[llvm] [OFFLOAD] Stricter enforcement of user offload disable (PR #133470)

[via llvm-commits]

https://github.com/adurang created https://github.com/llvm/llvm-project/pull/133470

If user specifies offload is disabled (e.g., OMP_TARGET_OFFLOAD=disable), disable library almost completely. This reduces resources spent to a minimum and ensures all APIs behave as if the only available device is the host device.

Currently some of the APIs behave as if there were devices avaible for offload even when under OMP_TARGET_OFFLOAD=disable.

>From c073357cce4bd2e877629eb0c53146265160249d Mon Sep 17 00:00:00 2001
From: "Duran, Alex" <alejandro.duran at intel.com>
Date: Fri, 28 Mar 2025 02:31:24 -0700
Subject: [PATCH] [OFFLOAD] Stricter enforcement of user offload disable

If user specifies offload is disabled (e.g., OMP_TARGET_OFFLOAD=disable),
disable library almost completely. This reduces resources spent
to a minimum and ensures all APIs behave as if the only available device
is the host device.

Currently some of the APIs behave as if there were devices
avaible for offload even when under OMP_TARGET_OFFLOAD=disable.
---
 offload/include/OffloadPolicy.h     | 22 ++++++++++++++++++++--
 offload/libomptarget/OffloadRTL.cpp |  7 +++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/offload/include/OffloadPolicy.h b/offload/include/OffloadPolicy.h
index 858d9c323b15a..7f57d4da8b364 100644
--- a/offload/include/OffloadPolicy.h
+++ b/offload/include/OffloadPolicy.h
@@ -26,7 +26,7 @@ extern "C" int __kmpc_get_target_offload(void) __attribute__((weak));
 
 class OffloadPolicy {
 
-  OffloadPolicy(PluginManager &PM) {
+  OffloadPolicy() {
     // TODO: Check for OpenMP.
     switch ((kmp_target_offload_kind_t)__kmpc_get_target_offload()) {
     case tgt_disabled:
@@ -36,6 +36,17 @@ class OffloadPolicy {
       Kind = MANDATORY;
       return;
     default:
+      // delay DEFAULT policy until PluginManager is ready
+      UserValue = false;
+      return;
+    };
+  }
+
+  OffloadPolicy(PluginManager &PM) {
+    const OffloadPolicy &OP = get();
+    if (!OP.UserValue) {
+      // User didn't specify a policy, decide
+      // based on number of devices discovered
       if (PM.getNumDevices()) {
         DP("Default TARGET OFFLOAD policy is now mandatory "
            "(devices were found)\n");
@@ -46,10 +57,16 @@ class OffloadPolicy {
         Kind = DISABLED;
       }
       return;
-    };
+    }
+    Kind = OP.Kind;
   }
 
 public:
+  static const OffloadPolicy &get() {
+    static OffloadPolicy OP;
+    return OP;
+  }
+
   static const OffloadPolicy &get(PluginManager &PM) {
     static OffloadPolicy OP(PM);
     return OP;
@@ -58,6 +75,7 @@ class OffloadPolicy {
   enum OffloadPolicyKind { DISABLED, MANDATORY };
 
   OffloadPolicyKind Kind = MANDATORY;
+  bool UserValue = true;
 };
 
 #endif // OMPTARGET_OFFLOAD_POLICY_H
diff --git a/offload/libomptarget/OffloadRTL.cpp b/offload/libomptarget/OffloadRTL.cpp
index 29b573a27d087..ecf7b501efcbc 100644
--- a/offload/libomptarget/OffloadRTL.cpp
+++ b/offload/libomptarget/OffloadRTL.cpp
@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "OffloadPolicy.h"
 #include "OpenMP/OMPT/Callback.h"
 #include "PluginManager.h"
 
@@ -31,6 +32,12 @@ void initRuntime() {
   if (PM == nullptr)
     PM = new PluginManager();
 
+  if (OffloadPolicy::get().Kind == OffloadPolicy::DISABLED) {
+    DP("Offload is disabled. Skipping library initialization\n");
+    // Do only absolutely needed initialization
+    return;
+  }
+
   RefCount++;
   if (RefCount == 1) {
     DP("Init offload library!\n");