[gcc/devel/omp/gcc-16] Default compute dimensions (compile time)

https://gcc.gnu.org/g:dcd8a542c270f1bc0fe9593403e98204c63ce826

commit dcd8a542c270f1bc0fe9593403e98204c63ce826
Author: Nathan Sidwell <[email protected]>
Date:   Sun Apr 5 21:28:12 2026 +0000

    Default compute dimensions (compile time)
    
    This patch was previously combined with an unrelated change and testcases
    that was upstreamed separately ("Add '-Wopenacc-parallelism'",
    22cff118f7526bec195ed6e41452980820fdf3a8).
    
    gcc/ChangeLog
            * doc/invoke.texi (fopenacc-dim): Document syntax for using
            runtime value from environment variable.
            * omp-offload.cc (oacc_parse_default_dims): Implement it.
    
    libgomp/ChangeLog
            * testsuite/libgomp.oacc-c-c++-common/loop-default-compile.c: New.
    
    Co-Authored-By: Tom de Vries  <[email protected]>
    Co-Authored-By: Thomas Schwinge  <[email protected]>
    Co-Authored-By: Julian Brown  <[email protected]>

Diff:
---
 gcc/doc/invoke.texi                                |  6 +++++-
 gcc/omp-offload.cc                                 | 25 ++++++++++++++--------
 .../loop-default-compile.c                         | 13 +++++++++++
 3 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 2fe30324693c..799bdac1f12f 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -5552,7 +5552,11 @@ have support for @option{-pthread}.
 Specify default compute dimensions for parallel offload regions that do
 not explicitly specify them.  The @var{geom} value is a triple of
 @samp{:}-separated sizes, in order @var{gang}, @var{worker}, and @var{vector}.
-A size can be omitted, to use a target-specific default value.
+If a size is to be deferred until execution @samp{-} can be used;
+alternatively a size can be omitted to use a target-specific default value.
+When deferring to runtime, the environment variable @env{GOMP_OPENACC_DIM}
+can be set.  It has the same format as the option value, except that
+@samp{-} is not permitted.
 
 @opindex fopenmp
 @opindex fno-openmp
diff --git a/gcc/omp-offload.cc b/gcc/omp-offload.cc
index e66f4b072f86..64b3401dd500 100644
--- a/gcc/omp-offload.cc
+++ b/gcc/omp-offload.cc
@@ -876,8 +876,9 @@ oacc_get_min_dim (int dim)
 }
 
 /* Parse the default dimension parameter.  This is a set of
-   :-separated optional compute dimensions.  Each specified dimension
-   is a positive integer.  When device type support is added, it is
+   :-separated optional compute dimensions.  Each dimension is either
+   a positive integer, or '-' for a dynamic value computed at
+   runtime.  When device type support is added, it is
    planned to be a comma separated list of such compute dimensions,
    with all but the first prefixed by the colon-terminated device
    type.  */
@@ -912,14 +913,20 @@ oacc_parse_default_dims (const char *dims)
 
 	  if (*pos != ':')
 	    {
-	      long val;
-	      const char *eptr;
+	      long val = 0;
 
-	      errno = 0;
-	      val = strtol (pos, const_cast<char **> (&eptr), 10);
-	      if (errno || val <= 0 || (int) val != val)
-		goto malformed;
-	      pos = eptr;
+	      if (*pos == '-')
+		pos++;
+	      else
+		{
+		  const char *eptr;
+
+		  errno=0;
+		  val = strtol (pos, const_cast<char **> (&eptr), 10);
+		  if (errno || val <= 0 || (int) val != val)
+		    goto malformed;
+		  pos = eptr;
+		}
 	      oacc_default_dims[ix] = (int) val;
 	    }
 	}
diff --git a/libgomp/testsuite/libgomp.oacc-c-c++-common/loop-default-compile.c b/libgomp/testsuite/libgomp.oacc-c-c++-common/loop-default-compile.c
new file mode 100644
index 000000000000..6c479e4eb258
--- /dev/null
+++ b/libgomp/testsuite/libgomp.oacc-c-c++-common/loop-default-compile.c
@@ -0,0 +1,13 @@
+/* { dg-additional-options "-fopenacc-dim=16:16" } */
+/* This code uses nvptx inline assembly guarded with acc_on_device, which is
+   not optimized away at -O0, and then confuses the target assembler.
+   { dg-skip-if "" { *-*-* } { "-O0" } { "" } } */
+/* { dg-set-target-env-var "GOMP_OPENACC_DIM" "8:8" } */
+
+#include "loop-default.h"
+
+int main ()
+{
+  /* Environment should be ignored.  */
+  return test_1 (16, 16, 32);
+}