lld-link: Allow backward references between associated comdats

[thakis]

Summary

References between associated comdats are invalid per COFF spec, but the newest Windows SDK contains obj files that have these references (https://bugs.chromium.org/p/chromium/issues/detail?id=925943#c13). So add back support for them and add tests for them. The old code handled them fine.

This makes lld-link match the behavior of newer link.exe versions as far as I can tell. (The behavior before this change matched the behavior of older link.exe versions.)

This mostly reverts r352254.

[hans]

lgtm

Maybe mention link.exe's behaviour in the commit message too?

lld/COFF/InputFiles.cpp

if (Parent == PendingComdat) {
     // without any symbols.
     Diag();
     return;
   }
 
   // Check whether the parent is prevailing. If it is, so are we, and we read
   // the section; otherwise mark it as discarded.
   if (Parent) {
-    if (Parent->Selection == IMAGE_COMDAT_SELECT_ASSOCIATIVE) {
-      Diag();
-      return;
-    }
-
     SectionChunk *C = readSection(SectionNumber, Def, "");
-    C->Selection = IMAGE_COMDAT_SELECT_ASSOCIATIVE;
     SparseChunks[SectionNumber] = C;
-    if (SparseChunks[SectionNumber])
-      Parent->addAssociative(SparseChunks[SectionNumber]);
+    if (C) {
+      C->Selection = IMAGE_COMDAT_SELECT_ASSOCIATIVE;
+      Parent->addAssociative(C);
+    }
   } else {
     SparseChunks[SectionNumber] = nullptr;
   }
 }
 
 void ObjFile::recordPrevailingSymbolForMingw(
     COFFSymbolRef Sym, DenseMap<StringRef, uint32_t> &PrevailingSectionMap) {
   // For comdat symbols in executable sections, where this is the copy

lld/test/COFF/associative-comdat-order.test

 # Tests that an associative comdat being associated with another
 # associated comdat later in the file produces an error.
 # RUN: sed -e s/ASSOC1/2/ -e s/ASSOC2/3/ %s | yaml2obj > %t.obj
 # RUN: not lld-link /include:symbol /dll /noentry /nodefaultlib %t.obj /out:%t.exe 2>&1 | FileCheck --check-prefix=FORWARD %s
 # FORWARD: lld-link: error: {{.*}}: associative comdat .text$ac1 (sec 1) has invalid reference to section .text$ac2 (sec 2)
 # FORWARD-NOT: lld-link: error:
 
 # Tests that an associative comdat being associated with another
-# associated comdat earlier in the file produces an error.
+# associated comdat earlier in the file works.
 # RUN: sed -e s/ASSOC1/3/ -e s/ASSOC2/1/ %s | yaml2obj > %t.obj
-# RUN: not lld-link /include:symbol /dll /noentry /nodefaultlib %t.obj /out:%t.exe 2>&1 | FileCheck --check-prefix=BACKWARD %s
-# BACKWARD: lld-link: error: {{.*}}: associative comdat .text$ac2 (sec 2) has invalid reference to section .text$ac1 (sec 1)
-# BACKWARD-NOT: lld-link: error:
+
+# RUN: lld-link /include:symbol /dll /noentry /nodefaultlib %t.obj /out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=BACKWARD %s
+# BACKWARD: Contents of section .text:
+# BACKWARD:   180001000 01000000 02000000 03000000 ............
+
+# RUN: lld-link /dll /noentry /nodefaultlib %t.obj /out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=BACKWARDDROP %s
+# BACKWARDDROP-NOT: Contents of section .text:
 
 --- !COFF
 header:          
   Machine:         IMAGE_FILE_MACHINE_AMD64
   Characteristics: [  ]
 sections:        
   - Name:            '.text$ac1'
     Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
     Alignment:       1
     SectionData:     '01000000'
   - Name:            '.text$ac2'
     Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
     Alignment:       1
-    SectionData:     '01000000'
+    SectionData:     '02000000'
   - Name:            '.text$nm'
     Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
     Alignment:       1
-    SectionData:     '01000000'
+    SectionData:     '03000000'
 symbols:         
   - Name:            '.text$ac1'
     Value:           0
     SectionNumber:   1
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_NULL
     StorageClass:    IMAGE_SYM_CLASS_STATIC
     SectionDefinition: