+2003-12-30 Geoffrey Keating <geoffk@greed.local>
+
+ * doc/tm.texi (PREFERRED_RELOAD_CLASS): Describe use of NO_REGS
+ with constants.
+
2003-12-30 Mark Mitchell <mark@codesourcery.com>
* stor-layout.c (layout_decl): Turn bitfields into ordinary
@code{DATA_REGS} as long as @var{class} includes the data registers.
Requiring a data register guarantees that a @samp{moveq} will be used.
-If @var{x} is a @code{const_double}, by returning @code{NO_REGS}
-you can force @var{x} into a memory constant. This is useful on
-certain machines where immediate floating values cannot be loaded into
-certain kinds of registers.
+One case where @code{PREFERRED_RELOAD_CLASS} must not return
+@var{class} is if @var{x} is a legitimate constant which cannot be
+loaded into some register class. By returning @code{NO_REGS} you can
+force @var{x} into a memory location. For example, rs6000 can load
+immediate values into general-purpose registers, but does not have an
+instruction for loading an immediate value into a floating-point
+register, so @code{PREFERRED_RELOAD_CLASS} returns @code{NO_REGS} when
+@var{x} is a floating-point constant. If the constant can't be loaded
+into any kind of register, code generation will be better if
+@code{LEGITIMATE_CONSTANT_P} makes the constant illegitimate instead
+of using @code{PREFERRED_RELOAD_CLASS}.
@end defmac
@defmac PREFERRED_OUTPUT_RELOAD_CLASS (@var{x}, @var{class})