[PATCHv3 6/8] gdb/arch: assert that X86_XSTATE_MPX is not set for x32

[Andrew Burgess <aburgess@redhat.com>]

While trying to merge this commit:

  commit 4bb20a6244b7091a9a7a2ae35dfbd7e8db27550a
  Date:   Wed Mar 20 04:13:18 2024 -0700

      gdbserver: Clear X86_XSTATE_MPX bits in xcr0 on x32

With this patch series of mine:

  https://inbox.sourceware.org/gdb-patches/cover.1706801009.git.aburgess@redhat.com

I worried that there could be other paths that could result in an xcr0
value that has X86_XSTATE_MPX set in x32 mode.  As everyone eventually
calls amd64_create_target_description to build their target
description, I figured we could assert in here that if X86_XSTATE_MPX
is set then we should not be an x32 target, this should uncover any
other bugs in this area.

I'm not currently able to build/run any x32 binaries, so I have no way
to test this.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31511
---
 gdb/arch/amd64.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/gdb/arch/amd64.c b/gdb/arch/amd64.c
index b8ff4270079..f182e686dda 100644
--- a/gdb/arch/amd64.c
+++ b/gdb/arch/amd64.c
@@ -66,8 +66,12 @@ amd64_create_target_description (uint64_t xcr0, bool is_x32, bool is_linux,
   if (xcr0 & X86_XSTATE_AVX)
     regnum = create_feature_i386_64bit_avx (tdesc.get (), regnum);
 
-  if ((xcr0 & X86_XSTATE_MPX) && !is_x32)
-    regnum = create_feature_i386_64bit_mpx (tdesc.get (), regnum);
+  if (xcr0 & X86_XSTATE_MPX)
+    {
+      /* MPX is not available on x32.  */
+      gdb_assert (!is_x32);
+      regnum = create_feature_i386_64bit_mpx (tdesc.get (), regnum);
+    }
 
   if (xcr0 & X86_XSTATE_AVX512)
     regnum = create_feature_i386_64bit_avx512 (tdesc.get (), regnum);
-- 
2.25.4