middle-end: Drop __builtin_prefetch calls in autovectorization [PR114061]

At present the autovectorizer fails to vectorize simple loops
involving calls to `__builtin_prefetch'.  A simple example of such
loop is given below:

void foo(double * restrict a, double * restrict b, int n){
  int i;
  for(i=0; i<n; ++i){
    a[i] = a[i] + b[i];
    __builtin_prefetch(&(b[i+8]));
  }
}

The failure stems from two issues:

1. Given that it is typically not possible to fully reason about a
   function call due to the possibility of side effects, the
   autovectorizer does not attempt to vectorize loops which make such
   calls.

   Given the memory reference passed to `__builtin_prefetch', in the
   absence of assurances about its effect on the passed memory
   location the compiler deems the function unsafe to vectorize,
   marking it as clobbering memory in `vect_find_stmt_data_reference'.
   This leads to the failure in autovectorization.

2. Notwithstanding the above issue, though the prefetch statement
   would be classed as `vect_unused_in_scope', the loop invariant that
   is used in the address of the prefetch is the scalar loop's and not
   the vector loop's IV. That is, it still uses `i' and not `vec_iv'
   because the instruction wasn't vectorized, causing DCE to think the
   value is live, such that we now have both the vector and scalar loop
   invariant actively used in the loop.

This patch addresses both of these:

1. About the issue regarding the memory clobber, data prefetch does
   not generate faults if its address argument is invalid and does not
   write to memory.  Therefore, it does not alter the internal state
   of the program or its control flow under any circumstance.  As
   such, it is reasonable that the function be marked as not affecting
   memory contents.

   To achieve this, we add the necessary logic to
   `get_references_in_stmt' to ensure that builtin functions are given
   given the same treatment as internal functions.  If the gimple call
   is to a builtin function and its function code is
   `BUILT_IN_PREFETCH', we mark `clobbers_memory' as false.

2. Finding precedence in the way clobber statements are handled,
   whereby the vectorizer drops these from both the scalar and
   vectorized versions of a given loop, we choose to drop prefetch
   hints in a similar fashion.  This seems appropriate given how
   software prefetch hints are typically ignored by processors across
   architectures, as they seldom lead to performance gain over their
   hardware counterparts.

gcc/ChangeLog:

	PR tree-optimization/114061
	* tree-data-ref.cc (get_references_in_stmt): set
	`clobbers_memory' to false for __builtin_prefetch.
	* tree-vect-loop.cc (vect_transform_loop): Drop all
	__builtin_prefetch calls from loops.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-prefetch-drop.c: New test.
	* gcc.target/aarch64/vect-prefetch-drop.c: Likewise.

0 files changed, 0 insertions, 0 deletions