[llvm-branch-commits] [lld] Discard SHT_LLVM_LTO sections in relocatable links (PR #92825)

[via llvm-branch-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-elf

Author: Paul Kirth (ilovepi)

<details>
<summary>Changes</summary>

So long as ld -r links using bitcode always result in an ELF object, and
not a merged bitcode object, the output form a relocatable link using
FatLTO objects should not have a .llvm.lto section. Prior to this, using
the object code sections would cause the bitcode section in the output
of a relocatable link to be corrupted, by concatenating all the .llvm.lto
sections together.

This patch discards SHT_LLVM_LTO sections when not using
--fat-lto-objects, so that the relocatable ELF output won't contain
inalid bitcode.


---
Full diff: https://github.com/llvm/llvm-project/pull/92825.diff


2 Files Affected:

- (modified) lld/ELF/InputFiles.cpp (+9) 
- (modified) lld/test/ELF/fatlto/fatlto.test (+2-5) 


``````````diff
diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index 1f496026d3ae2..0ac49761601c4 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -832,6 +832,15 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats,
       this->sections[i] =
           createInputSection(i, sec, check(obj.getSectionName(sec, shstrtab)));
       break;
+    case SHT_LLVM_LTO:
+      // When doing a relocatable link with FatLTO objects, if we're not using
+      // the bitcode, discard it, since it will be concatenated together when
+      // handling orphan sections, and which will be an invalid bitcode object.
+      if (config->relocatable && !config->fatLTOObjects) {
+        sections[i] = &InputSection::discarded;
+        break;
+      }
+      LLVM_FALLTHROUGH;
     default:
       this->sections[i] =
           createInputSection(i, sec, check(obj.getSectionName(sec, shstrtab)));
diff --git a/lld/test/ELF/fatlto/fatlto.test b/lld/test/ELF/fatlto/fatlto.test
index e250325dc54f4..d2c96c3c51b98 100644
--- a/lld/test/ELF/fatlto/fatlto.test
+++ b/lld/test/ELF/fatlto/fatlto.test
@@ -50,10 +50,6 @@
 ; RUN: cmp %t/foo-fatLTO.archive %t/foo-LTO
 
 ;; Test FatLTO works with relocatable links using PIC objects
-;; Currently, with PIC relocatable links, FatLTO sections are treated as
-;; orphan sections and incorrectly concatenated together. This test verifies
-;; the current behavior, but should be fixed to either merge those sections
-;; correctly, or to drop them altogether.
 ; RUN: opt < %t/a-LTO.ll -passes="embed-bitcode<thinlto;emit-summary>" | llc --relocation-model=pic --filetype=obj -o %t/a-fat-pic.o
 ; RUN: llvm-readobj -S %t/a-fat-pic.o | FileCheck --check-prefix=HAS_LLVM_LTO %s
 
@@ -64,9 +60,10 @@
 ; RUN: llvm-readobj -S %t/fat.pic.archive | FileCheck --check-prefix=HAS_LLVM_LTO %s
 
 ; RUN: ld.lld --whole-archive %t/fat.pic.archive -r -o %t/fat-pic-relocatable.o
-; RUN: llvm-readobj -S %t/fat-pic-relocatable.o | FileCheck --check-prefix=HAS_LLVM_LTO %s
+; RUN: llvm-readobj -S %t/fat-pic-relocatable.o | FileCheck --check-prefix=CHECK-NON-LTO-TARGET %s
 
 ; HAS_LLVM_LTO: Name: .llvm.lto
+; HAS_LLVM_LTO: Type: SHT_LLVM_LTO
 
 ;--- a-LTO.ll
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"

``````````

</details>


https://github.com/llvm/llvm-project/pull/92825