Use __builtin_assume_aligned on untagged Lisp vals

* src/conf_post.h (__has_builtin, __builtin_assume_aligned):
New macros, for compilers not already defining them.
(__has_builtin___builtin_assume_aligned): New macro.
* src/lisp.h (lisp_h_XUNTAG): Use __builtin_assume_aligned.
This shrinks text space by 0.2% on x86-64 with GCC 6.1.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 1fc6130be0b..4042f4decb1 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -341,7 +341,9 @@ error !;
      (struct Lisp_Symbol *) ((intptr_t) XLI (a) - Lisp_Symbol \
                              + (char *) lispsym))
 # define lisp_h_XTYPE(a) ((enum Lisp_Type) (XLI (a) & ~VALMASK))
-# define lisp_h_XUNTAG(a, type) ((void *) (intptr_t) (XLI (a) - (type)))
+# define lisp_h_XUNTAG(a, type) \
+    __builtin_assume_aligned ((void *) (intptr_t) (XLI (a) - (type)), \
+                              GCALIGNMENT)
 #endif
 
 /* When compiling via gcc -O0, define the key operations as macros, as