[lld] [LLD][ELF] Fix SHF_MERGE misalignment when spilled (PR #119289)

[Daniel Thornburgh via llvm-commits]

https://github.com/mysterymath created https://github.com/llvm/llvm-project/pull/119289

Section merging can increase section alignment after potential spill sections are created. Since this operation is never performed on spill sections, they can keep their earlier, smaller, alignment, which produces a misalignment if a spill occurs.

This change propagates alignment increases forward after merging.

>From 2f4b8f5d5b5ff66bb1bcec1d0fa5417abc354ba7 Mon Sep 17 00:00:00 2001
From: Daniel Thornburgh <dthorn at google.com>
Date: Thu, 5 Dec 2024 16:24:28 -0800
Subject: [PATCH] [LLD][ELF] Fix SHF_MERGE misalignment when spilled

Section merging can increase section alignment after potential spill
sections are created. Since this operation is never performed on spill
sections, they can keep their earlier, smaller, alignment, which
produces a misalignment if a spill occurs.

This change propagates alignment increases forward after merging.
---
 lld/ELF/OutputSections.cpp                   | 17 ++++++++++++++++-
 lld/test/ELF/linkerscript/section-class.test | 10 ++++++----
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/lld/ELF/OutputSections.cpp b/lld/ELF/OutputSections.cpp
index 546dc58b4bc843..5e77eeb1ed5988 100644
--- a/lld/ELF/OutputSections.cpp
+++ b/lld/ELF/OutputSections.cpp
@@ -249,12 +249,27 @@ void OutputSection::finalizeInputSections() {
     // catch misuses.
     isd->sectionBases.clear();
 
+
     // Some input sections may be removed from the list after ICF.
     for (InputSection *s : isd->sections)
       commitSection(s);
   }
-  for (auto *ms : mergeSections)
+  for (auto *ms : mergeSections) {
+    // Merging may have increased the alignment of a spillable section. Update
+    // the alignment of potential spill sections and their containing output
+    // sections.
+    if (!script->potentialSpillLists.empty()) {
+      if (auto it = script->potentialSpillLists.find(ms);
+          it != script->potentialSpillLists.end()) {
+        for (PotentialSpillSection *s = it->second.head; s; s = s->next) {
+          s->addralign = std::max(s->addralign, ms->addralign);
+          s->parent->addralign = std::max(s->parent->addralign, s->addralign);
+        }
+      }
+    }
+
     ms->finalizeContents();
+  }
 }
 
 static void sortByOrder(MutableArrayRef<InputSection *> in,
diff --git a/lld/test/ELF/linkerscript/section-class.test b/lld/test/ELF/linkerscript/section-class.test
index 7fce13bfe3e025..ae1a5026ef0490 100644
--- a/lld/test/ELF/linkerscript/section-class.test
+++ b/lld/test/ELF/linkerscript/section-class.test
@@ -310,6 +310,7 @@ SECTIONS {
 .byte 0x12, 0x34
 
 .section .b,"aM", at progbits,1
+.p2align 1
 .byte 0x12
 
 # RUN: llvm-mc -n -filetype=obj -triple=x86_64 merge.s -o merge.o
@@ -317,10 +318,11 @@ SECTIONS {
 #--- spill-merge.lds
 ## SHF_MERGE sections are spilled according to the class refs of the first
 ## merged input section (the one giving the resulting section its name).
+## Spills take into account increases in section alignment due to merging.
 MEMORY {
   a : ORIGIN = 0, LENGTH = 1
-  b : ORIGIN = 1, LENGTH = 2
-  c : ORIGIN = 3, LENGTH = 2
+  b : ORIGIN = 1, LENGTH = 4
+  c : ORIGIN = 5, LENGTH = 4
 }
 
 SECTIONS {
@@ -336,8 +338,8 @@ SECTIONS {
 
 # SPILL-MERGE:      Name          Type     Address          Off    Size
 # SPILL-MERGE:      .first  PROGBITS 0000000000000000 000190 000000
-# SPILL-MERGE-NEXT: .second PROGBITS 0000000000000001 001001 000002
-# SPILL-MERGE-NEXT: .third  PROGBITS 0000000000000003 001003 000000
+# SPILL-MERGE-NEXT: .second PROGBITS 0000000000000002 001002 000003
+# SPILL-MERGE-NEXT: .third  PROGBITS 0000000000000006 001006 000000
 
 #--- link-order.s
 .section .a,"a", at progbits