[patch, fortran] PR20923 gfortran slow for large array constructors

[Jerry DeLisle]

Hi folks,

This patch cuts the compilation time for some programs with large array 
constructors in half.  It does this by traversing the constructor and only 
calling the more complicated expand_constructor for expression types that are 
EXPR_ARRAY.

Since gfc_constant_ac can be called recursively, this simplification indeed has 
significant benefit.

I do not see a real need for another test case since all it would accomplish is 
lengthen the time it takes to run the test suite.  One interesting side effect 
of this was an ICE in gfc_conv_array_constructor_expr, at 
fortran/trans-expr.c:3832.  This occurs with the following test case where the 
limit of 65535 is exceeded.

integer, parameter :: n=65536
integer, parameter :: i(n)=(/(mod(k,2),k=1,n)/)
integer, parameter :: m(n)=i(n:1:-1)
print *, i(1), m(1), i(n), m(n)
end

This takes several minutes before hitting the error:

integer, parameter :: i(n)=(/(mod(k,2),k=1,n)/)
                            1
Error: The number of elements in the array constructor at (1) requires an 
increase of the allowed 65535 upper limit.   See -fmax-array-constructor option

The ICE occurs after the error message.

I tried several ways to avoid the ICE with no avail.  I opted to change the 
error to a gfc_fatal_error.  If one thinks about it, this really is a fatal error.

The test case which shows the improvement is:

program sel
     implicit none
     integer,parameter              :: n=1000
     integer                        :: i,j
     real,dimension(n*n) :: vect
     vect(:) = (/ ((( (i+j+3)),i=1,n),j=1,n) /)
end

$ time gfc44 pr20923.f90

real	0m18.616s
user	0m17.173s
sys	0m0.063s

$ time gfc pr20923.f90

real	0m9.467s
user	0m8.764s
sys	0m0.044s

Regression tested on x86-64=linux-gnu.  OK for trunk?  Is this sufficient to 
close the PR or shall I leave it open?

Regards,

Jerry

2009-12-04  Jerry DeLisle  <jvdelisle@gcc.gnu.org>

	PR fortran/20923
	* trans-array.c (gfc_conv_array_initializer): Change gfc_error_now to
	gfc_fatal_error.
	* array.c (count_elements): Whitespace. (extract_element): Whitespace.
	(is_constant_element): Changed name from constant_element.
	(gfc_constant_element): Only use expand_construuctor for expression
	types of EXPR_ARRAY.  If expression type is EXPR_CONSTANT, no need to
	call gfc_is_constant_expr.
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