GNU Compiler Collection (GCC) 4.2 Internals

14.22.1.2 Substitution in Mode Macros

If an .md file construct uses mode macros, each version of the construct will often need slightly different strings or modes. For example:

• When a define_expand defines several addm3 patterns (see Standard Names), each expander will need to use the appropriate mode name for m.
• When a define_insn defines several instruction patterns, each instruction will often use a different assembler mnemonic.
• When a define_insn requires operands with different modes, using a macro for one of the operand modes usually requires a specific mode for the other operand(s).

GCC supports such variations through a system of “mode attributes”. There are two standard attributes: mode, which is the name of the mode in lower case, and MODE, which is the same thing in upper case. You can define other attributes using:

     (define_mode_attr name [(mode1 "value1") ... (moden "valuen")])

where name is the name of the attribute and valuei is the value associated with modei.

When GCC replaces some :macro with :mode, it will scan each string and mode in the pattern for sequences of the form <macro:attr>, where attr is the name of a mode attribute. If the attribute is defined for mode, the whole <...> sequence will be replaced by the appropriate attribute value.

For example, suppose an .md file has:

     (define_mode_macro P [(SI "Pmode == SImode") (DI "Pmode == DImode")])
     (define_mode_attr load [(SI "lw") (DI "ld")])

If one of the patterns that uses :P contains the string "<P:load>\t%0,%1", the SI version of that pattern will use "lw\t%0,%1" and the DI version will use "ld\t%0,%1".

Here is an example of using an attribute for a mode:

     (define_mode_macro LONG [SI DI])
     (define_mode_attr SHORT [(SI "HI") (DI "SI")])
     (define_insn ...
       (sign_extend:LONG (match_operand:<LONG:SHORT> ...)) ...)

The macro: prefix may be omitted, in which case the substitution will be attempted for every macro expansion.