diff --git a/lld/COFF/InputFiles.h b/lld/COFF/InputFiles.h
--- a/lld/COFF/InputFiles.h
+++ b/lld/COFF/InputFiles.h
@@ -255,9 +255,10 @@
   // match the existing symbol and its selection. If either old or new
   // symbol have selection IMAGE_COMDAT_SELECT_LARGEST, Sym might replace
   // the existing leader. In that case, Prevailing is set to true.
-  void handleComdatSelection(COFFSymbolRef sym,
-                             llvm::COFF::COMDATType &selection,
-                             bool &prevailing, DefinedRegular *leader);
+  void
+  handleComdatSelection(COFFSymbolRef sym, llvm::COFF::COMDATType &selection,
+                        bool &prevailing, DefinedRegular *leader,
+                        const llvm::object::coff_aux_section_definition *def);
 
   llvm::Optional<Symbol *>
   createDefined(COFFSymbolRef sym,
diff --git a/lld/COFF/InputFiles.cpp b/lld/COFF/InputFiles.cpp
--- a/lld/COFF/InputFiles.cpp
+++ b/lld/COFF/InputFiles.cpp
@@ -472,8 +472,23 @@
   return symtab->addUndefined(name, this, sym.isWeakExternal());
 }
 
-void ObjFile::handleComdatSelection(COFFSymbolRef sym, COMDATType &selection,
-                                    bool &prevailing, DefinedRegular *leader) {
+static const coff_aux_section_definition *findSectionDef(COFFObjectFile *obj,
+                                                         int32_t section) {
+  uint32_t numSymbols = obj->getNumberOfSymbols();
+  for (uint32_t i = 0; i < numSymbols; ++i) {
+    COFFSymbolRef sym = check(obj->getSymbol(i));
+    if (sym.getSectionNumber() != section)
+      continue;
+    if (const coff_aux_section_definition *def = sym.getSectionDefinition())
+      return def;
+  }
+  return nullptr;
+}
+
+void ObjFile::handleComdatSelection(
+    COFFSymbolRef sym, COMDATType &selection, bool &prevailing,
+    DefinedRegular *leader,
+    const llvm::object::coff_aux_section_definition *def) {
   if (prevailing)
     return;
   // There's already an existing comdat for this symbol: `Leader`.
@@ -540,8 +555,16 @@
     break;
 
   case IMAGE_COMDAT_SELECT_SAME_SIZE:
-    if (leaderChunk->getSize() != getSection(sym)->SizeOfRawData)
-      symtab->reportDuplicate(leader, this);
+    if (leaderChunk->getSize() != getSection(sym)->SizeOfRawData) {
+      if (!config->mingw) {
+        symtab->reportDuplicate(leader, this);
+      } else {
+        const coff_aux_section_definition *leaderDef = findSectionDef(
+            leaderChunk->file->getCOFFObj(), leaderChunk->getSectionNumber());
+        if (!leaderDef || leaderDef->Length != def->Length)
+          symtab->reportDuplicate(leader, this);
+      }
+    }
     break;
 
   case IMAGE_COMDAT_SELECT_EXACT_MATCH: {
@@ -657,7 +680,7 @@
     COMDATType selection = (COMDATType)def->Selection;
 
     if (leader->isCOMDAT)
-      handleComdatSelection(sym, selection, prevailing, leader);
+      handleComdatSelection(sym, selection, prevailing, leader, def);
 
     if (prevailing) {
       SectionChunk *c = readSection(sectionNumber, def, getName());
diff --git a/lld/test/COFF/Inputs/comdat-binutils.yaml b/lld/test/COFF/Inputs/comdat-binutils.yaml
new file mode 100644
--- /dev/null
+++ b/lld/test/COFF/Inputs/comdat-binutils.yaml
@@ -0,0 +1,30 @@
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [ IMAGE_FILE_RELOCS_STRIPPED, IMAGE_FILE_LINE_NUMS_STRIPPED ]
+sections:
+  - Name:            '.rdata$mysymbol'
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ ]
+    Alignment:       16
+    SectionData:     2A000000000000000000000000000000
+symbols:
+  - Name:            '.rdata$mysymbol'
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          1
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+      Selection:       IMAGE_COMDAT_SELECT_SAME_SIZE
+  - Name:            mysymbol
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...
diff --git a/lld/test/COFF/Inputs/comdat-llvm.yaml b/lld/test/COFF/Inputs/comdat-llvm.yaml
new file mode 100644
--- /dev/null
+++ b/lld/test/COFF/Inputs/comdat-llvm.yaml
@@ -0,0 +1,30 @@
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:
+  - Name:            '.rdata$mysymbol'
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     2A
+symbols:
+  - Name:            '.rdata$mysymbol'
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          1
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        3686517206
+      Number:          1
+      Selection:       IMAGE_COMDAT_SELECT_SAME_SIZE
+  - Name:            mysymbol
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...
diff --git a/lld/test/COFF/comdat-gcc-compatibility-size.test b/lld/test/COFF/comdat-gcc-compatibility-size.test
new file mode 100644
--- /dev/null
+++ b/lld/test/COFF/comdat-gcc-compatibility-size.test
@@ -0,0 +1,15 @@
+# RUN: yaml2obj %p/Inputs/comdat-llvm.yaml > %t.llvm.o
+# RUN: yaml2obj %p/Inputs/comdat-binutils.yaml > %t.binutils.o
+# RUN: lld-link -lldmingw -noentry -dll %t.llvm.o %t.binutils.o -out:%t.dll
+# RUN: lld-link -lldmingw -noentry -dll %t.binutils.o %t.llvm.o -out:%t.dll
+# RUN: not lld-link -noentry -dll %t.llvm.o %t.binutils.o -out:%t.dll
+# RUN: not lld-link -noentry -dll %t.binutils.o %t.llvm.o -out:%t.dll
+
+# The test object files have been generated by assembling the following
+# snippet using binutils and llvm.
+
+#        .section .rdata$mysymbol, "dr"
+#        .linkonce same_size
+#        .globl mysymbol
+#mysymbol:
+#        .byte 42