Do not create multiple NOTE segments.

[ruiu]

Summary

Previously, we didn't try to make effort to put .note sections next
to each other in the output file, so two .note sections were likely
to be stored to two separate NOTE segments. That's undesirable because
we should create as few segments as possible in general.

[ruiu]

• reverted a change against .note.b section.

ELF/Writer.cpp

 // We compute a rank for each section. The rank indicates where the
 // section should be placed in the file.  Instead of using simple
 // numbers (0,1,2...), we use a series of flags. One for each decision
 // point when placing the section.
 // Using flags has two key properties:
 // * It is easy to check if a give branch was taken.
 // * It is easy two see how similar two ranks are (see getRankProximity).
 enum RankFlags {
-  RF_NOT_ADDR_SET = 1 << 16,
-  RF_NOT_INTERP = 1 << 15,
-  RF_NOT_ALLOC = 1 << 14,
-  RF_WRITE = 1 << 13,
-  RF_EXEC_WRITE = 1 << 12,
-  RF_EXEC = 1 << 11,
-  RF_NON_TLS_BSS = 1 << 10,
-  RF_NON_TLS_BSS_RO = 1 << 9,
-  RF_NOT_TLS = 1 << 8,
-  RF_BSS = 1 << 7,
+  RF_NOT_ADDR_SET = 1 << 18,
+  RF_NOT_INTERP = 1 << 17,
+  RF_NOT_ALLOC = 1 << 16,
+  RF_WRITE = 1 << 15,
+  RF_EXEC_WRITE = 1 << 13,
+  RF_EXEC = 1 << 12,
+  RF_NON_TLS_BSS = 1 << 11,
+  RF_NON_TLS_BSS_RO = 1 << 10,
+  RF_NOT_TLS = 1 << 9,
+  RF_BSS = 1 << 8,
+  RF_NOTE = 1 << 7,
   RF_PPC_NOT_TOCBSS = 1 << 6,
   RF_PPC_OPD = 1 << 5,
   RF_PPC_TOCL = 1 << 4,
   RF_PPC_TOC = 1 << 3,
   RF_PPC_BRANCH_LT = 1 << 2,
   RF_MIPS_GPREL = 1 << 1,
   RF_MIPS_NOT_GOT = 1 << 0
 };
static unsigned getSectionRank(const OutputSection *Sec) {
   if (!IsTls)
     Rank |= RF_NOT_TLS;
 
   // Within the TLS initialization block, the non-nobits sections need to appear
   // first.
   if (IsNoBits)
     Rank |= RF_BSS;
 
+  // We create a NOTE segment for contiguous .note sections, so make
+  // them contigous if there are more than one .note section with the
+  // same attributes.
+  if (Sec->Type == SHT_NOTE)
+    Rank |= RF_NOTE;
+
   // Some architectures have additional ordering restrictions for sections
   // within the same PT_LOAD.
   if (Config->EMachine == EM_PPC64) {
     // PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections
     // that we would like to make sure appear is a specific order to maximize
     // their coverage by a single signed 16-bit offset from the TOC base
     // pointer. Conversely, the special .tocbss section should be first among
     // all SHT_NOBITS sections. This will put it next to the loaded special
if (Name == ".toc1")
       Rank |= RF_PPC_TOCL;
 
     if (Name == ".toc")
       Rank |= RF_PPC_TOC;
 
     if (Name == ".branch_lt")
       Rank |= RF_PPC_BRANCH_LT;
   }
+
   if (Config->EMachine == EM_MIPS) {
     // All sections with SHF_MIPS_GPREL flag should be grouped together
     // because data in these sections is addressable with a gp relative address.
     if (Sec->Flags & SHF_MIPS_GPREL)
       Rank |= RF_MIPS_GPREL;
 
     if (Sec->Name != ".got")
       Rank |= RF_MIPS_NOT_GOT;

test/ELF/note-contiguous.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 
 // RUN: ld.lld %t.o -o %t1
 // RUN: llvm-readobj -program-headers %t1 | FileCheck %s
 
 // CHECK:      Type: PT_NOTE
 // CHECK-NEXT: Offset:
 // CHECK-NEXT: VirtualAddress:
 // CHECK-NEXT: PhysicalAddress:
-// CHECK-NEXT: FileSize: 8
-// CHECK-NEXT: MemSize: 8
+// CHECK-NEXT: FileSize: 16
+// CHECK-NEXT: MemSize: 16
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   PF_R
 // CHECK-NEXT: ]
 // CHECK-NEXT: Alignment: 1
+// CHECK-NOT:  Type: PT_NOTE
 
 // RUN: echo "SECTIONS { .note : { *(.note.a) *(.note.b) } }" > %t.script
 // RUN: ld.lld %t.o --script %t.script -o %t2
 // RUN: llvm-readobj -program-headers %t2 | FileCheck -check-prefix=SCRIPT %s
 
 // SCRIPT:      Type: PT_NOTE
 // SCRIPT-NEXT: Offset:
 // SCRIPT-NEXT: VirtualAddress:
 // SCRIPT-NEXT: PhysicalAddress:
 // SCRIPT-NEXT: FileSize: 16
 // SCRIPT-NEXT: MemSize: 16
 // SCRIPT-NEXT: Flags [
 // SCRIPT-NEXT:   PF_R
 // SCRIPT-NEXT: ]
 // SCRIPT-NEXT: Alignment: 1
+// SCRIPT-NOT:  Type: PT_NOTE
 
 .section .note.a, "a", @note
 .quad 0
 
 .section .foo, "a"
 .quad 0
 
 .section .note.b, "a", @note
 .quad 0