[jitlink][ELF] Add zero-fill blocks for symbols in section SHN_COMMON

Symbols with special section index SHN_COMMON (0xfff2) haven't been handled so far and caused an invalid section error.

This is a more or less straightforward use of the code commented out at the end of the function. I checked with the ELF spec, that the symbol value gives the alignment.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D89795

llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp

@@ -510,6 +510,17 @@ private:
         // I am not sure on If this is going to hold as an invariant. Revisit.
         if (!Name)
           return Name.takeError();
+
+        if (SymRef.isCommon()) {
+          // Symbols in SHN_COMMON refer to uninitialized data. The st_value
+          // field holds alignment constraints.
+          Symbol &S =
+              G->addCommonSymbol(*Name, Scope::Default, getCommonSection(), 0,
+                                 SymRef.st_size, SymRef.getValue(), false);
+          JITSymbolTable[SymbolIndex] = &S;
+          continue;
+        }
+
         // TODO: weak and hidden
         if (SymRef.isExternal())
           bindings = {Linkage::Strong, Scope::Default};
@@ -550,18 +561,12 @@ private:
           JITSymbolTable[SymbolIndex] = &S;
         }
 
-        //  }
         // TODO: The following has to be implmented.
         // leaving commented out to save time for future patchs
         /*
           G->addAbsoluteSymbol(*Name, SymRef.getValue(), SymRef.st_size,
           Linkage::Strong, Scope::Default, false);
-
-          if(SymRef.isCommon()) {
-            G->addCommonSymbol(*Name, Scope::Default, getCommonSection(), 0, 0,
-          SymRef.getValue(), false);
-          }
-  */
+        */
       }
     }
     return Error::success();

llvm/test/ExecutionEngine/JITLink/X86/ELF_x86-64_common.s

@@ -0,0 +1,25 @@
+# RUN: rm -rf %t && mkdir -p %t
+# RUN: llvm-mc -triple=x86_64-unknown-linux -position-independent -filetype=obj -o %t/elf_common.o %s
+# RUN: llvm-jitlink -entry=load_common -noexec -check %s %t/elf_common.o
+
+        .text
+        .file   "elf_common.c"
+        .globl  load_common
+        .p2align        4, 0x90
+        .type   load_common,@function
+
+load_common:
+# Check that common variable GOT entry is synthesized correctly. In order to
+# prevent the optimizer from relaxing the edge, we use a movl instruction.
+# jitlink-check: decode_operand(load_common, 4) = \
+# jitlink-check:   got_addr(elf_common.o, common_data) - next_pc(load_common)
+# jitlink-check: *{8}(got_addr(elf_common.o, common_data)) = common_data
+        movl    common_data@GOTPCREL(%rip), %eax
+        ret
+.Lfunc_end0:
+        .size   load_common, .Lfunc_end0-load_common
+
+# Check that common is zero-filled.
+# jitlink-check: *{4}(common_data) = 0
+        .type   common_data,@object
+        .comm   common_data,4,4