Fix JITLink for ELF X86 so that it accepts a .bss section by adding a call to createZeroFillBlock

[• drmeister]

Summary

Add an invocation to createZeroFillBlock to allow the .bss section to be loaded.
Also, enhance two error messages so that they print the name of the section that they could not find.

[• drmeister]

Fixed the formatting suggestions from my first submission.

[lhames]

Looks great -- thanks @drmeister.

All of Lint's comments about formatting are valid. If you want an easy way to avoid formatting comments in the future you can use the git-clang-format utility. Just make sure clang is in your path, then run:

/path/to/clang/tools/clang-format/git-clang-format <commit>

e.g.

/path/to/clang/tools/clang-format/git-clang-format HEAD

That will reformat your patch to match clang's formatting guides. (In this patch you'll also need to fix the case on one of your variables: I don't believe clang-format fixes case for you).

This also needs an llvm-jitlink regression test. I think the following change to llvm/test/ExecutionEngine/JITLink/X86/ELF_x86-64_relocations.s should work:

diff --git a/llvm/test/ExecutionEngine/JITLink/X86/ELF_x86-64_relocations.s b/llvm/test/ExecutionEngine/JITLink/X86/ELF_x86-64_relocations.s
index ca9c926b32d..0eef1111026 100644
--- a/llvm/test/ExecutionEngine/JITLink/X86/ELF_x86-64_relocations.s
+++ b/llvm/test/ExecutionEngine/JITLink/X86/ELF_x86-64_relocations.s
@@ -46,6 +46,17 @@ named_data:
         .long   42
         .size   named_data, 4
 
+# Test BSS / zero-fill section handling.
+# llvm-jitlink: *{4}bss_variable = 0
+
+       .type   bss_variable,@object
+       .bss
+       .globl  bss_variable
+       .p2align        2
+bss_variable:
+       .long   0
+       .size   bss_variable, 4
+
         .ident  "clang version 10.0.0-4ubuntu1 "
         .section        ".note.GNU-stack","",@progbits

I have verified that it fails without your patch -- if it runs with it then this is a viable test.

[lhames]

You beat me to it with the formatting. Assuming the test case works for you this is good to land.

llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp

for (auto &SecRef : sections) {
         // for now everything is
         auto &section = G->createSection(*Name, Prot);
         // Do this here because we have it, but move it into graphify later
         G->createContentBlock(section, StringRef(Data, Size), Address,
                               Alignment, 0);
         if (SecRef.sh_type == ELF::SHT_SYMTAB)
           // TODO: Dynamic?
           SymTab = SecRef;
+      } else {
+        auto &Section = G->createSection(*Name, Prot);
+        G->createZeroFillBlock(Section, Size, Address, Alignment, 0);
       }
     }
 
     return Error::success();
   }
 
   Error addRelocations() {
     LLVM_DEBUG(dbgs() << "Adding relocations\n");
for (auto SecRef : sections) {
       auto StringTable = Obj.getStringTable(**StrTabSec);
       if (!StringTable)
         return StringTable.takeError();
       auto Name = Obj.getSectionName(SecRef);
       if (!Name)
         return Name.takeError();
       auto Section = G->findSectionByName(*Name);
       if (!Section)
-        return make_error<llvm::StringError>("Could not find a section",
+        return make_error<llvm::StringError>("Could not find a section " +
+                                             *Name,
                                              llvm::inconvertibleErrorCode());
       // we only have one for now
       auto blocks = Section->blocks();
       if (blocks.empty())
         return make_error<llvm::StringError>("Section has no block",
                                              llvm::inconvertibleErrorCode());
       int SymbolIndex = -1;
       for (auto SymRef : *Symbols) {
for (auto SecRef : sections) {
             return DefinedSection.takeError();
           auto sectName = Obj.getSectionName(**DefinedSection);
           if (!sectName)
             return Name.takeError();
 
           auto JitSection = G->findSectionByName(*sectName);
           if (!JitSection)
             return make_error<llvm::StringError>(
-                "Could not find a section", llvm::inconvertibleErrorCode());
+                "Could not find the JitSection " + *sectName,
+                llvm::inconvertibleErrorCode());
           auto bs = JitSection->blocks();
           if (bs.empty())
             return make_error<llvm::StringError>(
                 "Section has no block", llvm::inconvertibleErrorCode());
 
           auto B = *bs.begin();
           LLVM_DEBUG({ dbgs() << "  " << *Name << ": "; });
           if (SymRef.getType() == ELF::STT_SECTION)

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

.Lend_test_gotpcrel:
 
         .type   named_data,@object
         .data
         .p2align        2
 named_data:
         .long   42
         .size   named_data, 4
 
+# Test BSS / zero-fill section handling.
+# llvm-jitlink: *{4}bss_variable = 0
+
+  .type bss_variable,@object
+  .bss
+  .globl  bss_variable
+  .p2align  2
+bss_variable:
+  .long 0
+  .size bss_variable, 4
+
         .ident  "clang version 10.0.0-4ubuntu1 "
         .section        ".note.GNU-stack","",@progbits
-        .addrsig
+        .addrsig