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

Makefile.inc1

@@ -103,8 +103,11 @@ MK_GCC_BOOTSTRAP=	no
 # here since we will always have the right make, unlike in src/Makefile
 # Don't include bsd.linker.mk yet until XBINUTILS is handled (after src.opts.mk)
 _NO_INCLUDE_LINKERMK=	t
+# We also want the X_COMPILER* variables if we are using an external toolchain.
+_WANT_TOOLCHAIN_CROSS_VARS=	t
 .include "share/mk/bsd.compiler.mk"
 .undef _NO_INCLUDE_LINKERMK
+.undef _WANT_TOOLCHAIN_CROSS_VARS
 # src.opts.mk depends on COMPILER_FEATURES
 .include "share/mk/src.opts.mk"
 
@@ -205,7 +208,10 @@ X${BINUTIL}?=	${${BINUTIL}}
 MK_LLD_BOOTSTRAP=	no
 .endif
 
+# We also want the X_LINKER* variables if we are using an external toolchain.
+_WANT_TOOLCHAIN_CROSS_VARS=	t
 .include "share/mk/bsd.linker.mk"
+.undef _WANT_TOOLCHAIN_CROSS_VARS
 
 # Begin WITH_SYSTEM_COMPILER / WITH_SYSTEM_LD
 

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

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