tm.texi: document STACK_SAVEAREA_MODE.

md.texi: update save_stack_* pattern description.

From-SVN: r20844

gcc/md.texi

@@ -2412,14 +2412,15 @@ such required data.
 @end enumerate
 
 When saving the stack pointer, operand 0 is the save area and operand 1
-is the stack pointer.  The mode used to allocate the save area is the
-mode of operand 0.  You must specify an integral mode, or
-@code{VOIDmode} if no save area is needed for a particular type of save
-(either because no save is needed or because a machine-specific save
-area can be used).  Operand 0 is the stack pointer and operand 1 is the
-save area for restore operations.  If @samp{save_stack_block} is
-defined, operand 0 must not be @code{VOIDmode} since these saves can be
-arbitrarily nested.
+is the stack pointer.  The mode used to allocate the save area defaults
+to @code{Pmode} but you can override that choice by defining the
+@code{STACK_SAVEAREA_MODE} macro (@pxref{Storage_Layout}).  You must
+specify an integral mode, or @code{VOIDmode} if no save area is needed
+for a particular type of save (either because no save is needed or
+because a machine-specific save area can be used).  Operand 0 is the
+stack pointer and operand 1 is the save area for restore operations.  If
+@samp{save_stack_block} is defined, operand 0 must not be
+@code{VOIDmode} since these saves can be arbitrarily nested.
 
 A save area is a @code{mem} that is at a constant offset from
 @code{virtual_stack_vars_rtx} when the stack pointer is saved for use by

gcc/tm.texi

@@ -979,6 +979,19 @@ 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.
 
+@findex STACK_SAVEAREA_MODE
+@item STACK_SAVEAREA_MODE (@var{save_level})
+If defined, an expression of type @code{enum machine_mode} that
+specifies the mode of a @code{save_stack_@var{level}} named pattern
+(@pxref{Standard Names}).  @var{save_level} is one of @code{SAVE_BLOCK},
+@code{SAVE_FUNCTION}, or @code{SAVE_NONLOCAL} and selects which of the
+three named patterns is having its mode specified.
+
+You need not define this macro if it always returns @code{Pmode}.  You
+would most commonly define this macro if the
+@code{save_stack_@var{level}} patterns need to support both a 32- and a
+64-bit mode.
+
 @findex CHECK_FLOAT_VALUE
 @item CHECK_FLOAT_VALUE (@var{mode}, @var{value}, @var{overflow})
 A C statement to validate the value @var{value} (of type