[PATCH] D65242: [ELF] More dynamic relocation packing

[Vic Yang via Phabricator via llvm-commits]

victoryang created this revision.
victoryang added a reviewer: ruiu.
Herald added subscribers: llvm-commits, mgrang, MaskRay, kristof.beyls, arichardson, javed.absar, emaste.
Herald added a reviewer: espindola.
Herald added a project: LLVM.

Currently, with Android dynamic relocation packing, only relative
relocations are grouped together.  This patch implements similar
packing for non-relative relocations.

The implementation groups non-relative relocations with the same
r_info.  By requiring a minimum group size of 3, this achieves
smaller relocation sections.  Linking libmonochrome in Chromium for
Android, targeting ARM32, I see a .rel.dyn of size 127697 bytes, as
compared to 150532 bytes without this change.

Grouping by r_info also allows the runtime dynamic linker to implement
an 1-entry cache to reduce the number of symbol lookup required.  Such
a cache would hit 11167 times for libmonochrome, as compared to 4107
times without this change.


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D65242

Files:
  lld/ELF/SyntheticSections.cpp


Index: lld/ELF/SyntheticSections.cpp
===================================================================
--- lld/ELF/SyntheticSections.cpp
+++ lld/ELF/SyntheticSections.cpp
@@ -1679,6 +1679,35 @@
       relativeGroups.emplace_back(std::move(group));
   }
 
+  // Sort by r_info first so that we can group relocations with the same r_info.
+  // The cost of each group varies based on r_info, but we can approximate the
+  // cost analysis by the number of values encoded.  Each group adds 3 values to
+  // be encoded, and each relocation in a group encodes one less value, so we
+  // only group relocations if there are 3 or more of them with the same r_info.
+  llvm::sort(nonRelatives, [](const Elf_Rela &a, const Elf_Rela &b) {
+    return a.r_info < b.r_info;
+  });
+
+  std::vector<Elf_Rela> ungroupedNonRelatives;
+  std::vector<std::vector<Elf_Rela>> nonRelativeGroups;
+  for (auto i = nonRelatives.begin(), e = nonRelatives.end(); i != e;) {
+    std::vector<Elf_Rela> group;
+    do {
+      group.push_back(*i++);
+    } while (i != e && (i - 1)->r_info == i->r_info);
+
+    if (group.size() < 3)
+      ungroupedNonRelatives.insert(ungroupedNonRelatives.end(), group.begin(),
+                                   group.end());
+    else
+      nonRelativeGroups.emplace_back(std::move(group));
+  }
+
+  // Sort ungrouped relocations by offset to minimize the encoded length.
+  llvm::sort(ungroupedNonRelatives, [](const Elf_Rela &a, const Elf_Rela &b) {
+    return a.r_offset < b.r_offset;
+  });
+
   unsigned hasAddendIfRela =
       config->isRela ? RELOCATION_GROUP_HAS_ADDEND_FLAG : 0;
 
@@ -1733,14 +1762,26 @@
     }
   }
 
-  // Finally the non-relative relocations.
-  llvm::sort(nonRelatives, [](const Elf_Rela &a, const Elf_Rela &b) {
-    return a.r_offset < b.r_offset;
-  });
-  if (!nonRelatives.empty()) {
-    add(nonRelatives.size());
+  // Grouped non-relatives.
+  for (std::vector<Elf_Rela> &g : nonRelativeGroups) {
+    add(g.size());
+    add(RELOCATION_GROUPED_BY_INFO_FLAG | hasAddendIfRela);
+    add(g[0].r_info);
+    for (Elf_Rela &r : g) {
+      add(r.r_offset - offset);
+      offset = r.r_offset;
+      if (config->isRela) {
+        add(r.r_addend - addend);
+        addend = r.r_addend;
+      }
+    }
+  }
+
+  // Finally the ungrouped non-relative relocations.
+  if (!ungroupedNonRelatives.empty()) {
+    add(ungroupedNonRelatives.size());
     add(hasAddendIfRela);
-    for (Elf_Rela &r : nonRelatives) {
+    for (Elf_Rela &r : ungroupedNonRelatives) {
       add(r.r_offset - offset);
       offset = r.r_offset;
       add(r.r_info);


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