Only compute the X_COMPILER_*/X_LINKER_* variables when needed

When building CheriBSD we have to set XLD/XCC/XCFLAGS on the command line.
This triggers the $XCC != $CC case in bsd.compiler.mk (and the same for LD
in bsd.linker.mk) which causes it to call ${XCC} --version and
${XLD} --version (plus various awk+sed+echo calls) in every subdirectory.
For incremental builds and stages that only walk the source tree this is
often the majority of the time spent in that directory.

By only computing the value of the X_COMPILER_*/X_LINKER_* variables if
_WANT_TOOLCHAIN_CROSS_VARS is set we can reduce the number of cc/ld calls
to once per build stage instead of once per recursive make.

With this change (and no changes to the sources) the `make includes` stage
now takes 28 seconds at -j1 instead of 86 seconds.

Approved By:	brooks (mentor)
Differential Revision: https://reviews.freebsd.org/D17046

2 files changed, 17 insertions, 2 deletions

diff --git a/share/mk/bsd.compiler.mk b/share/mk/bsd.compiler.mk
index 3bbca41fdf86..a528bb02b2b3 100644
--- a/share/mk/bsd.compiler.mk
+++ b/share/mk/bsd.compiler.mk
@@ -116,7 +116,16 @@ ccache-print-options: .PHONY
 .endif	# exists(${CCACHE_BIN})
 .endif	# ${MK_CCACHE_BUILD} == "yes"
 
-.for cc X_ in CC $${_empty_var_} XCC X_
+_cc_vars=CC $${_empty_var_}
+.if !empty(_WANT_TOOLCHAIN_CROSS_VARS)
+# Only the toplevel makefile needs to compute the X_COMPILER_* variables.
+# Skipping the computation of the unused X_COMPILER_* in the subdirectory
+# makefiles can save a noticeable amount of time when walking the whole source
+# tree (e.g. during make includes, etc.).
+_cc_vars+=XCC X_
+.endif
+
+.for cc X_ in ${_cc_vars}
 .if ${cc} == "CC" || !empty(XCC)
 # Try to import COMPILER_TYPE and COMPILER_VERSION from parent make.
 # The value is only used/exported for the same environment that impacts
diff --git a/share/mk/bsd.linker.mk b/share/mk/bsd.linker.mk
index caf4fccbae0f..35cfe7128202 100644
--- a/share/mk/bsd.linker.mk
+++ b/share/mk/bsd.linker.mk
@@ -26,7 +26,13 @@
 .if !target(__<bsd.linker.mk>__)
 __<bsd.linker.mk>__:
 
-.for ld X_ in LD $${_empty_var_} XLD X_
+_ld_vars=LD $${_empty_var_}
+.if !empty(_WANT_TOOLCHAIN_CROSS_VARS)
+# Only the toplevel makefile needs to compute the X_LINKER_* variables.
+_ld_vars+=XLD X_
+.endif
+
+.for ld X_ in ${_ld_vars}
 .if ${ld} == "LD" || !empty(XLD)
 # Try to import LINKER_TYPE and LINKER_VERSION from parent make.
 # The value is only used/exported for the same environment that impacts