fix(bench): separate vlibc-under-test TU from host harness TUs

benchmarks/Makefile.am

@@ -10,18 +10,30 @@ EXTRA_PROGRAMS = bench_vlibc bench_string
 bench_vlibc_SOURCES = bench_vlibc.c
 bench_string_SOURCES = bench_string.c
 
-# bench_string is a system-headers-only translation unit (it times whichever
-# libc it is linked against), so it must NOT inherit AM_CPPFLAGS: -I include
-# would pull vlibc's self-contained <stddef.h>/<string.h> into the same TU as
-# the system <stdio.h>/<time.h> and double-define size_t/NULL/offsetof.
-# bench_vlibc keeps the inherited -I include.
+# Both harness TUs are system-headers-only translation units (they time
+# whichever libc they are linked against), so they must NOT inherit
+# AM_CPPFLAGS: -I include would pull vlibc's self-contained
+# <stdarg.h>/<stddef.h>/<string.h> into the same TU as the system
+# <stdio.h>/<time.h> and double-define size_t/NULL/offsetof (or hide the
+# compiler's internal <stdarg.h> behind vlibc's). The vlibc function under
+# test is reached through the adapter TU bench_vlibc_under.c, which is
+# compiled with vlibc's headers and exposes the call to the harness through
+# its own declaration (see that file).
+bench_vlibc_CPPFLAGS =
 bench_string_CPPFLAGS =
 
+# Adapter TU for bench_vlibc: vlibc's headers only, so it must NOT inherit
+# the (empty) per-target CPPFLAGS above; it gets the include path back
+# explicitly.
+bench_vlibc_under.o: bench_vlibc_under.c
+	$(AM_V_CC)$(CC) -I$(top_srcdir)/include $(CPPFLAGS) $(AM_CFLAGS) \
+	    $(CFLAGS) -c -o $@ $<
+
 # By default benchmarks link against vlibc itself. --with-libc=glibc links the
 # same harness against the system glibc; --with-libc=musl builds it with
 # musl-gcc for the musl reference.
 if BENCH_LINK_VLIBC
-bench_vlibc_LDADD = ../libvlibc.la
+bench_vlibc_LDADD = bench_vlibc_under.o ../libvlibc.la
 bench_string_LDADD = ../libvlibc.la
 else
 bench_vlibc_LDADD =
@@ -38,6 +50,12 @@ endif
 # binary from the previous configuration and the wrong libc would be measured.
 EXTRA_bench_string_DEPENDENCIES = $(top_builddir)/config.status
 EXTRA_bench_vlibc_DEPENDENCIES = $(top_builddir)/config.status
+bench_vlibc_DEPENDENCIES = bench_vlibc_under.o
+
+# The adapter TU is built by a hand-rolled rule (per-TU include paths), so it
+# is not picked up by automake's automatic distribution/cleaning.
+EXTRA_DIST = bench_vlibc_under.c
+CLEANFILES = bench_vlibc_under.o
 
 # bench_vlibc.c calls vlibc_version(), which exists only in vlibc — under
 # --with-libc=glibc/musl it cannot link, so it is built and run only in the