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Diffstat (limited to 'gcc/doc/md.texi')
| -rw-r--r-- | gcc/doc/md.texi | 75 |
1 files changed, 41 insertions, 34 deletions
diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi index 2f2325e..1d16e29 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: |