http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56937
Bug #: 56937
Summary: Unnecessarily temporary with array-vector assignments
Classification: Unclassified
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: fortran
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: bur...@gcc.gnu.org
For assignments of the form
array(vec_idx) = array(vec_idx) + nonaliasing_expr
no temporary should be generated.
In other words, the following program should print "1.0" and not "0.5". It
prints "1.0" with PGI and Intel (which do this optimization) and prints "0.5"
with pathf95, g95, gfortran and NAG, which don't:
real :: r(4), p(4)
integer :: idx(4)
p = [0.25, 0.25, 0.25, 0.25]
idx = [2,2,3,2]
r = 0
r(idx) = r(idx) + p
print *, sum(r)
end
The reason for the value is that gfortran (et al.) create a temporary and use:
tmp(2) = r(1) + 0.25 ! r(1:4) == 0
tmp(2) = r(2) + 0.25
tmp(3) = r(3) + 0.25
tmp(2) = r(4) + 0.25
r(2) = tmp(2) ! = 0.25
r(2) = tmp(2) ! = 0.25
r(3) = tmp(3) ! = 0.25
r(2) = tmp(2) ! = 0.25
-> sum(r) = 0.5
Without temporary one gets:
r(2) = 0 + 0.25
r(2) = (0.25) + 0.25
r(3) = 0 + 0.25
r(4) = (0.75) + 0.25
-> sum(r) = 1.0
The reason that the optimization is valid is given by the following restriction
in the Fortran standard:
"If a vector subscript has two or more elements with the same value, an array
section with that vector subscript shall not appear in a variable definition
context (16.6.7)." (Fortran 2008, 6.5.3.3.2 Vector subscript, paragraph 3)
