[gdb/testsuite] Add have_mpx in lib/gdb.exp

The sources for the test-cases gdb.arch/i386-mpx*.exp contain have_mpx
functions that test whether the processor supports mpx instructions.

OTOH, the test-cases are compiled using -mmpx -fcheck-pointer-bounds, which
instrument all functions with mpx instructions.

So, the function that is supposed to test whether mpx instruction are
supported contains mpx instructions, which is a bit odd.

We could fix this by:
- factoring out the have_mpx function into a single source file, and
- compiling it without "-mmpx -fcheck-pointer-bounds".

But having the mpx support test as part of the test-cases seems like an
unnecessary complication that makes the test-cases more difficult to analyze,
reason about and modify.

So we go one step further and factor out the mpx support test in into a
gdb_caching_proc.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-12  Tom de Vries  <tdevries@suse.de>

	* gdb.arch/i386-mpx-call.c (have_mpx): Remove.
	(main): Remove call to have_mpx.
	* gdb.arch/i386-mpx-call.exp: Use have_mpx.
	* gdb.arch/i386-mpx-map.c (have_mpx): Remove.
	(main): Remote call to have_mpx.
	* gdb.arch/i386-mpx-map.exp: Use have_mpx.
	* gdb.arch/i386-mpx-sigsegv.c (have_mpx): Remove.
	(main): Remove call to have_mpx.
	* gdb.arch/i386-mpx-sigsegv.exp: Use have_mpx.
	* gdb.arch/i386-mpx-simple_segv.c (have_mpx): Remove.
	(main): Remove call to have_mpx.
	* gdb.arch/i386-mpx-simple_segv.exp: Use have_mpx.
	* gdb.arch/i386-mpx.c (have_mpx): Remove.
	(main): Remote call to have_mpx.
	* gdb.arch/i386-mpx.exp: Use have_mpx.
	* lib/gdb.exp (have_mpx): New proc.

1 files changed, 5 insertions, 18 deletions

diff --git a/gdb/testsuite/gdb.arch/i386-mpx-map.exp b/gdb/testsuite/gdb.arch/i386-mpx-map.exp
index a98194d..55b70d1 100644
--- a/gdb/testsuite/gdb.arch/i386-mpx-map.exp
+++ b/gdb/testsuite/gdb.arch/i386-mpx-map.exp
@@ -27,6 +27,11 @@ if { ![supports_mpx_check_pointer_bounds] } {
     return -1
 }
 
+if { ![have_mpx] } {
+    unsupported "processor does not support MPX"
+    return -1
+}
+
 set comp_flags "-mmpx -fcheck-pointer-bounds -I${srcdir}/../nat/"
 
 if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile} \
@@ -39,24 +44,6 @@ if ![runto_main] {
 	return -1
 }
 
-set supports_mpx 0
-set test "probe MPX support"
-
-gdb_test_multiple "print have_mpx()" $test {
-    -re ".. = 1\r\n$gdb_prompt $" {
-        pass $test
-        set supports_mpx 1
-    }
-    -re ".. = 0\r\n$gdb_prompt $" {
-        pass $test
-    }
-}
-
-if { !$supports_mpx } {
-    unsupported "processor does not support MPX"
-    return
-}
-
 gdb_breakpoint [ gdb_get_line_number "after-decl" ]
 gdb_breakpoint [ gdb_get_line_number "after-alloc" ]
 gdb_breakpoint [ gdb_get_line_number "after-assign" ]