Two weeks ago I submitted the patch, available at:
http://gcc.gnu.org/ml/fortran/2013-10/msg00022.html ; while the ME patch
is not yet approved, the C FE was approved (latest C/ME patch:
http://gcc.gnu.org/ml/gcc-patches/2013-10/msg01752.html).
Additionally, I'd like to early ping for:
http://gcc.gnu.org/ml/fortran/2013-10/msg00068.html
Tobias
PS: Actually, safelen (i.e. "GCC ivdep", "omp simd", Cilk's "simd") does
not require that the result is independent of the loop-waking order.
Just (quoting from my C patch for GCC ivdep): "With this pragma, the
programmer asserts that there are no loop-carried dependencies which
would prevent that consecutive iterations of the following loop can be
executed concurrently with SIMD (single instruction multiple data)
instructions."
Fortran's do concurrent requires more: "The DO CONCURRENT construct
provides a means for the program to specify that individual loop
iterations have no interdependencies." (F2008, Introduction). Still, in
terms of optimizing "do concurrent", setting safelen gives the ME all
required information.
On Oct 08, 2013, Tobias Burnus wrote:
This patch requires my pending ME and C FE patch:
http://gcc.gnu.org/ml/gcc-patches/2013-10/msg00514.html
Using C/C++'s #pragma ivdep or – with the attached Fortran patch – "do
concurrent", the loop condition is annotated such that later the
loop's vectorization safelen is set to infinity (well, INT_MAX). The
main purpose is to tell the compiler that the result is independent of
the order in which the loop is walked. The typical case is pointer
aliasing, in which the compiler either doesn't vectorize or adds a
run-time aliasing check (loop versioning). With the annotation, the
compiler simply assumes that there is no aliasing and avoids the
versioning. – Contrary to C++ which does not even have the "restrict"
qualifier (gcc and others do support __restrict) and cases where
C/C++'s __restrict qualifier isn't sufficient/applicable, the effect
on typical Fortran code should be smaller as most variables cannot
alias. Still, in some cases it can help. (See test case for an example.)
There is an alternative to ivdep, which is more lower level [1]:
OpenMPv4's "omp simd" (with safelen=) for C/C++/Fortran and – for
C/C++ – Cilk Plus's #pragma simd.
Build and regtested on x86-64-gnu-linux.
OK for the trunk?
Tobias
PS: I think the same annotation could be also used with FORALL and
implied loops with whole-array/array-section assignments, when the FE
knows that there is no aliasing between the LHS and RHS side. (In some
cases, the FE knows this while the ME doesn't.)
PPS: My personal motivation is my long-standing wish to pass this
information to the middle end for DO CONCURRENT but also to use the
pragma for a specific C++ code.
[1] The OpenMPv4 support for C/C++ will be merged soon, for Fortran it
will take a while (maybe still in 4.9, maybe only later). See
http://gcc.gnu.org/ml/gcc-patches/2013-10/msg00502.html / The relevant
Cilk Plus patch has been posted at
http://gcc.gnu.org/ml/gcc-patches/2013-08/msg01626.html
