[lld] b6e4aae - [ELF] --gc-sections: retain dependent sections of non-SHF_ALLOC sections

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2020-10-21T10:11:26-07:00
New Revision: b6e4aae2cc269bbdce9263cd49dcd582840b333a

URL: https://github.com/llvm/llvm-project/commit/b6e4aae2cc269bbdce9263cd49dcd582840b333a
DIFF: https://github.com/llvm/llvm-project/commit/b6e4aae2cc269bbdce9263cd49dcd582840b333a.diff

LOG: [ELF] --gc-sections: retain dependent sections of non-SHF_ALLOC sections

Fix http://lists.llvm.org/pipermail/llvm-dev/2020-October/145908.html

Currently non-SHF_ALLOC SHT_REL[A] (due to --emit-relocs) and SHF_LINK_ORDER are not
marked live.

Reviewed By: grimar, psmith

Differential Revision: https://reviews.llvm.org/D89841

Added: 
    

Modified: 
    lld/ELF/MarkLive.cpp
    lld/test/ELF/gc-sections-linkorder2.s

Removed: 
    


################################################################################
diff  --git a/lld/ELF/MarkLive.cpp b/lld/ELF/MarkLive.cpp
index af6c08c21581..fe5d32c80bfe 100644
--- a/lld/ELF/MarkLive.cpp
+++ b/lld/ELF/MarkLive.cpp
@@ -339,16 +339,16 @@ template <class ELFT> void elf::markLive() {
 
   // Otherwise, do mark-sweep GC.
   //
-  // The -gc-sections option works only for SHF_ALLOC sections
-  // (sections that are memory-mapped at runtime). So we can
-  // unconditionally make non-SHF_ALLOC sections alive except
-  // SHF_LINK_ORDER and SHT_REL/SHT_RELA sections.
+  // The -gc-sections option works only for SHF_ALLOC sections (sections that
+  // are memory-mapped at runtime). So we can unconditionally make non-SHF_ALLOC
+  // sections alive except SHF_LINK_ORDER, SHT_REL/SHT_RELA sections, and
+  // sections in a group.
   //
   // Usually, non-SHF_ALLOC sections are not removed even if they are
-  // unreachable through relocations because reachability is not
-  // a good signal whether they are garbage or not (e.g. there is
-  // usually no section referring to a .comment section, but we
-  // want to keep it.).
+  // unreachable through relocations because reachability is not a good signal
+  // whether they are garbage or not (e.g. there is usually no section referring
+  // to a .comment section, but we want to keep it.) When a non-SHF_ALLOC
+  // section is retained, we also retain sections dependent on it.
   //
   // Note on SHF_LINK_ORDER: Such sections contain metadata and they
   // have a reverse dependency on the InputSection they are linked with.
@@ -370,8 +370,11 @@ template <class ELFT> void elf::markLive() {
     bool isLinkOrder = (sec->flags & SHF_LINK_ORDER);
     bool isRel = (sec->type == SHT_REL || sec->type == SHT_RELA);
 
-    if (!isAlloc && !isLinkOrder && !isRel && !sec->nextInSectionGroup)
+    if (!isAlloc && !isLinkOrder && !isRel && !sec->nextInSectionGroup) {
       sec->markLive();
+      for (InputSection *isec : sec->dependentSections)
+        isec->markLive();
+    }
   }
 
   // Follow the graph to mark all live sections.

diff  --git a/lld/test/ELF/gc-sections-linkorder2.s b/lld/test/ELF/gc-sections-linkorder2.s
index b22bc589f56e..2024972b3937 100644
--- a/lld/test/ELF/gc-sections-linkorder2.s
+++ b/lld/test/ELF/gc-sections-linkorder2.s
@@ -14,3 +14,9 @@ _start:
 .quad 0
 .section .zed,"ao", at progbits,.foo
 .quad 0
+
+.section .nonalloc
+.quad 0
+
+.section .nonalloc_linkorder,"o", at progbits,.nonalloc
+.quad 0