MC: prevent early DCE of empty sections

Prevent the early elimination of sections in the object writer.  There may be
references to the section itself by other symbols, which may potentially not be
possible to resolve.  ML (Visual Studio's Macro Assembler) also seems to retain
empty sections.

The elimination of symbols and sections which are unused should really occur at
the link phase.  This will not cause any change in the resulting binary, simply
in the generated object files.

The adjustments to the other unit tests account for the fluctuating section
index caused by the appearance of sections which were previously discarded.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210373 91177308-0d34-0410-b5e6-96231b3b80d8

11 files changed, 76 insertions, 63 deletions

diff --git a/lib/MC/WinCOFFObjectWriter.cpp b/lib/MC/WinCOFFObjectWriter.cpp
index 6f9c73b9cc..2d4b75858a 100644
--- a/lib/MC/WinCOFFObjectWriter.cpp
+++ b/lib/MC/WinCOFFObjectWriter.cpp
@@ -833,13 +833,9 @@ void WinCOFFObjectWriter::WriteObject(MCAssembler &Asm,
 
   DenseMap<COFFSection *, uint16_t> SectionIndices;
   for (auto & Section : Sections) {
-    if (Layout.getSectionAddressSize(Section->MCData) > 0) {
-      size_t Number = ++Header.NumberOfSections;
-      SectionIndices[Section.get()] = Number;
-      MakeSectionReal(*Section, Number);
-    } else {
-      Section->Number = -1;
-    }
+    size_t Number = ++Header.NumberOfSections;
+    SectionIndices[Section.get()] = Number;
+    MakeSectionReal(*Section, Number);
   }
 
   Header.NumberOfSymbols = 0;
diff --git a/test/MC/COFF/alias.s b/test/MC/COFF/alias.s
index dc4f65acf7..eb5398a4ed 100644
--- a/test/MC/COFF/alias.s
+++ b/test/MC/COFF/alias.s
@@ -1,4 +1,5 @@
-// RUN: llvm-mc -filetype=obj -triple i686-pc-win32 %s -o - | llvm-readobj -t -r | FileCheck %s
+// RUN: llvm-mc -filetype=obj -triple i686-pc-win32 %s -o - \
+// RUN:   | llvm-readobj -t -r | FileCheck %s
 
 local1:
 external_aliased_to_local = local1
@@ -36,7 +37,7 @@ weak_aliased_to_external = external2
 // CHECK-NEXT:     AuxSymbolCount: 1
 // CHECK:        }
 // CHECK:        Symbol {
-// CHECK-NEXT:     Name: local1
+// CHECK:          Name: local1
 // CHECK-NEXT:     Value: 0
 // CHECK-NEXT:     Section: .text (1)
 // CHECK-NEXT:     BaseType: Null (0x0)
@@ -89,7 +90,7 @@ weak_aliased_to_external = external2
 // CHECK-NEXT:     StorageClass: WeakExternal (0x69)
 // CHECK-NEXT:     AuxSymbolCount: 1
 // CHECK-NEXT:     AuxWeakExternal {
-// CHECK-NEXT:       Linked: external2 (9)
+// CHECK-NEXT:       Linked: external2 (13)
 // CHECK-NEXT:       Search: Library (0x2)
 // CHECK-NEXT:       Unused: (00 00 00 00 00 00 00 00 00 00)
 // CHECK-NEXT:     }
diff --git a/test/MC/COFF/basic-coff-64.s b/test/MC/COFF/basic-coff-64.s
index 89d17452d0..38a9e578a4 100644
--- a/test/MC/COFF/basic-coff-64.s
+++ b/test/MC/COFF/basic-coff-64.s
@@ -25,10 +25,10 @@ _main:                                  # @main
 
 // CHECK: ImageFileHeader {
 // CHECK:   Machine: IMAGE_FILE_MACHINE_AMD64
-// CHECK:   SectionCount: 2
+// CHECK:   SectionCount: 3
 // CHECK:   TimeDateStamp: {{[0-9]+}}
 // CHECK:   PointerToSymbolTable: 0x{{[0-9A-F]+}}
-// CHECK:   SymbolCount: 6
+// CHECK:   SymbolCount: 8
 // CHECK:   OptionalHeaderSize: 0
 // CHECK:   Characteristics [ (0x0)
 // CHECK:   ]
diff --git a/test/MC/COFF/basic-coff.s b/test/MC/COFF/basic-coff.s
index 9b299707a1..38bfa6d101 100644
--- a/test/MC/COFF/basic-coff.s
+++ b/test/MC/COFF/basic-coff.s
@@ -25,10 +25,10 @@ L_.str:                                 # @.str
 
 // CHECK: ImageFileHeader {
 // CHECK:   Machine: IMAGE_FILE_MACHINE_I386
-// CHECK:   SectionCount: 2
+// CHECK:   SectionCount: 3
 // CHECK:   TimeDateStamp: {{[0-9]+}}
 // CHECK:   PointerToSymbolTable: 0x{{[0-9A-F]+}}
-// CHECK:   SymbolCount: 6
+// CHECK:   SymbolCount: 8
 // CHECK:   OptionalHeaderSize: 0
 // CHECK:   Characteristics [ (0x0)
 // CHECK:   ]
diff --git a/test/MC/COFF/early-dce.s b/test/MC/COFF/early-dce.s
new file mode 100644
index 0000000000..ec1a9bda76
--- /dev/null
+++ b/test/MC/COFF/early-dce.s
@@ -0,0 +1,16 @@
+# RUN: llvm-mc -triple i686-windows -g -filetype obj -o - %s \
+# RUN:   | llvm-readobj -s -t | FileCheck %s
+
+	.section .rdata
+
+	.align 8
+	.global data
+data:
+	.quad 0
+
+# CHECK: Sections [
+# CHECK:  Section {
+# CHECK:    Name: .text
+# CHECK:  }
+# CHECK: ]
+
diff --git a/test/MC/COFF/linkonce.s b/test/MC/COFF/linkonce.s
index b6e81ad4b6..f2e350645b 100644
--- a/test/MC/COFF/linkonce.s
+++ b/test/MC/COFF/linkonce.s
@@ -91,41 +91,41 @@
 // CHECK: Symbols [
 // CHECK:   Symbol {
 // CHECK:     Name: s1
-// CHECK:     Section: s1 (1)
+// CHECK:     Section: s1 (4)
 // CHECK:     AuxSectionDef {
-// CHECK:       Number: 1
+// CHECK:       Number: 4
 // CHECK:       Selection: Any (0x2)
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: s2
-// CHECK:     Section: s2 (2)
+// CHECK:     Section: s2 (5)
 // CHECK:     AuxSectionDef {
-// CHECK:       Number: 2
+// CHECK:       Number: 5
 // CHECK:       Selection: NoDuplicates (0x1)
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: s3
-// CHECK:     Section: s3 (3)
+// CHECK:     Section: s3 (6)
 // CHECK:     AuxSectionDef {
-// CHECK:       Number: 3
+// CHECK:       Number: 6
 // CHECK:       Selection: Any (0x2)
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: s4
-// CHECK:     Section: s4 (4)
+// CHECK:     Section: s4 (7)
 // CHECK:     AuxSectionDef {
-// CHECK:       Number: 4
+// CHECK:       Number: 7
 // CHECK:       Selection: SameSize (0x3)
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: s5
-// CHECK:     Section: s5 (5)
+// CHECK:     Section: s5 (8)
 // CHECK:     AuxSectionDef {
-// CHECK:       Number: 5
+// CHECK:       Number: 8
 // CHECK:       Selection: ExactMatch (0x4)
 // CHECK:     }
 // CHECK:   }
@@ -134,17 +134,17 @@
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: s7
-// CHECK:     Section: s7 (7)
+// CHECK:     Section: s7 (10)
 // CHECK:     AuxSectionDef {
-// CHECK:       Number: 7
+// CHECK:       Number: 10
 // CHECK:       Selection: Largest (0x6)
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: s8
-// CHECK:     Section: s8 (8)
+// CHECK:     Section: s8 (11)
 // CHECK:     AuxSectionDef {
-// CHECK:       Number: 8
+// CHECK:       Number: 11
 // CHECK:       Selection: Newest (0x7)
 // CHECK:     }
 // CHECK:   }
diff --git a/test/MC/COFF/section-comdat.s b/test/MC/COFF/section-comdat.s
index a0ea7d0928..8a7de1a1d9 100644
--- a/test/MC/COFF/section-comdat.s
+++ b/test/MC/COFF/section-comdat.s
@@ -41,56 +41,56 @@ Symbol7:
 
 // CHECK: Sections [
 // CHECK:   Section {
-// CHECK:     Number: 1
+// CHECK:     Number: 4
 // CHECK:     Name: assocSec
 // CHECK:     Characteristics [
 // CHECK:       IMAGE_SCN_LNK_COMDAT
 // CHECK:     ]
 // CHECK:   }
 // CHECK:   Section {
-// CHECK:     Number: 2
+// CHECK:     Number: 5
 // CHECK:     Name: secName
 // CHECK:     Characteristics [
 // CHECK:       IMAGE_SCN_LNK_COMDAT
 // CHECK:     ]
 // CHECK:   }
 // CHECK:   Section {
-// CHECK:     Number: 3
+// CHECK:     Number: 6
 // CHECK:     Name: secName
 // CHECK:     Characteristics [
 // CHECK:       IMAGE_SCN_LNK_COMDAT
 // CHECK:     ]
 // CHECK:   }
 // CHECK:   Section {
-// CHECK:     Number: 4
+// CHECK:     Number: 7
 // CHECK:     Name: SecName
 // CHECK:     Characteristics [
 // CHECK:       IMAGE_SCN_LNK_COMDAT
 // CHECK:     ]
 // CHECK:   }
 // CHECK:   Section {
-// CHECK:     Number: 5
+// CHECK:     Number: 8
 // CHECK:     Name: SecName
 // CHECK:     Characteristics [
 // CHECK:       IMAGE_SCN_LNK_COMDAT
 // CHECK:     ]
 // CHECK:   }
 // CHECK:   Section {
-// CHECK:     Number: 6
+// CHECK:     Number: 9
 // CHECK:     Name: SecName
 // CHECK:     Characteristics [
 // CHECK:       IMAGE_SCN_LNK_COMDAT
 // CHECK:     ]
 // CHECK:   }
 // CHECK:   Section {
-// CHECK:     Number: 7
+// CHECK:     Number: 10
 // CHECK:     Name: SecName
 // CHECK:     Characteristics [
 // CHECK:       IMAGE_SCN_LNK_COMDAT
 // CHECK:     ]
 // CHECK:   }
 // CHECK:   Section {
-// CHECK:     Number: 8
+// CHECK:     Number: 11
 // CHECK:     Name: SecName
 // CHECK:     Characteristics [
 // CHECK:       IMAGE_SCN_LNK_COMDAT
@@ -100,7 +100,7 @@ Symbol7:
 // CHECK: Symbols [
 // CHECK:   Symbol {
 // CHECK:     Name: assocSec
-// CHECK:     Section: assocSec (1)
+// CHECK:     Section: assocSec (4)
 // CHECK:     AuxSectionDef {
 // CHECK:       Selection: Any
 // CHECK:     }
@@ -111,40 +111,40 @@ Symbol7:
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: secName
-// CHECK:     Section: secName (2)
+// CHECK:     Section: secName (5)
 // CHECK:     AuxSectionDef {
 // CHECK:       Selection: Any
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: Symbol1
-// CHECK:     Section: secName (2)
+// CHECK:     Section: secName (5)
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: secName
-// CHECK:     Section: secName (3)
+// CHECK:     Section: secName (6)
 // CHECK:     AuxSectionDef {
 // CHECK:       Selection: NoDuplicates
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: Symbol2
-// CHECK:     Section: secName (3)
+// CHECK:     Section: secName (6)
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: SecName
-// CHECK:     Section: SecName (4)
+// CHECK:     Section: SecName (7)
 // CHECK:     AuxSectionDef {
 // CHECK:       Selection: SameSize
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: Symbol3
-// CHECK:     Section: SecName (4)
+// CHECK:     Section: SecName (7)
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: SecName
-// CHECK:     Section: SecName (5)
+// CHECK:     Section: SecName (8)
 // CHECK:     AuxSymbolCount: 1
 // CHECK:     AuxSectionDef {
 // CHECK:       Selection: ExactMatch
@@ -152,40 +152,40 @@ Symbol7:
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: Symbol4
-// CHECK:     Section: SecName (5)
+// CHECK:     Section: SecName (8)
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: SecName
-// CHECK:     Section: SecName (6)
+// CHECK:     Section: SecName (9)
 // CHECK:     AuxSectionDef {
 // CHECK:       Selection: Associative
-// CHECK:       AssocSection: assocSec (1)
+// CHECK:       AssocSection: assocSec (4)
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: SecName
-// CHECK:     Section: SecName (7)
+// CHECK:     Section: SecName (10)
 // CHECK:     AuxSectionDef {
 // CHECK:       Selection: Largest
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: Symbol6
-// CHECK:     Section: SecName (7)
+// CHECK:     Section: SecName (10)
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: SecName
-// CHECK:     Section: SecName (8)
+// CHECK:     Section: SecName (11)
 // CHECK:     AuxSectionDef {
 // CHECK:       Selection: Newest (0x7)
 // CHECK:     }
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: Symbol7
-// CHECK:     Section: SecName (8)
+// CHECK:     Section: SecName (11)
 // CHECK:   }
 // CHECK:   Symbol {
 // CHECK:     Name: Symbol5
-// CHECK:     Section: SecName (6)
+// CHECK:     Section: SecName (9)
 // CHECK:   }
 // CHECK: ]
diff --git a/test/MC/COFF/section-name-encoding.s b/test/MC/COFF/section-name-encoding.s
index 74cd490bd3..7edd6d7446 100644
--- a/test/MC/COFF/section-name-encoding.s
+++ b/test/MC/COFF/section-name-encoding.s
@@ -10,11 +10,11 @@
 // Raw encoding
 
 // CHECK:   Section {
-// CHECK:     Number: 1
+// CHECK:     Number: 4
 // CHECK:     Name: s (73 00 00 00 00 00 00 00)
 // CHECK:   }
 // CHECK:   Section {
-// CHECK:     Number: 2
+// CHECK:     Number: 5
 // CHECK:     Name: s1234567 (73 31 32 33 34 35 36 37)
 // CHECK:   }
 .section s;        .long 1
@@ -25,7 +25,7 @@
 
 // /4
 // CHECK:   Section {
-// CHECK:     Number: 3
+// CHECK:     Number: 6
 // CHECK:     Name: s12345678 (2F 34 00 00 00 00 00 00)
 // CHECK:   }
 .section s12345678; .long 1
@@ -57,7 +57,7 @@ pad_sections aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 //     "s12345678\0"     # of pad sections
 //
 // CHECK:   Section {
-// CHECK:     Number: 9
+// CHECK:     Number: 12
 // CHECK:     Name: seven_digit (2F 31 30 30 30 30 32 39)
 // CHECK:   }
 .section seven_digit; .long 1
@@ -82,7 +82,7 @@ pad_sections_ex aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 // "2F 2F 41 41 6D 4A 61 34" is "//AAmJa4", which decodes to "0 0 38 9 26 56".
 //
 // CHECK:   Section {
-// CHECK:     Number: 15
+// CHECK:     Number: 18
 // CHECK:     Name: double_slash (2F 2F 41 41 6D 4A 61 34)
 // CHECK:   }
 .section double_slash; .long 1
diff --git a/test/MC/COFF/seh.s b/test/MC/COFF/seh.s
index 72d42f4b2e..cd884b4fa0 100644
--- a/test/MC/COFF/seh.s
+++ b/test/MC/COFF/seh.s
@@ -35,13 +35,13 @@
 // CHECK-NEXT: ]
 
 // CHECK-NEXT: Relocations [
-// CHECK-NEXT:   Section (2) .xdata {
+// CHECK-NEXT:   Section (4) .xdata {
 // CHECK-NEXT:     0x14 IMAGE_REL_AMD64_ADDR32NB __C_specific_handler
 // CHECK-NEXT:     0x20 IMAGE_REL_AMD64_ADDR32NB func
 // CHECK-NEXT:     0x24 IMAGE_REL_AMD64_ADDR32NB func
 // CHECK-NEXT:     0x28 IMAGE_REL_AMD64_ADDR32NB .xdata
 // CHECK-NEXT:   }
-// CHECK-NEXT:   Section (3) .pdata {
+// CHECK-NEXT:   Section (5) .pdata {
 // CHECK-NEXT:     0x0 IMAGE_REL_AMD64_ADDR32NB func
 // CHECK-NEXT:     0x4 IMAGE_REL_AMD64_ADDR32NB func
 // CHECK-NEXT:     0x8 IMAGE_REL_AMD64_ADDR32NB .xdata
diff --git a/test/MC/COFF/symbol-fragment-offset-64.s b/test/MC/COFF/symbol-fragment-offset-64.s
index b8244709aa..deac88869b 100644
--- a/test/MC/COFF/symbol-fragment-offset-64.s
+++ b/test/MC/COFF/symbol-fragment-offset-64.s
@@ -36,10 +36,10 @@ _main:                                  # @main
 
 // CHECK: {
 // CHECK:   Machine:                   IMAGE_FILE_MACHINE_AMD64
-// CHECK:   SectionCount:              2
+// CHECK:   SectionCount:              3
 // CHECK:   TimeDateStamp:             {{[0-9]+}}
 // CHECK:   PointerToSymbolTable:      0x{{[0-9A-F]+}}
-// CHECK:   SymbolCount:               7
+// CHECK:   SymbolCount:               9
 // CHECK:   OptionalHeaderSize:        0
 // CHECK:   Characteristics [ (0x0)
 // CHECK:   ]
diff --git a/test/MC/COFF/symbol-fragment-offset.s b/test/MC/COFF/symbol-fragment-offset.s
index 71b1703972..b09c5af1b6 100644
--- a/test/MC/COFF/symbol-fragment-offset.s
+++ b/test/MC/COFF/symbol-fragment-offset.s
@@ -36,10 +36,10 @@ L_.str2:
 
 // CHECK: {
 // CHECK:   Machine:                   IMAGE_FILE_MACHINE_I386 (0x14C)
-// CHECK:   SectionCount:              2
+// CHECK:   SectionCount:              3
 // CHECK:   TimeDateStamp:             {{[0-9]+}}
 // CHECK:   PointerToSymbolTable:      0x{{[0-9A-F]+}}
-// CHECK:   SymbolCount:               7
+// CHECK:   SymbolCount:               9
 // CHECK:   OptionalHeaderSize:        0
 // CHECK:   Characteristics [ (0x0)
 // CHECK:   ]