Remove intermediate relocatable code stored in __nv_relfatbin sections, if objcopy is at least version 2.26 (which added support for --update-sections).

The intermediate code is a result of separate compilation and linking, and removing it reduces TF's GPU wheel size.

PiperOrigin-RevId: 317081343
Change-Id: I603477b4499344aeec653765be78de11f392eac6

third_party/nccl/build_defs.bzl.tpl

@@ -169,35 +169,94 @@ _device_link = rule(
 )
 """Links device code and generates source code for kernel registration."""
 
+def _prune_relocatable_code_impl(ctx):
+    """Clears __nv_relfatbin section containing relocatable device code."""
+    empty_file = ctx.actions.declare_file(ctx.attr.name + "__nv_relfatbin")
+    ctx.actions.write(empty_file, "")
+
+    # Parse 'objcopy --version' and update section if it's at least v2.26.
+    # Otherwise, simply copy the file without changing it.
+    # TODO(csigg): version parsing is brittle, can we do better?
+    command = r"""
+        objcopy=$1                                         \
+        section=$2                                         \
+        input=$3                                           \
+        output=$4                                          \
+        args=""                                            \
+        pattern='([0-9])\.([0-9]+)';                       \
+        if [[ $($objcopy --version) =~ $pattern ]] && {    \
+            [ ${BASH_REMATCH[1]} -gt 2 ] ||                \
+            [ ${BASH_REMATCH[2]} -ge 26 ]; }; then         \
+          args="--update-section __nv_relfatbin=$section"; \
+        fi;                                                \
+        $objcopy $args $input $output
+    """
+    cc_toolchain = find_cpp_toolchain(ctx)
+    outputs = []
+    for src in ctx.files.srcs:
+        out = ctx.actions.declare_file("pruned_" + src.basename, sibling = src)
+        ctx.actions.run_shell(
+            inputs = [empty_file] + ctx.files.srcs,  # + ctx.files._crosstool,
+            outputs = [out],
+            arguments = [
+                cc_toolchain.objcopy_executable,
+                empty_file.path,
+                src.path,
+                out.path,
+            ],
+            command = command,
+        )
+        outputs.append(out)
+    return DefaultInfo(files = depset(outputs))
+
+_prune_relocatable_code = rule(
+    implementation = _prune_relocatable_code_impl,
+    attrs = {
+        "srcs": attr.label_list(mandatory = True, allow_files = True),
+        "_cc_toolchain": attr.label(
+            default = "@bazel_tools//tools/cpp:current_cc_toolchain",
+        ),
+        # "_crosstool": attr.label_list(
+        #     cfg = "host",
+        #     default = ["@bazel_tools//tools/cpp:crosstool"]
+        # ),
+    },
+)
+
 def _merge_archive_impl(ctx):
     # Generate an mri script to the merge archives in srcs and pass it to 'ar'.
     # See https://stackoverflow.com/a/23621751.
     files = _pic_only(ctx.files.srcs)
     mri_script = "create " + ctx.outputs.out.path
     for f in files:
-        mri_script += "\\naddlib " + f.path
-    mri_script += "\\nsave\\nend"
+        mri_script += r"\naddlib " + f.path
+    mri_script += r"\nsave\nend"
 
     cc_toolchain = find_cpp_toolchain(ctx)
     ctx.actions.run_shell(
         inputs = ctx.files.srcs,  # + ctx.files._crosstool,
         outputs = [ctx.outputs.out],
-        command = "printf \"%s\" | %s -M" % (mri_script, cc_toolchain.ar_executable),
+        command = "echo -e \"%s\" | %s -M" % (mri_script, cc_toolchain.ar_executable),
     )
 
 _merge_archive = rule(
     implementation = _merge_archive_impl,
     attrs = {
         "srcs": attr.label_list(mandatory = True, allow_files = True),
-        "_cc_toolchain": attr.label(default = "@bazel_tools//tools/cpp:current_cc_toolchain"),
-        # "_crosstool": attr.label_list(cfg = "host", default = ["@bazel_tools//tools/cpp:crosstool"]),
+        "_cc_toolchain": attr.label(
+            default = "@bazel_tools//tools/cpp:current_cc_toolchain",
+        ),
+        # "_crosstool": attr.label_list(
+        #     cfg = "host",
+        #     default = ["@bazel_tools//tools/cpp:crosstool"]
+        # ),
     },
     outputs = {"out": "lib%{name}.a"},
 )
 """Merges srcs into a single archive."""
 
 def cuda_rdc_library(name, hdrs = None, copts = None, linkstatic = True, **kwargs):
-    """Produces a cuda_library using separate compilation and linking.
+    r"""Produces a cuda_library using separate compilation and linking.
 
     CUDA separate compilation and linking allows device function calls across
     translation units. This is different from the normal whole program
@@ -239,17 +298,24 @@ def cuda_rdc_library(name, hdrs = None, copts = None, linkstatic = True, **kwarg
 
     The steps marked with '*' are implemented in the _device_link rule.
 
+    The intermediate relocatable device code in xy.a is no longer needed at
+    this point and the corresponding section is replaced with an empty one using
+    objcopy. We do not remove the section completely because it is referenced by
+    relocations, and removing those as well breaks fatbin registration.
+
     The object files in both xy.a and dlink.a reference symbols defined in the
     other archive. The separate archives are a side effect of using two
     cc_library targets to implement a single compilation trajectory. We could
     fix this once bazel supports C++ sandwich. For now, we just merge the two
     archives to avoid unresolved symbols:
 
-    xy.a      dlink.a
-        \    /           merge archive
-      xy_dlink.a
-           |             cc_library (or alternatively, cc_import)
-     final target
+                    xy.a
+                     |         objcopy --update-section __nv_relfatbin=''
+    dlink.a     xy_pruned.a
+         \           /         merge archive
+          xy_merged.a
+              |                cc_library (or alternatively, cc_import)
+         final target
 
     Another complication is that cc_library produces (depending on the
     configuration) both PIC and non-PIC archives, but the distinction
@@ -313,19 +379,26 @@ def cuda_rdc_library(name, hdrs = None, copts = None, linkstatic = True, **kwarg
         linkstatic = linkstatic,
     )
 
+    # Remove intermediate relocatable device code.
+    pruned = name + "_pruned"
+    _prune_relocatable_code(
+        name = pruned,
+        srcs = [lib],
+    )
+
     # Repackage the two libs into a single archive. This is required because
     # both libs reference symbols defined in the other one. For details, see
     # https://eli.thegreenplace.net/2013/07/09/library-order-in-static-linking
-    archive = name + "_a"
+    merged = name + "_merged"
     _merge_archive(
-        name = archive,
-        srcs = [lib, dlink],
+        name = merged,
+        srcs = [pruned, dlink],
     )
 
     # Create cc target from archive.
     native.cc_library(
         name = name,
-        srcs = [archive],
+        srcs = [merged],
         hdrs = hdrs,
         linkstatic = linkstatic,
     )