libctf: don't run tests requiring deduplicating linker unless one is in use

Before now, we were relying on gcc -B to repoint GCC at our freshly-built
linker, and assuming we could use this to run tests that do linker dedup.

This is, alas, not reliable (building --with-ld=/an/absolute/path bypasses
-B searches for the linker), and we would like the libctf tests in
particular to not fail if the compiler uses a linker that cannot deduplicate
(simply skipping such tests).

Add a new testsuite function that tries to link one of the lookup tests'
testcases (which is known to produce conflicting types, i.e. a dict with
per-translation-unit children) and then does an objdump and greps it for
signs of the per-translation-unit children.  If there are none, the linker
probably just concatenated the sections: i.e., it is not a linker that
can deduplicate CTF.

libctf/
	PR libctf/33162
	* testsuite/config/default.exp (objdump): Introduce.
	(OBJDUMPFLAGS): Likewise.
	* testsuite/lib/ctf-lib.exp (check_ctf_linker_dedup): New.
	* testsuite/libctf-lookup/lookup.exp: Use it.
	* testsuite/libctf-regression/libctf-repeat-cu.exp: Likewise.
	* testsuite/libctf-regression/regression.exp: Likewise.

libctf/testsuite/config/default.exp

@@ -34,6 +34,10 @@ if ![info exists as] then {
     set as [findfile $base_dir/../gas/as-new $base_dir/../gas/as-new [transform as]]
 }
 
+if ![info exists objdump] then {
+    set objdump [findfile $base_dir/../binutils/objdump]
+}
+
 remote_exec host "mkdir -p tmpdir"
 
 # Make symlinks from tmpdir/libctf to the linker and assembler in the
@@ -63,10 +67,12 @@ if {![info exists CFLAGS_FOR_TARGET]} {
 if ![info exists AR] then {
     set AR [findfile $base_dir/../binutils/ar]
 }
-
 if {![info exists OBJDUMP]} {
     set OBJDUMP [findfile $base_dir/../binutils/objdump]
 }
+if ![info exists OBJDUMPFLAGS] then {
+    set OBJDUMPFLAGS {}
+}
 
 # load the utility procedures
 load_lib ctf-lib.exp

libctf/testsuite/lib/ctf-lib.exp

@@ -20,6 +20,51 @@
 
 load_file $srcdir/../../ld/testsuite/lib/ld-lib.exp
 
+# Returns true if the target linker deduplicates CTF.
+proc check_ctf_linker_dedup { } {
+    global ctf_linker_dedup_saved
+
+    if {![info exists ctf_linker_dedup_saved]} {
+	set ctf_linker_dedup_saved 0
+
+	if ([check_ctf_available]) {
+	    global objdump srcdir
+
+	    set basename "tmpdir/ctf_linker_dedups[pid]"
+	    compile_one_cc $srcdir/libctf-lookup/ambiguous-struct-A.c ${basename}-A.o "-gctf -fPIC -c"
+	    compile_one_cc $srcdir/libctf-lookup/ambiguous-struct-B.c ${basename}-B.o "-gctf -fPIC -c"
+	    compile_one_cc "${basename}-A.o ${basename}-B.o" $basename.so "-gctf -fPIC -shared"
+	    if {! [remote_file host exists $basename.so] } {
+		return 0
+	    }
+
+	    # Don't use run_host_cmd: it dumps the entire output into the log,
+	    # even on success.
+	    set cmdret [remote_exec host [concat sh -c [list "$objdump --ctf $basename.so >dump.out 2>dump.err"]] "" "/dev/null"]
+	    set cmdret [lindex $cmdret 0]
+	    remote_upload host "dump.out"
+	    remote_upload host "dump.err"
+	    set dump_out [prune_warnings [file_contents "dump.out"]]
+	    set dump_err [prune_warnings [file_contents "dump.err"]]
+	    remote_file host delete "dump.out" "dump.err"
+	    remote_file build delete "dump.out" "dump.err"
+
+	    if {$cmdret != 0} {
+		verbose -log "failed with $cmdret: stderr: $dump_err"
+		verbose -log "output: $dump_out"
+		return 0;
+	    }
+
+	    remote_file host delete $basename.so ${basename}-A.o ${basename}-B.o
+	    if [regexp {CTF archive member: } $dump_out] {
+		set ctf_linker_dedup_saved 1
+	    }
+	}
+    }
+    return $ctf_linker_dedup_saved
+}
+
+
 proc run_native_host_cmd { command } {
     global link_output
     global ld

libctf/testsuite/libctf-lookup/lookup.exp

@@ -28,6 +28,11 @@ if {![check_ctf_available]} {
     return 0
 }
 
+if {![check_ctf_linker_dedup]} {
+    unsupported "no CTF deduplication support in the linker"
+    return 0
+}
+
 if {[info exists env(LC_ALL)]} {
     set old_lc_all $env(LC_ALL)
 }

libctf/testsuite/libctf-regression/libctf-repeat-cu.exp

@@ -37,6 +37,11 @@ if {![check_ctf_available]} {
     return 0
 }
 
+if {![check_ctf_linker_dedup]} {
+    unsupported "no CTF deduplication support in the linker"
+    return 0
+}
+
 if {[info exists env(LC_ALL)]} {
     set old_lc_all $env(LC_ALL)
 }

libctf/testsuite/libctf-regression/regression.exp

@@ -23,6 +23,11 @@ if {![check_ctf_available]} {
     return 0
 }
 
+if {![check_ctf_linker_dedup]} {
+    unsupported "no CTF deduplication support in the linker"
+    return 0
+}
+
 if ![is_elf_format] {
     unsupported "CTF needs bfd changes to be emitted on non-ELF"
     return 0