defaults.h: Default MAX_FIXED_MODE_SIZE to MAX (BITS_PER_WORD * 2, 64)

The old GET_MODE_SIZE (DImode) (i.e. 64) made sense before
64-bitters.  Now the default is just a trap: when using the
default 64, things like TImode (128 bits) still mostly works,
but general corner cases related to computing large-size objects
numbers, like (1 << 64)/8 break, as exposed by
gcc.dg/pr105094.c.

So, keep the floor at 64 for 32-bitters and smaller targets, but
for larger targets, make it 2 * BITS_PER_WORD.  Also, express it
more directly with focus on BITS_PER_WORD, not the size of a
mode.  Add "by GCC internally" in an attempt to tell that this
is when gcc cooks something up, not when plain input uses a type
with such a mode.

	* defaults.h (MAX_FIXED_MODE_SIZE): Default to 2 * BITS_PER_WORD
	for larger-than-32-bitters.
	* doc/tm.texi.in (MAX_FIXED_MODE_SIZE): Adjust accordingly.  Tweak
	wording.
	* doc/tm.texi: Regenerate.

Author: Hans-Peter Nilsson

gcc/defaults.h

@@ -1158,7 +1158,7 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 #endif
 
 #ifndef MAX_FIXED_MODE_SIZE
-#define MAX_FIXED_MODE_SIZE GET_MODE_BITSIZE (DImode)
+#define MAX_FIXED_MODE_SIZE MAX (BITS_PER_WORD * 2, 64)
 #endif
 
 /* Nonzero if structures and unions should be returned in memory.

gcc/doc/tm.texi

@@ -1424,10 +1424,10 @@ the smaller of @var{computed} and @code{BIGGEST_ALIGNMENT}
 
 @defmac MAX_FIXED_MODE_SIZE
 An integer expression for the size in bits of the largest integer
-machine mode that should actually be used.  All integer machine modes of
-this size or smaller can be used for structures and unions with the
-appropriate sizes.  If this macro is undefined, @code{GET_MODE_BITSIZE
-(DImode)} is assumed.
+machine mode that should actually be used by GCC internally.
+All integer machine modes of this size or smaller can be used for
+structures and unions with the appropriate sizes.  If this macro is
+undefined, @code{MAX (BITS_PER_WORD * 2, 64)} is assumed.
 @end defmac
 
 @defmac STACK_SAVEAREA_MODE (@var{save_level})

gcc/doc/tm.texi.in

@@ -1251,10 +1251,10 @@ the smaller of @var{computed} and @code{BIGGEST_ALIGNMENT}
 
 @defmac MAX_FIXED_MODE_SIZE
 An integer expression for the size in bits of the largest integer
-machine mode that should actually be used.  All integer machine modes of
-this size or smaller can be used for structures and unions with the
-appropriate sizes.  If this macro is undefined, @code{GET_MODE_BITSIZE
-(DImode)} is assumed.
+machine mode that should actually be used by GCC internally.
+All integer machine modes of this size or smaller can be used for
+structures and unions with the appropriate sizes.  If this macro is
+undefined, @code{MAX (BITS_PER_WORD * 2, 64)} is assumed.
 @end defmac
 
 @defmac STACK_SAVEAREA_MODE (@var{save_level})