diff --git a/llvm/lib/ProfileData/GCOV.cpp b/llvm/lib/ProfileData/GCOV.cpp
index 35d40db11716..71ea44a1a722 100644
--- a/llvm/lib/ProfileData/GCOV.cpp
+++ b/llvm/lib/ProfileData/GCOV.cpp
@@ -175,16 +175,24 @@ bool GCOVFile::readGCDA(GCOVBuffer &buf) {
     if (tag == GCOV_TAG_OBJECT_SUMMARY) {
       buf.readInt(RunCount);
       buf.readInt(dummy);
+      // clang<11 uses a fake 4.2 format which sets length to 9.
+      if (length == 9)
+        buf.readInt(RunCount);
     } else if (tag == GCOV_TAG_PROGRAM_SUMMARY) {
-      buf.readInt(dummy);
-      buf.readInt(dummy);
-      buf.readInt(RunCount);
+      // clang<11 uses a fake 4.2 format which sets length to 0.
+      if (length > 0) {
+        buf.readInt(dummy);
+        buf.readInt(dummy);
+        buf.readInt(RunCount);
+      }
       ++ProgramCount;
     } else if (tag == GCOV_TAG_FUNCTION) {
       if (length == 0) // Placeholder
         continue;
       // As of GCC 10, GCOV_TAG_FUNCTION_LENGTH has never been larger than 3.
-      if ((length != 2 && length != 3) || !buf.readInt(ident))
+      // However, clang<11 uses a fake 4.2 format which may set length larger
+      // than 3.
+      if (length < 2 || !buf.readInt(ident))
         return false;
       auto It = IdentToFunction.find(ident);
       uint32_t linenoChecksum, cfgChecksum = 0;
diff --git a/llvm/test/tools/llvm-cov/Inputs/gcov-fake-4.2.gcda b/llvm/test/tools/llvm-cov/Inputs/gcov-fake-4.2.gcda
new file mode 100644
index 000000000000..613e3a85c026
Binary files /dev/null and b/llvm/test/tools/llvm-cov/Inputs/gcov-fake-4.2.gcda differ
diff --git a/llvm/test/tools/llvm-cov/Inputs/gcov-fake-4.2.gcno b/llvm/test/tools/llvm-cov/Inputs/gcov-fake-4.2.gcno
new file mode 100644
index 000000000000..24f1c82476b2
Binary files /dev/null and b/llvm/test/tools/llvm-cov/Inputs/gcov-fake-4.2.gcno differ
diff --git a/llvm/test/tools/llvm-cov/gcov-fake-4.2.c b/llvm/test/tools/llvm-cov/gcov-fake-4.2.c
new file mode 100644
index 000000000000..7e8eb2f2a5ff
--- /dev/null
+++ b/llvm/test/tools/llvm-cov/gcov-fake-4.2.c
@@ -0,0 +1,39 @@
+/// Test that llvm-cov supports a fake gcov 4.2 format used before clang 11.
+
+// RUN: rm -rf %t && mkdir %t && cd %t
+// RUN: llvm-cov gcov test. --gcno=%S/Inputs/gcov-fake-4.2.gcno --gcda=%S/Inputs/gcov-fake-4.2.gcda | FileCheck %s
+// RUN: FileCheck %s --check-prefix=C < test.cpp.gcov
+// RUN: FileCheck %s --check-prefix=H < test.h.gcov
+
+// CHECK:       File 'test.cpp'
+// CHECK-NEXT:  Lines executed:84.21% of 38
+// CHECK-NEXT:  Creating 'test.cpp.gcov'
+// CHECK-EMPTY:
+// CHECK-NEXT:  File './test.h'
+// CHECK-NEXT:  Lines executed:100.00% of 1
+// CHECK-NEXT:  Creating 'test.h.gcov'
+// CHECK-EMPTY:
+
+//      C:        -:    0:Source:test.cpp
+// C-NEXT:        -:    0:Graph:{{.*}}gcov-fake-4.2.gcno
+// C-NEXT:        -:    0:Data:{{.*}}gcov-fake-4.2.gcda
+/// `Runs` is stored in GCOV_TAG_OBJECT_SUMMARY with a length of 9.
+// C-NEXT:        -:    0:Runs:2
+// C-NEXT:        -:    0:Programs:1
+// C-NEXT:        -:    1:
+// C-NEXT:        -:    2:
+// C-NEXT:        -:    3:
+// C-NEXT:        -:    4:
+// C-NEXT:        -:    5:
+// C-NEXT:        -:    6:
+// C-NEXT:        -:    7:
+// C-NEXT:        -:    8:
+// C-NEXT:        -:    9:
+// C-NEXT:8589934592:   10:
+
+//      H:        -:    0:Source:./test.h
+// H-NEXT:        -:    0:Graph:{{.*}}gcov-fake-4.2.gcno
+// H-NEXT:        -:    0:Data:{{.*}}gcov-fake-4.2.gcda
+// H-NEXT:        -:    0:Runs:2
+// H-NEXT:        -:    0:Programs:1
+// H-NEXT:        4:    1: