[ELF] Fix llvm_unreachable failure when COMMON is placed in SHT_PROGBITS output section

Fix a regression in aa27bab: COMMON in an
SHT_PROGBITS output section caused llvm_unreachable failure.

Author: MaskRay

lld/ELF/SyntheticSections.h

@@ -186,9 +186,7 @@ class BuildIdSection : public SyntheticSection {
 class BssSection final : public SyntheticSection {
 public:
   BssSection(StringRef name, uint64_t size, uint32_t alignment);
-  void writeTo(uint8_t *) override {
-    llvm_unreachable("unexpected writeTo() call for SHT_NOBITS section");
-  }
+  void writeTo(uint8_t *) override {}
   bool isNeeded() const override { return size != 0; }
   size_t getSize() const override { return size; }
 

lld/test/ELF/linkerscript/common.s

@@ -1,47 +1,44 @@
 # REQUIRES: x86
-# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
-# RUN: echo "SECTIONS { . = SIZEOF_HEADERS; .common : { *(COMMON) } }" > %t.script
-# RUN: ld.lld -o %t1 --script %t.script %t
-# RUN: llvm-readobj -S --symbols %t1 | FileCheck %s
-
-# CHECK:       Section {
-# CHECK:         Index:
-# CHECK:         Name: .common
-# CHECK-NEXT:    Type: SHT_NOBITS
-# CHECK-NEXT:    Flags [
-# CHECK-NEXT:      SHF_ALLOC
-# CHECK-NEXT:      SHF_WRITE
-# CHECK-NEXT:    ]
-# CHECK-NEXT:    Address: 0x200
-# CHECK-NEXT:    Offset: 0x
-# CHECK-NEXT:    Size: 384
-# CHECK-NEXT:    Link: 0
-# CHECK-NEXT:    Info: 0
-# CHECK-NEXT:    AddressAlignment: 256
-# CHECK-NEXT:    EntrySize: 0
-# CHECK-NEXT:  }
-# CHECK:       Symbol {
-# CHECK:         Name: q1
-# CHECK-NEXT:    Value: 0x200
-# CHECK-NEXT:    Size: 128
-# CHECK-NEXT:    Binding: Global
-# CHECK-NEXT:    Type: Object
-# CHECK-NEXT:    Other: 0
-# CHECK-NEXT:    Section: .common
-# CHECK-NEXT:  }
-# CHECK-NEXT:  Symbol {
-# CHECK-NEXT:    Name: q2
-# CHECK-NEXT:    Value: 0x300
-# CHECK-NEXT:    Size: 128
-# CHECK-NEXT:    Binding: Global
-# CHECK-NEXT:    Type: Object
-# CHECK-NEXT:    Other: 0
-# CHECK-NEXT:    Section: .common
-# CHECK-NEXT:  }
+## Test that COMMON matches common symbols.
 
+# RUN: rm -rf %t && split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %t/a.s -o %t/a.o
+# RUN: ld.lld -T %t/1.t %t/a.o -o %t/a1
+# RUN: llvm-readelf -S -s %t/a1 | FileCheck %s --check-prefix=CHECK1
+# RUN: ld.lld -T %t/2.t %t/a.o -o %t/a2
+# RUN: llvm-readelf -S -s %t/a2 | FileCheck %s --check-prefix=CHECK2
+
+# CHECK1:      [Nr] Name    Type     Address  Off      Size   ES Flg Lk Inf Al
+# CHECK1-NEXT: [ 0]         NULL     [[#%x,]] [[#%x,]] 000000 00      0   0  0
+# CHECK1-NEXT: [ 1] .text   PROGBITS [[#%x,]] [[#%x,]] 000005 00  AX  0   0  4
+# CHECK1-NEXT: [ 2] .data   PROGBITS [[#%x,]] [[#%x,]] 000001 00  WA  0   0  1
+# CHECK1-NEXT: [ 3] .common NOBITS   [[#%x,]] [[#%x,]] 000180 00  WA  0   0 256
+# CHECK1:         Value          Size Type    Bind   Vis     Ndx   Name
+# CHECK1-DAG:  [[#%x,]]           128 OBJECT  GLOBAL DEFAULT [[#]] q1
+# CHECK1-DAG:  [[#%x,]]           128 OBJECT  GLOBAL DEFAULT [[#]] q2
+
+# CHECK2:      [Nr] Name    Type     Address  Off      Size   ES Flg Lk Inf Al
+# CHECK2-NEXT: [ 0]         NULL     [[#%x,]] [[#%x,]] 000000 00      0   0  0
+# CHECK2-NEXT: [ 1] .text   PROGBITS [[#%x,]] [[#%x,]] 000005 00  AX  0   0  4
+# CHECK2-NEXT: [ 2] .data   PROGBITS [[#%x,]] [[#%x,]] 000180 00  WA  0   0 256
+# CHECK2:         Value          Size Type    Bind   Vis     Ndx   Name
+# CHECK2-DAG:  [[#%x,]]           128 OBJECT  GLOBAL DEFAULT [[#]] q1
+# CHECK2-DAG:  [[#%x,]]           128 OBJECT  GLOBAL DEFAULT [[#]] q2
+
+#--- a.s
 .globl _start
 _start:
   jmp _start
 
+.section .data,"aw",@progbits
+.byte 0
+
 .comm q1,128,8
 .comm q2,128,256
+
+#--- 1.t
+SECTIONS { . = SIZEOF_HEADERS; .common : { *(COMMON) } }
+
+#--- 2.t
+## COMMON can be placed in a SHT_PROGBITS section.
+SECTIONS { . = SIZEOF_HEADERS; .data : { *(.data) *(COMMON) } }