[Patch, fortran] PR4773 - [4.6 Regression] Unnecessary temporaries increase the runtime for channel.f90 by ~70%

[Paul Richard Thomas]

Revision 161670 affected an optimization used for

array_lhs = array_valued_function (args...).

In order to prevent aliasing with the result, which is passed by
reference, a fairly conservative test was applied to generate a
temporary, if needed.  Unfortunately, this reduced the performance of
gfortran with some code; most notably with channel.f90.

This patch tests if the 'array_lhs' has ever been host associated, on
condition that 'array_valued_function' is contained, in addition to
checking that the result is not use-associated, a pointer or a target.
 If this is not the case, no temporary is generated.

Clearly some further optimization would be possible, if the
'array_lhs' were specifically not host associated to
'array_valued_function' OR ANY OF IT'S CALLEEs.   This latter
condition is rather difficult to implement, which is why I have not
even tried it.

Bootstrapped and regtested on RHEL5.3/i686 - OK for trunk and.....?

Cheers

Paul

2010-07-09  Paul Thomas  <pault@gcc.gnu.org>

	PR fortran/44773
	* trans-expr.c (arrayfunc_assign_needs_temporary): No temporary
	if the lhs has never been host associated, as well as not being
	use associated, a pointer or a target.
	* resolve.c (resolve_variable): Mark variables that are host
	associated.
	* gfortran.h: Add the host_assoc bit to the symbol_attribute
	structure.
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