https://gcc.gnu.org/g:86cf7857bb2599e3d7d35199afbd9c37d36e1099
commit r16-5668-g86cf7857bb2599e3d7d35199afbd9c37d36e1099 Author: Andrew Stubbs <[email protected]> Date: Tue Nov 11 15:41:04 2025 +0000 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: --- gcc/config/gcn/gcn.cc | 10 +++++++++ gcc/config/gcn/mkoffload.cc | 54 ++++++++++++++++++++++++++++++++++++++------- 2 files changed, 56 insertions(+), 8 deletions(-) diff --git a/gcc/config/gcn/gcn.cc b/gcc/config/gcn/gcn.cc index 1e04074d78b7..a729ea4de36b 100644 --- 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 222adb2cd41d..d9d89c64f950 100644 --- 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 (®counts_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);
