Re: Machine Description '.md'

[Alan Lehotsky <lehotsky at tiac dot net>]

At 12:31 PM -0700 6/7/00, Mike Stump wrote:
>..

..

>Yes or no, depending upon the design.  There are companies now that
>have reconfigurable CPU architectures, that use gcc for
>compilations.  One way to do it, would be to just add predicates to
>instructions (for example the entire floating point unit):
>
>/* Nonzero if we should generate code to use the fpu.  */
>#define MASK_FPU 1
>#define TARGET_FPU (target_flags & MASK_FPU)
>
>(define_expand "cmpsf"
>   ;; The 96 here isn't ever used by anyone.
>   [(set (reg:CCFP 96)
>   (compare:CCFP (match_operand:SF 0 "register_operand" "")
>		      (match_operand:SF 1 "register_operand" "")))]
>   "TARGET_FPU"
>   "

	Good concept, but in general a predefined NAMED pattern has
	to have a compile-time constant predicate, because they
	tend to appear in


	#if HAVE_XXXXXX
	...
	#endif

	preprocessor blocks.  I'm guessing that you've extracted code
	from a real .md file, so clearly the FP instructions aren't
	subject to this limitation....

	You can of course have the DEFINE_EXPAND's code body
	make runtime decisions about what sort of RTL to generate,
	and you can always have unnamed DEFINE_INSN patterns that
	are conditional on executable test expressions.

	RELATED THOUGHT:  It seems to me that there is NO circumstance
			where an anonymous DEFINE_EXPAND can be
			useful.  Right?

-- Al

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