tree-parloops: Enable runtime thread detection with -ftree-parallelize-loops

This patch adds runtime thread count detection to auto-parallelization.
-ftree-parallelize-loops option generates parallelized loops without
specifying a fixed thread count, deferring this decision to program execution
time where it is controlled by the OMP_NUM_THREADS environment variable.

Bootstrap and regression tested on aarch64-linux.  Compiled SPEC HPC pot3d
https://www.spec.org/hpc2021/docs/benchmarks/628.pot3d_s.html with
-ftree-parallelize-loops and tested without having OMP_NUM_THREADS set in the
environment and with OMP_NUM_THREADS set to different values.

gcc/ChangeLog:

	* doc/invoke.texi (ftree-parallelize-loops): Update.
	* common.opt (ftree-parallelize-loops): Add alias that maps to
	special value INT_MAX for runtime thread detection.
	* tree-parloops.cc (create_parallel_loop): Use INT_MAX for runtime
	detection.  Call gimple_build_omp_parallel without building a
	OMP_CLAUSE_NUM_THREADS clause.
	(gen_parallel_loop): For auto-detection, use a conservative
	estimate of 2 threads.
	(parallelize_loops): Same.

gcc/testsuite/ChangeLog:

	* gcc.dg/autopar/runtime-auto.c: New test.

Signed-off-by: Sebastian Pop <spop@nvidia.com>

gcc/common.opt

@@ -3303,6 +3303,10 @@ ftree-parallelize-loops=
 Common Joined RejectNegative UInteger Var(flag_tree_parallelize_loops) Init(1) Optimization
 -ftree-parallelize-loops=<number>	Enable automatic parallelization of loops.
 
+ftree-parallelize-loops
+Common Alias(ftree-parallelize-loops=,2147483647,1)
+Enable automatic parallelization of loops.
+
 ftree-phiprop
 Common Var(flag_tree_phiprop) Init(1) Optimization
 Enable hoisting loads from conditional pointers.

gcc/doc/invoke.texi

@@ -659,7 +659,7 @@ Objective-C and Objective-C++ Dialects}.
 -ftree-phiprop  -ftree-loop-distribution  -ftree-loop-distribute-patterns
 -ftree-loop-ivcanon  -ftree-loop-linear  -ftree-loop-optimize
 -ftree-loop-vectorize
--ftree-parallelize-loops=@var{n}  -ftree-pre  -ftree-partial-pre  -ftree-pta
+-ftree-parallelize-loops[=@var{n}]  -ftree-pre  -ftree-partial-pre  -ftree-pta
 -ftree-reassoc  -ftree-scev-cprop  -ftree-sink  -ftree-slsr  -ftree-sra
 -ftree-switch-conversion  -ftree-tail-merge
 -ftree-ter  -ftree-vectorize  -ftree-vrp  -ftrivial-auto-var-init
@@ -14691,8 +14691,9 @@ variable merging and induction variable elimination) on trees.
 Enabled by default at @option{-O1} and higher.
 
 @opindex ftree-parallelize-loops
-@item -ftree-parallelize-loops=n
-Parallelize loops, i.e., split their iteration space to run in n threads.
+@item -ftree-parallelize-loops
+@itemx -ftree-parallelize-loops=@var{n}
+Parallelize loops, i.e., split their iteration space to run in multiple threads.
 This is only possible for loops whose iterations are independent
 and can be arbitrarily reordered.  The optimization is only
 profitable on multiprocessor machines, for loops that are CPU-intensive,
@@ -14700,6 +14701,17 @@ rather than constrained e.g.@: by memory bandwidth.  This option
 implies @option{-pthread}, and thus is only supported on targets
 that have support for @option{-pthread}.
 
+When a positive value @var{n} is specified, the number of threads is fixed
+at compile time and cannot be changed after compilation. The compiler
+generates ``#pragma omp parallel num_threads(@var{n})''.
+
+When used without @code{=@var{n}} (i.e., @option{-ftree-parallelize-loops}),
+the number of threads is determined at program execution time via the
+@env{OMP_NUM_THREADS} environment variable. If @env{OMP_NUM_THREADS} is not
+set, the OpenMP runtime automatically detects the number of available
+processors and uses that value. This enables creating binaries that
+adapt to different hardware configurations without recompilation.
+
 @opindex ftree-pta
 @item -ftree-pta
 Perform function-local points-to analysis on trees.  This flag is

gcc/testsuite/gcc.dg/autopar/runtime-auto.c

@@ -0,0 +1,53 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -ftree-parallelize-loops -fdump-tree-parloops2-details" } */
+
+void abort (void);
+
+#define N 1000
+
+int a[N], b[N], c[N];
+
+void
+test_parallel_loop (void)
+{
+  int i;
+
+  /* This loop should be auto-parallelized when -ftree-parallelize-loops
+     (without =number) is used for runtime thread detection via OMP_NUM_THREADS.  */
+  for (i = 0; i < N; i++)
+    a[i] = b[i] + c[i];
+}
+
+int
+main (void)
+{
+  int i;
+
+  for (i = 0; i < N; i++)
+    {
+      b[i] = i;
+      c[i] = i * 2;
+    }
+
+  test_parallel_loop ();
+
+  for (i = 0; i < N; i++)
+    {
+      if (a[i] != b[i] + c[i])
+	abort ();
+    }
+
+  return 0;
+}
+
+/* Check that the loop is parallelized with runtime thread detection.  */
+/* { dg-final { scan-tree-dump "parallelizing" "parloops2" } } */
+
+/* Check that "#pragma omp parallel" is generated.  */
+/* { dg-final { scan-tree-dump "pragma omp parallel" "parloops2" } } */
+
+/* Check that instead of generating a num_threads(x) clause, the compiler calls
+   "__builtin_omp_get_num_threads" that will set the number of threads at
+   program execution time.  */
+/* { dg-final { scan-tree-dump "__builtin_omp_get_num_threads" "parloops2" } } */
+

gcc/tree-parloops.cc

@@ -2601,10 +2601,19 @@ create_parallel_loop (class loop *loop, tree loop_fn, tree data,
       gsi = gsi_last_bb (paral_bb);
 
       gcc_checking_assert (n_threads != 0);
-      t = build_omp_clause (loc, OMP_CLAUSE_NUM_THREADS);
-      OMP_CLAUSE_NUM_THREADS_EXPR (t)
-	= build_int_cst (integer_type_node, n_threads);
-      omp_par_stmt = gimple_build_omp_parallel (NULL, t, loop_fn, data);
+      if (n_threads == INT_MAX)
+	/* No hardcoded thread count, let OpenMP runtime decide.  */
+	omp_par_stmt = gimple_build_omp_parallel (NULL, NULL_TREE, loop_fn,
+						  data);
+      else
+	{
+	  /* Build the OMP_CLAUSE_NUM_THREADS clause only if we have a fixed
+	     thread count.  */
+	  t = build_omp_clause (loc, OMP_CLAUSE_NUM_THREADS);
+	  OMP_CLAUSE_NUM_THREADS_EXPR (t)
+	    = build_int_cst (integer_type_node, n_threads);
+	  omp_par_stmt = gimple_build_omp_parallel (NULL, t, loop_fn, data);
+	}
       gimple_set_location (omp_par_stmt, loc);
 
       gsi_insert_after (&gsi, omp_par_stmt, GSI_NEW_STMT);
@@ -2812,7 +2821,6 @@ gen_parallel_loop (class loop *loop,
   struct clsn_data clsn_data;
   location_t loc;
   gimple *cond_stmt;
-  unsigned int m_p_thread=2;
 
   /* From
 
@@ -2885,15 +2893,14 @@ gen_parallel_loop (class loop *loop,
 
   if (!oacc_kernels_p)
     {
-      if (loop->inner)
-	m_p_thread=2;
-      else
-	m_p_thread=MIN_PER_THREAD;
-
       gcc_checking_assert (n_threads != 0);
+      /* For runtime thread detection, use a conservative estimate of 2 threads
+	 for the many iterations condition check.  */
+      unsigned threads = (n_threads == INT_MAX) ? 2 : n_threads;
+      unsigned m_p_thread = loop->inner ? 2 : MIN_PER_THREAD;
       many_iterations_cond =
 	fold_build2 (GE_EXPR, boolean_type_node,
-		     nit, build_int_cst (type, m_p_thread * n_threads - 1));
+		     nit, build_int_cst (type, m_p_thread * threads - 1));
 
       many_iterations_cond
 	= fold_build2 (TRUTH_AND_EXPR, boolean_type_node,
@@ -3905,14 +3912,15 @@ parallelize_loops (bool oacc_kernels_p)
       estimated = estimated_loop_iterations_int (loop);
       if (estimated == -1)
 	estimated = get_likely_max_loop_iterations_int (loop);
+      /* For runtime thread detection, use an estimate of 2 threads.  */
+      unsigned threads = (n_threads == INT_MAX) ? 2 : n_threads;
+      unsigned m_p_thread = loop->inner ? 2 : MIN_PER_THREAD;
       /* FIXME: Bypass this check as graphite doesn't update the
 	 count and frequency correctly now.  */
       if (!flag_loop_parallelize_all
 	  && !oacc_kernels_p
 	  && ((estimated != -1
-	       && (estimated
-		   < ((HOST_WIDE_INT) n_threads
-		      * (loop->inner ? 2 : MIN_PER_THREAD) - 1)))
+	       && (estimated < ((HOST_WIDE_INT) threads * m_p_thread - 1)))
 	      /* Do not bother with loops in cold areas.  */
 	      || optimize_loop_nest_for_size_p (loop)))
 	continue;