Fix exec platform auto-detection for cross-compilation

cuda/platform_alias_extension.bzl

@@ -111,13 +111,60 @@ def _emit_outer_constraint_alias(build_content, alias_name, target_name, platfor
     build_content.append(")")
     build_content.append("")
 
+def _detect_host_platform(ctx):
+    """Detect the host platform where Bazel is running.
+
+    This is used to set a sensible default for exec platform when auto-detection
+    fallback is used.
+
+    Args:
+        ctx: Repository rule context
+
+    Returns:
+        A string representing the detected platform (e.g., "linux-x86_64")
+    """
+    os_name = ctx.os.name.lower()
+    arch = ctx.os.arch.lower()
+
+    # Normalize OS name
+    if os_name.startswith("linux"):
+        os_name = "linux"
+    elif os_name.startswith("windows"):
+        os_name = "windows"
+    elif os_name.startswith("mac"):
+        os_name = "macos"
+    else:
+        # Unknown OS, default to linux
+        os_name = "linux"
+
+    # Normalize architecture
+    if arch in ["x86_64", "amd64", "x64"]:
+        arch = "x86_64"
+    elif arch in ["aarch64", "arm64"]:
+        arch = "aarch64"
+    else:
+        # Unknown arch, default to x86_64
+        arch = "x86_64"
+
+    platform = "{}-{}".format(os_name, arch)
+
+    # Map to a supported CUDA platform
+    # For aarch64 on Linux, default to sbsa (server-class)
+    if platform == "linux-aarch64":
+        platform = "linux-sbsa"
+
+    return platform
+
 def _platform_alias_repo_impl(ctx):
     """Implementation of the platform_alias_repo repository rule.
 
     Args:
         ctx: Repository context with attributes x86_repo, arm64_repo, and targets.
     """
 
+    # Detect the host platform for exec platform default
+    host_platform = _detect_host_platform(ctx)
+
     # Generate BUILD.bazel content with platform-specific aliases
     build_content = ["# Generated by platform_alias_repo rule", ""]
 
@@ -197,12 +244,22 @@ def _platform_alias_repo_impl(ctx):
         )
 
         if platform_type == "exec":
-            # Outer alias: explicit --exec_platform flag wins; the default
-            # branch falls through to constraint-based auto-detection in
-            # the private :auto_<name> alias below.
+            # Outer alias for exec components (nvcc, cicc, etc.)
+            # Priority order:
+            #   1. Explicit --@rules_cuda//cuda:exec_platform flag
+            #   2. Detected host platform (where Bazel is running)
+            #   3. Constraint-based auto-detection (for RBE scenarios)
+            #
+            # The host platform detection happens at repository instantiation time
+            # and provides a sensible default that works for local builds and
+            # cross-compilation.
+            host_platform_suffix = host_platform.replace("-", "_")
+
             build_content.append("alias(")
             build_content.append('    name = "{}",'.format(target_name))
             build_content.append("    actual = select({")
+
+            # Explicit flag overrides
             for platform in SUPPORTED_PLATFORMS:
                 platform_suffix = platform.replace("-", "_")
                 build_content.append(
@@ -212,14 +269,23 @@ def _platform_alias_repo_impl(ctx):
                     build_content.append('            ":{}_{}",'.format(platform_suffix, target_name))
                 else:
                     build_content.append('            "{}",'.format(dummy_target))
-            build_content.append('        "//conditions:default": ":auto_{}",'.format(target_name))
+
+            # Default: use detected host platform
+            if host_platform in platforms_available:
+                build_content.append('        "//conditions:default": ":{}_{}",'.format(host_platform_suffix, target_name))
+            else:
+                # Host platform not available, fall back to auto-detection alias
+                build_content.append('        "//conditions:default": ":auto_{}",'.format(target_name))
+
             build_content.append("    }),")
             build_content.append('    visibility = ["//visibility:public"],')
             build_content.append(")")
             build_content.append("")
 
-            # Auto-detection fallback. Private — only the outer flag-select
-            # in this same package needs to reach it.
+            # Auto-detection fallback for when host platform is not available.
+            # This is a private target only used when the detected host platform
+            # doesn't have this component available.
+            # Uses constraint-based detection which works for RBE scenarios.
             _emit_outer_constraint_alias(
                 build_content,
                 "auto_" + target_name,