[LLD][ELF] Fix SHF_MERGE misalignment when spilled (llvm#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.

Author: mysterymath

lld/ELF/OutputSections.cpp

@@ -253,8 +253,20 @@ void OutputSection::finalizeInputSections() {
     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 (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,

lld/test/ELF/linkerscript/section-class.test

@@ -310,17 +310,19 @@ SECTIONS {
 .byte 0x12, 0x34
 
 .section .b,"aM",@progbits,1
+.p2align 3
 .byte 0x12
 
 # RUN: llvm-mc -n -filetype=obj -triple=x86_64 merge.s -o merge.o
 
 #--- 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 = 16
+  c : ORIGIN = 17, LENGTH = 16
 }
 
 SECTIONS {
@@ -332,12 +334,14 @@ SECTIONS {
 }
 
 # RUN: ld.lld -T spill-merge.lds merge.o -o spill-merge
-# RUN: llvm-readelf -S spill-merge | FileCheck %s --check-prefix=SPILL-MERGE
+# RUN: llvm-readelf -S -x .second spill-merge | FileCheck %s --check-prefix=SPILL-MERGE
 
-# SPILL-MERGE:      Name          Type     Address          Off    Size
+# 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 0000000000000008 001008 000009
+# SPILL-MERGE-NEXT: .third  PROGBITS 0000000000000018 001018 000000
+# SPILL-MERGE:      Hex dump of section '.second':
+# SPILL-MERGE-NEXT: 0x00000008 12000000 00000000 34 .
 
 #--- link-order.s
 .section .a,"a",@progbits