[JITLink][COFF] Implement IMAGE_COMDAT_SELECT_LARGEST partially.

[sunho]

Summary

Implement IMAGE_COMDAT_SELECT_LARGEST partially. It's going to fail if larger symbol appears but this hasn't happened at least in vcruntime library.

We probably would not implement this properly as it requires complicated runtime patching which is not of nature of JIT. However, we'd like to validate if larger section appears and report to the user in the near future.

[lhames]

LGTM, at least as an initial step to bring up COFF support.

So the MSVC runtime requires this? Do you know what kind of symbols the runtime uses IMAGE_COMDAT_SELECT_LARGEST for? C common symbols?

[sunho]

@lhames Almost all comdat symbols in vc runtime uses largest selection type. I don't really see size difference between symbols thought, maybe it was to deal with backwards compatibility.

llvm/lib/ExecutionEngine/JITLink/COFFLinkGraphBuilder.cpp

Expected<Symbol *> COFFLinkGraphBuilder::createCOMDATExportRequest(
   case COFF::IMAGE_COMDAT_SELECT_EXACT_MATCH:
   case COFF::IMAGE_COMDAT_SELECT_SAME_SIZE: {
     // FIXME: Implement size/content validation when LinkGraph is able to
     // handle this.
     L = Linkage::Weak;
     break;
   }
   case COFF::IMAGE_COMDAT_SELECT_LARGEST: {
-    // FIXME: Support IMAGE_COMDAT_SELECT_LARGEST when LinkGraph is able to
-    // handle this.
-    return make_error<JITLinkError>(
-        "IMAGE_COMDAT_SELECT_LARGEST is not supported.");
+    // FIXME: Support IMAGE_COMDAT_SELECT_LARGEST properly when LinkGraph is
+    // able to handle this.
+    LLVM_DEBUG({
+      dbgs() << "    " << SymIndex
+             << ": Partially supported IMAGE_COMDAT_SELECT_LARGEST was used"
+                " in section "
+             << Symbol.getSectionNumber() << "\n";
+    });
+    L = Linkage::Weak;
+    break;
   }
   case COFF::IMAGE_COMDAT_SELECT_NEWEST: {
     // Even link.exe doesn't support this selection properly.
     return make_error<JITLinkError>(
         "IMAGE_COMDAT_SELECT_NEWEST is not supported.");
   }
   default: {
     return make_error<JITLinkError>("Invalid comdat selection type: " +

llvm/test/ExecutionEngine/JITLink/X86/COFF_comdat_largest.test

 # REQUIRES: asserts
-# XFAIL: *
 # RUN: yaml2obj %s -o %t
 # RUN: llvm-jitlink -noexec --debug-only=jitlink -noexec %t 2>&1
 # 
-# Check jitlink return an error when IMAGE_COMDAT_SELECT_LARGEST selection type is encountered.
+# Check jitlink handles largest selection type as plain weak symbol.
 #
+# CHECK: Creating graph symbols...
+# CHECK:      2: Creating defined graph symbol for COFF symbol ".text" in .text (index: 2)
+# CHECK-NEXT:   0x0 (block + 0x00000000): size: 0x00000001, linkage: strong, scope: local, dead  -   <anonymous symbol>
+# CHECK-NEXT: 4: Exporting COMDAT graph symbol for COFF symbol "func" in section 2
+# CHECK-NEXT:   0x0 (block + 0x00000000): size: 0x00000001, linkage: weak, scope: default, dead  -   func
 
 --- !COFF
 header:
   Machine:         IMAGE_FILE_MACHINE_AMD64
   Characteristics: [  ]
 sections:
   - Name:            .text
     Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]