEATURES=	${COMPILER_FEATURES}
+X_COMPILER_RESOURCE_DIR=	${COMPILER_RESOURCE_DIR}
 .endif	# ${cc} == "CC" || (${cc} == "XCC" && ${XCC} != ${CC})
 
 # Export the values so sub-makes don't have to look them up again, using the
diff --git a/share/mk/bsd.sys.mk b/share/mk/bsd.sys.mk
index d5cce6418387..1c90d6a62def 100644
--- a/share/mk/bsd.sys.mk
+++ b/share/mk/bsd.sys.mk
@@ -206,6 +206,21 @@ CWARNFLAGS+=	-Wno-unknown-pragmas
 # This warning is utter nonsense
 CFLAGS+=	-Wno-format-zero-length
 
+.if ${COMPILER_TYPE} == "clang"
+# The headers provided by clang are incompatible with the FreeBSD headers.
+# If the version of clang is not one that has been patched to omit the
+# incompatible headers, we need to compile with -nobuiltininc and add the
+# resource dir to the end of the search paths. This ensures that headers such as
+# immintrin.h are still found but stddef.h, etc. are picked up from FreeBSD.
+#
+# XXX: This is a hack to support complete external installs of clang while
+# we work to synchronize our decleration guards with those in the clang tree.
+.if ${MK_CLANG_BOOTSTRAP} == "no" && ${COMPILER_RESOURCE_DIR} != "unknown" && \
+    !defined(BOOTSTRAPPING)
+CFLAGS+=-nobuiltininc -idirafter ${COMPILER_RESOURCE_DIR}/include
+.endif
+.endif
+
 CLANG_OPT_SMALL= -mstack-alignment=8 -mllvm -inline-threshold=3\
 		 -mllvm -simplifycfg-dup-ret
 .if ${COMPILER_VERSION} >= 30500 && ${COMPILER_VERSION} < 30700