[PATCH] D15564: [ELF] - Optimize .eh_frame section: remove CIE if all FDEs referencing it were removed.

[George Rimar via llvm-commits]

grimar created this revision.
grimar added reviewers: ruiu, rafael.
grimar added subscribers: llvm-commits, grimar.

This patch performs little optimization for eh_frame section.
If all FDE`s that referenced CIE are removed then CIE is also removed from output. 
That can happen for example when dropping FDEs that point to dropped sections. Testcase showing that is included.
The same optimization was added to ld about 14 years ago: https://sourceware.org/ml/binutils/2001-12/msg00144.html, gold does not do that it seems.

http://reviews.llvm.org/D15564

Files:
  ELF/OutputSections.cpp
  test/ELF/eh-frame-opt.s

Index: test/ELF/eh-frame-opt.s
===================================================================
--- test/ELF/eh-frame-opt.s
+++ test/ELF/eh-frame-opt.s
@@ -0,0 +1,39 @@
+// REQUIRES: x86
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+// RUN: ld.lld --gc-sections %t.o -o %t
+// RUN: llvm-readobj -s -section-data %t | FileCheck %s
+
+// Here we check that if all FDEs referencing a CIE
+// were removed, CIE is also removed.
+// CHECK:        Section {
+// CHECK:        Index:
+// CHECK:        Name: .eh_frame
+// CHECK-NEXT:   Type: SHT_X86_64_UNWIND
+// CHECK-NEXT:   Flags [
+// CHECK-NEXT:     SHF_ALLOC
+// CHECK-NEXT:   ]
+// CHECK-NEXT:   Address: 0x10120
+// CHECK-NEXT:   Offset: 0x120
+// CHECK-NEXT:   Size: 0
+// CHECK-NEXT:   Link: 0
+// CHECK-NEXT:   Info: 0
+// CHECK-NEXT:   AddressAlignment: 8
+// CHECK-NEXT:   EntrySize: 0
+// CHECK-NEXT:   SectionData (
+// CHECK-NEXT:   )
+// CHECK-NEXT: }
+
+.section foo,"ax", at progbits
+.cfi_startproc
+ nop
+.cfi_endproc
+
+.section bar,"ax", at progbits
+.cfi_startproc
+ nop
+ nop
+.cfi_endproc
+
+.text
+.globl _start;
+_start:
Index: ELF/OutputSections.cpp
===================================================================
--- ELF/OutputSections.cpp
+++ ELF/OutputSections.cpp
@@ -941,7 +941,8 @@
   auto RelI = Rels.begin();
   auto RelE = Rels.end();
 
-  DenseMap<unsigned, unsigned> OffsetToIndex;
+  // Maps offset to Index/Length pair.
+  DenseMap<unsigned, std::pair<unsigned, uint32_t>> OffsetToData;
   while (!D.empty()) {
     if (D.size() < 4)
       error("Truncated CIE/FDE length");
@@ -974,21 +975,23 @@
 
       std::pair<StringRef, StringRef> CieInfo(Entry, Personality);
       auto P = CieMap.insert(std::make_pair(CieInfo, Cies.size()));
-      if (P.second) {
+      if (P.second)
         Cies.push_back(C);
-        this->Header.sh_size += Length;
-      }
-      OffsetToIndex[Offset] = P.first->second;
+      OffsetToData[Offset] = std::make_pair(P.first->second, Length);
     } else {
       if (!HasReloc)
         error("FDE doesn't reference another section");
       InputSectionBase<ELFT> *Target = S->getRelocTarget(*RelI);
       if (Target != &InputSection<ELFT>::Discarded && Target->isLive()) {
         uint32_t CieOffset = Offset + 4 - ID;
-        auto I = OffsetToIndex.find(CieOffset);
-        if (I == OffsetToIndex.end())
+        auto I = OffsetToData.find(CieOffset);
+        if (I == OffsetToData.end())
           error("Invalid CIE reference");
-        Cies[I->second].Fdes.push_back(EHRegion<ELFT>(S, Index));
+        std::pair<unsigned, uint32_t> &IndLen = I->second;
+        Cie<ELFT> &Cie = Cies[IndLen.first];
+        if (Cie.Fdes.empty())
+          this->Header.sh_size += IndLen.second;
+        Cie.Fdes.push_back(EHRegion<ELFT>(S, Index));
         this->Header.sh_size += Length;
       }
     }
@@ -1014,6 +1017,9 @@
   const endianness E = ELFT::TargetEndianness;
   size_t Offset = 0;
   for (const Cie<ELFT> &C : Cies) {
+    if (C.Fdes.empty())
+      continue;
+
     size_t CieOffset = Offset;
 
     StringRef CieData = C.data();


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