amdgcn: Auto-detect USM mode and set HSA_XNACK

The AMD GCN runtime must be set to the correct "XNACK" mode for Unified Shared
Memory and/or self-mapping to work, but this is not always clear at compile and
link time due to the split nature of the offload compilation pipeline.

When XNACK mode is enabled, the runtime will restart GPU load/store
instructions that fail due to memory exceptions caused by page-misses.  While
this is important for shared-memory systems that might experience swapping, we
are mostly interested in it because it is also used to implement page migration
between host and GPU memory, which is the basis of USM.

This patch checks that the XNACK mode is configured appropriately in the
compiler, and mkoffload then adds a runtime check into the final program to
ensure that the HSA_XNACK environment variable passes the correct mode to the
GPU.

The HSA_XNACK variable must be set before the HSA runtime is even loaded, so
it makes more sense to have this set within the constructor than at some point
later within libgomp or the GCN plugin.

Other toolchains require the end-user to set HSA_XNACK manually (or else wonder
why it's not working), so the constructor also checks that any existing manual
setting is compatible with the binary's requirements.

gcc/ChangeLog:

	* config/gcn/gcn.cc (gcn_init_cumulative_args): Emit a warning if the
	-mxnack setting looks wrong.
	* config/gcn/mkoffload.cc: Include tree.h and omp-general.h.
	(process_asm): Add omp_requires parameter.
	Emit HSA_XNACK code into mkoffload_setup, as required.
	(main): Modify HSACO_ATTR_OFF to preserve user-set -mxnack.
	Pass omp_requires to process_asm.

diff --git a/gcc/config/gcn/gcn.cc b/gcc/config/gcn/gcn.cc
index 1e04074d78b74d33da1f2682f420042ec1cff65e..a729ea4de36b00aa05d4134eda56a8b614024903 100644 (file)
--- a/gcc/config/gcn/gcn.cc
+++ b/gcc/config/gcn/gcn.cc
@@ -2940,6 +2940,16 @@ gcn_init_cumulative_args (CUMULATIVE_ARGS *cum /* Argument info to init */ ,
   if (!caller && cfun->machine->normal_function)
     gcn_detect_incoming_pointer_arg (fndecl);
 
+  if ((omp_requires_mask & (OMP_REQUIRES_UNIFIED_SHARED_MEMORY
+                           | OMP_REQUIRES_SELF_MAPS))
+      && gcn_devices[gcn_arch].xnack_default != HSACO_ATTR_UNSUPPORTED
+      && flag_xnack == HSACO_ATTR_OFF)
+    {
+      warning_at (UNKNOWN_LOCATION, 0,
+                 "Unified Shared Memory is enabled, but XNACK is disabled");
+      inform (UNKNOWN_LOCATION, "Try -foffload-options=-mxnack=any");
+    }
+
   reinit_regs ();
 }
 

diff --git a/gcc/config/gcn/mkoffload.cc b/gcc/config/gcn/mkoffload.cc
index 222adb2cd41d3ce5fb8094769feccd4cea442fa4..d9d89c64f950fb1b2a31077bb8c30efd4f8915ab 100644 (file)
--- a/gcc/config/gcn/mkoffload.cc
+++ b/gcc/config/gcn/mkoffload.cc
@@ -38,6 +38,9 @@
 #include "configargs.h"  /* For configure_default_options.  */
 #include "multilib.h"  /* For multilib_options.  */
 
+#include "tree.h"       /* Dependency of omp-general.h.  */
+#include "omp-general.h" /* For enum omp_requires.  */
+
 /* These probably won't (all) be in elf.h for a while.  */
 #undef  EM_AMDGPU
 #define EM_AMDGPU              0xe0;
@@ -441,10 +444,12 @@ copy_early_debug_info (const char *infile, const char *outfile)
    encoded as structured data.  */
 
 static void
-process_asm (FILE *in, FILE *out, FILE *cfile)
+process_asm (FILE *in, FILE *out, FILE *cfile, uint32_t omp_requires)
 {
   int fn_count = 0, var_count = 0, ind_fn_count = 0;
   int dims_count = 0, regcount_count = 0;
+  bool xnack_required = (omp_requires & (OMP_REQUIRES_UNIFIED_SHARED_MEMORY
+                                        | OMP_REQUIRES_SELF_MAPS));
   struct obstack fns_os, dims_os, regcounts_os;
   obstack_init (&fns_os);
   obstack_init (&dims_os);
@@ -469,6 +474,7 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
   fn_count += 2;
 
   char buf[1000];
+  char dummy;
   enum
     { IN_CODE,
       IN_METADATA,
@@ -549,7 +555,6 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
          }
        }
 
-      char dummy;
       if (sscanf (buf, " .section .gnu.offload_vars%c", &dummy) > 0)
        {
          state = IN_VARS;
@@ -615,11 +620,24 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
   struct oaccdims *dims = XOBFINISH (&dims_os, struct oaccdims *);
   struct regcount *regcounts = XOBFINISH (&regcounts_os, struct regcount *);
 
+  /* If the -mxnack setting has a definite value (not "any" or undefined), or
+     the program "requires unified_shared_memory" (in which case -mxnack might
+     be "any"), then we emit code to check the mode at runtime.  */
+  bool check_xnack = (TEST_XNACK_OFF (elf_flags)
+                     || TEST_XNACK_ON (elf_flags)
+                     || xnack_required);
+  if (TEST_XNACK_OFF (elf_flags) && xnack_required)
+    fatal_error (input_location,
+                "conflicting settings; XNACK is forced off but Unified "
+                "Shared Memory is on");
+
+  /* Start generating the C code.  */
   if (gcn_stack_size)
-    {
-      fprintf (cfile, "#include <stdlib.h>\n");
-      fprintf (cfile, "#include <stdbool.h>\n\n");
-    }
+    fprintf (cfile, "#include <stdbool.h>\n");
+  if (check_xnack)
+    fprintf (cfile, "#include <stdio.h>\n");
+  if (gcn_stack_size || check_xnack)
+    fprintf (cfile, "#include <stdlib.h>\n\n");
 
   fprintf (cfile, "static const int gcn_num_vars = %d;\n\n", var_count);
   fprintf (cfile, "static const int gcn_num_ind_funcs = %d;\n\n", ind_fn_count);
@@ -677,6 +695,25 @@ process_asm (FILE *in, FILE *out, FILE *cfile)
             "    setenv (\"GCN_STACK_SIZE\", \"%d\", true);\n",
             gcn_stack_size);
 
+  /* Emit a constructor function to set the HSA_XNACK environment variable.
+     This must be done before the ROCr runtime library is loaded.
+     We never override a user value (except empty string), but we do emit a
+     useful diagnostic in the wrong mode (the ROCr message is not good.  */
+  if (check_xnack)
+    fprintf (cfile,
+            "\n"
+            "  const char *xn_var = getenv (\"HSA_XNACK\");\n"
+            "  if (!xn_var || xn_var[0] == '\\0')\n"
+            "    setenv (\"HSA_XNACK\", \"%d\", true);\n"
+            "  else if (%s)\n"
+            "    fprintf (stderr, \"warning: HSA_XNACK=%%s is incompatible; "
+                          "the GPU kernel may revert to host fallback\\n\", "
+                          "xn_var);\n",
+            xnack_required || TEST_XNACK_ON (elf_flags),
+            (xnack_required || TEST_XNACK_ON (elf_flags)
+             ? "xn_var[0] != '1' || xn_var[1] != '\\0'"
+             : "xn_var[0] != '0' || xn_var[1] != '\\0'"));
+
   /* End of mkoffload_setup function.  */
   fprintf (cfile, "}\n\n");
 
@@ -1116,7 +1153,8 @@ main (int argc, char **argv)
 #define GCN_DEVICE(name, NAME, ELF, ISA, XNACK, SRAM, ...) \
     case ELF: XNACK; break;
 #define HSACO_ATTR_UNSUPPORTED SET_XNACK_UNSET (elf_flags)
-#define HSACO_ATTR_OFF SET_XNACK_OFF (elf_flags)
+#define HSACO_ATTR_OFF \
+      if (TEST_XNACK_UNSET (elf_flags)) SET_XNACK_OFF (elf_flags)
 #define HSACO_ATTR_ANY \
       if (TEST_XNACK_UNSET (elf_flags)) SET_XNACK_ANY (elf_flags)
 #include "gcn-devices.def"
@@ -1348,7 +1386,7 @@ main (int argc, char **argv)
       if (!out)
        fatal_error (input_location, "cannot open %qs", gcn_s2_name);
 
-      process_asm (in, out, cfile);
+      process_asm (in, out, cfile, omp_requires);
 
       fclose (in);
       fclose (out);