[clang] [LinkerWrapper] Do not link device code under a relocatable link (PR #79314)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 created https://github.com/llvm/llvm-project/pull/79314

Summary:
A relocatable link through `clang -r` can go through the
clang-linker-wrapper if offloading is enabled. This will have the effect
of linking the device code and creating the wrapper module. It will then
be merged into the final file. This is useful behavior on its own, but
is likely not what is expected for a `-r` job.

This patch makes the linker wrapper ignore the device code when doing a
reloctable link. This has the effect of the linker merging the
`.llvm.offloading` sections in the output object. These will then be
parsed as normal when the executable is finally created.

Even though this doesn't actually perform a reloctable link on the
device code itself, it has a similar effect of combining multiple files
into a single one.


>From 0f8d9bb329b6d50493286e117ea0fe45e0a49247 Mon Sep 17 00:00:00 2001
From: Joseph Huber <huberjn at outlook.com>
Date: Wed, 24 Jan 2024 09:41:15 -0600
Subject: [PATCH] [LinkerWrapper] Do not link device code under a relocatable
 link

Summary:
A relocatable link through `clang -r` can go through the
clang-linker-wrapper if offloading is enabled. This will have the effect
of linking the device code and creating the wrapper module. It will then
be merged into the final file. This is useful behavior on its own, but
is likely not what is expected for a `-r` job.

This patch makes the linker wrapper ignore the device code when doing a
reloctable link. This has the effect of the linker merging the
`.llvm.offloading` sections in the output object. These will then be
parsed as normal when the executable is finally created.

Even though this doesn't actually perform a reloctable link on the
device code itself, it has a similar effect of combining multiple files
into a single one.
---
 clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp | 6 ++++++
 clang/tools/clang-linker-wrapper/LinkerWrapperOpts.td   | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp b/clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp
index 5485a4b74bf8a8f..b682cc293d54b21 100644
--- a/clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp
+++ b/clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp
@@ -1356,6 +1356,12 @@ Expected<SmallVector<SmallVector<OffloadFile>>>
 getDeviceInput(const ArgList &Args) {
   llvm::TimeTraceScope TimeScope("ExtractDeviceCode");
 
+  // If the user is requesting a reloctable link we ignore the device code. The
+  // actual linker will merge the embedded device code sections so they can be
+  // linked when the executable is finally created.
+  if (Args.hasArg(OPT_relocatable))
+    return SmallVector<SmallVector<OffloadFile>>{};
+
   StringRef Root = Args.getLastArgValue(OPT_sysroot_EQ);
   SmallVector<StringRef> LibraryPaths;
   for (const opt::Arg *Arg : Args.filtered(OPT_library_path, OPT_libpath))
diff --git a/clang/tools/clang-linker-wrapper/LinkerWrapperOpts.td b/clang/tools/clang-linker-wrapper/LinkerWrapperOpts.td
index b6d3297987fffe5..c59cb0fb3e7cbfc 100644
--- a/clang/tools/clang-linker-wrapper/LinkerWrapperOpts.td
+++ b/clang/tools/clang-linker-wrapper/LinkerWrapperOpts.td
@@ -127,6 +127,9 @@ def version : Flag<["--", "-"], "version">, Flags<[HelpHidden]>, Alias<v>;
 def whole_archive : Flag<["--", "-"], "whole-archive">, Flags<[HelpHidden]>;
 def no_whole_archive : Flag<["--", "-"], "no-whole-archive">, Flags<[HelpHidden]>;
 
+def relocatable : Flag<["--", "-"], "relocatable">, Flags<[HelpHidden]>;
+def r : Flag<["-"], "r">, Alias<relocatable>, Flags<[HelpHidden]>;
+
 // link.exe-style linker options.
 def out : Joined<["/", "-", "/?", "-?"], "out:">, Flags<[HelpHidden]>;
 def libpath : Joined<["/", "-", "/?", "-?"], "libpath:">, Flags<[HelpHidden]>;