gcc/

	* doc/md.texi: Document "preferred_for_size" and "preferred_for_speed"
	attributes.
	* genattr.c (main): Handle "preferred_for_size" and
	"preferred_for_speed" in the same way as "enabled".
	* recog.h (bool_attr): New enum.
	(target_recog): Replace x_enabled_alternatives with x_bool_attr_masks.
	(get_preferred_alternatives, check_bool_attrs): Declare.
	* recog.c (have_bool_attr, get_bool_attr, get_bool_attr_mask_uncached)
	(get_bool_attr_mask, get_preferred_alternatives, check_bool_attrs):
	New functions.
	(get_enabled_alternatives): Use get_bool_attr_mask.
	* ira-costs.c (record_reg_classes): Use get_preferred_alternatives
	instead of recog_data.enabled_alternatives.
	* ira.c (ira_setup_alts): Likewise.
	* postreload.c (reload_cse_simplify_operands): Likewise.
	* config/i386/i386.c (ix86_legitimate_combined_insn): Likewise.
	* ira-lives.c (preferred_alternatives): New variable.
	(process_bb_node_lives): Set it.
	(check_and_make_def_conflict, make_early_clobber_and_input_conflicts)
	(single_reg_class, ira_implicitly_set_insn_hard_regs): Use it instead
	of recog_data.enabled_alternatives.
	* lra-int.h (lra_insn_recog_data): Replace enabled_alternatives
	to preferred_alternatives.
	* lra-constraints.c (process_alt_operands): Update accordingly.
	* lra.c (lra_set_insn_recog_data): Likewise.
	(lra_update_insn_recog_data): Assert check_bool_attrs.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@216554 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 41 insertions, 34 deletions

diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index 2f2325e907a..1d16e294711 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -1080,7 +1080,7 @@ the addressing register.
 * Class Preferences::   Constraints guide which hard register to put things in.
 * Modifiers::           More precise control over effects of constraints.
 * Machine Constraints:: Existing constraints for some particular machines.
-* Disable Insn Alternatives:: Disable insn alternatives using the @code{enabled} attribute.
+* Disable Insn Alternatives:: Disable insn alternatives using attributes.
 * Define Constraints::  How to define machine-specific constraints.
 * C Constraint Interface:: How to test constraints from C code.
 @end menu
@@ -4006,42 +4006,49 @@ Unsigned constant valid for BccUI instructions
 @subsection Disable insn alternatives using the @code{enabled} attribute
 @cindex enabled
 
-The @code{enabled} insn attribute may be used to disable insn
-alternatives that are not available for the current subtarget.
-This is useful when adding new instructions to an existing pattern
-which are only available for certain cpu architecture levels as
-specified with the @code{-march=} option.
+There are three insn attributes that may be used to selectively disable
+instruction alternatives:
 
-If an insn alternative is disabled, then it will never be used.  The
-compiler treats the constraints for the disabled alternative as
-unsatisfiable.
+@table @code
+@item enabled
+Says whether an alternative is available on the current subtarget.
 
-In order to make use of the @code{enabled} attribute a back end has to add
-in the machine description files:
+@item preferred_for_size
+Says whether an enabled alternative should be used in code that is
+optimized for size.
 
-@enumerate
-@item
-A definition of the @code{enabled} insn attribute.  The attribute is
-defined as usual using the @code{define_attr} command.  This
-definition should be based on other insn attributes and/or target flags.
-The attribute must be a static property of the subtarget; that is, it
-must not depend on the current operands or any other dynamic context
-(for example, the location of the insn within the body of a loop).
-
-The @code{enabled} attribute is a numeric attribute and should evaluate to
-@code{(const_int 1)} for an enabled alternative and to
-@code{(const_int 0)} otherwise.
-@item
-A definition of another insn attribute used to describe for what
-reason an insn alternative might be available or
-not.  E.g. @code{cpu_facility} as in the example below.
-@item
-An assignment for the second attribute to each insn definition
-combining instructions which are not all available under the same
-circumstances.  (Note: It obviously only makes sense for definitions
-with more than one alternative.  Otherwise the insn pattern should be
-disabled or enabled using the insn condition.)
-@end enumerate
+@item preferred_for_speed
+Says whether an enabled alternative should be used in code that is
+optimized for speed.
+@end table
+
+All these attributes should use @code{(const_int 1)} to allow an alternative
+or @code{(const_int 0)} to disallow it.  The attributes must be a static
+property of the subtarget; they cannot for example depend on the
+current operands, on the current optimization level, on the location
+of the insn within the body of a loop, on whether register allocation
+has finished, or on the current compiler pass.
+
+The @code{enabled} attribute is a correctness property.  It tells GCC to act
+as though the disabled alternatives were never defined in the first place.
+This is useful when adding new instructions to an existing pattern in
+cases where the new instructions are only available for certain cpu
+architecture levels (typically mapped to the @code{-march=} command-line
+option).
+
+In contrast, the @code{preferred_for_size} and @code{preferred_for_speed}
+attributes are strong optimization hints rather than correctness properties.
+@code{preferred_for_size} tells GCC which alternatives to consider when
+adding or modifying an instruction that GCC wants to optimize for size.
+@code{preferred_for_speed} does the same thing for speed.  Note that things
+like code motion can lead to cases where code optimized for size uses
+alternatives that are not preferred for size, and similarly for speed.
+
+Although @code{define_insn}s can in principle specify the @code{enabled}
+attribute directly, it is often clearer to have subsiduary attributes
+for each architectural feature of interest.  The @code{define_insn}s
+can then use these subsiduary attributes to say which alternatives
+require which features.  The example below does this for @code{cpu_facility}.
 
 E.g. the following two patterns could easily be merged using the @code{enabled}
 attribute: