[lld] [lld] Change `--lto-emit-llvm` to use the pre-codegen module (PR #97480)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 created https://github.com/llvm/llvm-project/pull/97480

Summary:
Currently the `--lto-emit-llvm` option writes out the
post-internalization bitcode. This is the bitcode before any
optimizations or other pipelines have been run on it. This patch changes
that to use the pre-codegen module, which is the state of the LLVM-IR
after the optimizations have been run.

I believe that this makes sense as the `--lto-emit-llvm` option seems to
imply that we should emit the final output of the LLVM pass as if it
were the desired output. This should include optimizations at the
requested optimization level. My main motivation for this change is to
be able to use this to link several LLVM-IR files into a single one that
I can then pass back to `ld.lld` later (for JIT purposes).


>From 7ab8dbaf235f65d91f40ae0eff5bf5a1ba77579a Mon Sep 17 00:00:00 2001
From: Joseph Huber <huberjn at outlook.com>
Date: Tue, 2 Jul 2024 15:53:09 -0500
Subject: [PATCH] [lld] Change `--lto-emit-llvm` to use the pre-codegen module

Summary:
Currently the `--lto-emit-llvm` option writes out the
post-internalization bitcode. This is the bitcode before any
optimizations or other pipelines have been run on it. This patch changes
that to use the pre-codegen module, which is the state of the LLVM-IR
after the optimizations have been run.

I believe that this makes sense as the `--lto-emit-llvm` option seems to
imply that we should emit the final output of the LLVM pass as if it
were the desired output. This should include optimizations at the
requested optimization level. My main motivation for this change is to
be able to use this to link several LLVM-IR files into a single one that
I can then pass back to `ld.lld` later (for JIT purposes).
---
 lld/ELF/LTO.cpp               | 2 +-
 lld/test/ELF/lto/emit-llvm.ll | 6 ++++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/lld/ELF/LTO.cpp b/lld/ELF/LTO.cpp
index 3d92007469263..935d0a9eab9ee 100644
--- a/lld/ELF/LTO.cpp
+++ b/lld/ELF/LTO.cpp
@@ -147,7 +147,7 @@ static lto::Config createConfig() {
   c.PGOWarnMismatch = config->ltoPGOWarnMismatch;
 
   if (config->emitLLVM) {
-    c.PostInternalizeModuleHook = [](size_t task, const Module &m) {
+    c.PreCodeGenModuleHook = [](size_t task, const Module &m) {
       if (std::unique_ptr<raw_fd_ostream> os =
               openLTOOutputFile(config->outputFile))
         WriteBitcodeToFile(m, *os, false);
diff --git a/lld/test/ELF/lto/emit-llvm.ll b/lld/test/ELF/lto/emit-llvm.ll
index 01f5a056e0c0d..37488016a4bc2 100644
--- a/lld/test/ELF/lto/emit-llvm.ll
+++ b/lld/test/ELF/lto/emit-llvm.ll
@@ -9,11 +9,13 @@
 ; RUN: ld.lld --plugin-opt=emit-llvm -mllvm -bitcode-flush-threshold=0 -o /dev/null %t.o
 ; RUN: ld.lld --lto-emit-llvm -mllvm -bitcode-flush-threshold=0 -o /dev/null %t.o
 
-; CHECK: define internal void @main()
+; CHECK: define hidden void @main()
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
-define void @main() {
+ at llvm.compiler.used = appending global [1 x ptr] [ptr @main], section "llvm.metadata"
+
+define hidden void @main() {
   ret void
 }