[ELF] Enable temporary labels symbol names

[lenykholodov]

Summary

After revision r236642 compiler prevents emitting of temp label names into object files to save memory. This creates problems for the binutils linker, which analyzes symbol names from the input object files and can skip some local compiler generated labels. As a result linker doesn't see the label names and therefore puts them all into the final binary as global symbols. Binutils require label names to have a prefix '.L'.

This patch fixes temporary label names generation for ELF.

[dexonsmith]

(Sorry for the delay; I'm still working through mail from Thanksgiving.)

+Rafael, since we worked on this change together, and I don't know much about ELF.

[• rafael]

I do think this is a bug in the linker. It should strip symbols that
don't have a name in the same way it strips .L in mergeable sections.

Even if we want to create temp names, we can do that as a very late
pass in the elf writer.

I will try to make sure lld does the right thing and open bugs on bfd
ld and gold.

Thanks,
Rafael

[• rafael]

OK, things were not that simple :-(

I fixed lld to drop unnamed symbols in cases it drops .L symbols (r255357).

The testcase you included finds another problem is llvm: the output
produced when going directly to a .o is different from what we get
when using a .s.

I reported pr25811 to track it. Unfortunately this area has bigger
problems that should probably be fixed first: pr18716.

Cheers,
Rafael

lib/CodeGen/LLVMTargetMachine.cpp

     if (!MCE || !MAB)
       return true;
 
-    // Don't waste memory on names of temp labels.
-    Context->setUseNamesOnTempLabels(false);
+    if (getObjFileLowering()->getObjectFileType() ==
+        MCObjectFileInfo::Environment::IsELF) {
+      // Don't waste memory on names of temp labels.
+      Context->setUseNamesOnTempLabels(false);
+    }
 
     Triple T(getTargetTriple().str());
     AsmStreamer.reset(getTarget().createMCObjectStreamer(

test/CodeGen/ARM/elf-debug-label-name.ll

+; RUN: llc < %s -mtriple=armv7l-linux-eabihf -filetype=obj -o %s.o && llvm-objdump -t %s.o | FileCheck %s --check-prefix=CHECK
+; CHECK: .debug_str 00000000 .Linfo_string0
+; Function Attrs: nounwind
+define arm_aapcs_vfpcc i32 @_Z1xi(i32 %a) #0 !dbg !4 {
+entry:
+  %a.addr = alloca i32, align 4
+  store i32 %a, i32* %a.addr, align 4
+  call void @llvm.dbg.declare(metadata i32* %a.addr, metadata !13, metadata !14), !dbg !15
+  %0 = load i32, i32* %a.addr, align 4, !dbg !16
+  ret i32 %0, !dbg !17
+}
+
+; Function Attrs: nounwind readnone
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!8, !9, !10, !11}
+!llvm.ident = !{!12}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: 1, enums: !2, subprograms: !3)
+!1 = !DIFile(filename: "file", directory: "dir")
+!2 = !{}
+!3 = !{!4}
+!4 = distinct !DISubprogram(name: "x", linkageName: "_Z1xi", scope: !1, file: !1, line: 1, type: !5, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: false, variables: !2)
+!5 = !DISubroutineType(types: !6)
+!6 = !{!7, !7}
+!7 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+!8 = !{i32 2, !"Dwarf Version", i32 4}
+!9 = !{i32 2, !"Debug Info Version", i32 3}
+!10 = !{i32 1, !"wchar_size", i32 4}
+!11 = !{i32 1, !"min_enum_size", i32 4}
+!12 = !{!"clang"}
+!13 = !DILocalVariable(name: "a", arg: 1, scope: !4, file: !1, line: 1, type: !7)
+!14 = !DIExpression()
+!15 = !DILocation(line: 1, column: 11, scope: !4)
+!16 = !DILocation(line: 3, column: 10, scope: !4)
+!17 = !DILocation(line: 3, column: 3, scope: !4)