Fix a crash with assembler source and -g.

[probinson]

Summary

llvm-mc or clang with -g normally produces debug info describing the
assembler source itself; however, if that source already contains some
.file/.loc directives, we should instead emit the debug info described
by those directives. For certain assembler sources seen in the wild
(particularly in the Chrome build) this was causing a crash due to
incorrect assumptions about legal sequences of assembler source text.

Fixes PR38994.

[probinson]

We do not precisely match gcc/gas behavior in some more-peculiar cases, but my assertion is that those should not occur "naturally" and so it's okay. For example:

foo:
    nop
    .file 1 "a.c"

This will cause gcc/gas to emit an error to the effect that file number 1 is already defined (implicitly, because of the line-table record for the first instruction). Clang/llvm-mc will not, we'll just emit an odd-looking line table. I can see how to cause clang/llvm-mc to emit this error, but it feels like it would be a hack done just to match gcc's (likely unintentional) error-detection behavior for an ill-formed assembler file.

[dblaikie]

Sounds alright to me

llvm/include/llvm/MC/MCDwarf.h

void setRootFile(StringRef Directory, StringRef FileName,
     RootFile.Name = FileName;
     RootFile.DirIndex = 0;
     RootFile.Checksum = Checksum;
     RootFile.Source = Source;
     trackMD5Usage(Checksum.hasValue());
     HasSource = Source.hasValue();
   }
 
+  void resetFileTable() {
+    MCDwarfDirs.clear();
+    MCDwarfFiles.clear();
+    RootFile.Name.clear();
+    resetMD5Usage();
+    HasSource = false;
+  }
+
 private:
   void emitV2FileDirTables(MCStreamer *MCOS) const;
   void emitV5FileDirTables(MCStreamer *MCOS, Optional<MCDwarfLineStr> &LineStr) const;
 };
 
 class MCDwarfDwoLineTable {
   MCDwarfLineTableHeader Header;
   bool HasSplitLineTable = false;
void setRootFile(StringRef Directory, StringRef FileName,
     Header.RootFile.Name = FileName;
     Header.RootFile.DirIndex = 0;
     Header.RootFile.Checksum = Checksum;
     Header.RootFile.Source = Source;
     Header.trackMD5Usage(Checksum.hasValue());
     Header.HasSource = Source.hasValue();
   }
 
-  void resetRootFile() {
-    assert(Header.MCDwarfFiles.empty());
-    Header.RootFile.Name.clear();
-    Header.resetMD5Usage();
-    Header.HasSource = false;
-  }
+  void resetFileTable() { Header.resetFileTable(); }
 
   bool hasRootFile() const { return !Header.RootFile.Name.empty(); }
 
   const MCDwarfFile &getRootFile() const { return Header.RootFile; }
 
   // Report whether MD5 usage has been consistent (all-or-none).
   bool isMD5UsageConsistent() const { return Header.isMD5UsageConsistent(); }
 

llvm/lib/MC/MCParser/AsmParser.cpp

if (FileNumber == -1) {
     // Ignore the directive if there is no number and the target doesn't support
     // numberless .file directives. This allows some portability of assembler
     // between different object file formats.
     if (getContext().getAsmInfo()->hasSingleParameterDotFile())
       getStreamer().EmitFileDirective(Filename);
   } else {
     // In case there is a -g option as well as debug info from directive .file,
     // we turn off the -g option, directly use the existing debug info instead.
-    // Also reset any implicit ".file 0" for the assembler source.
+    // Throw away any implicit file table for the assembler source.
     if (Ctx.getGenDwarfForAssembly()) {
-      Ctx.getMCDwarfLineTable(0).resetRootFile();
+      Ctx.getMCDwarfLineTable(0).resetFileTable();
       Ctx.setGenDwarfForAssembly(false);
     }
 
     Optional<MD5::MD5Result> CKMem;
     if (HasMD5) {
       MD5::MD5Result Sum;
       for (unsigned i = 0; i != 8; ++i) {
         Sum.Bytes[i] = uint8_t(MD5Hi >> ((7 - i) * 8));

llvm/test/MC/AsmParser/directive_file-g.s

+## Make sure that using -g (or equivalent) on an asm file that already has
+## debug-info directives in it will correctly ignore the -g and produce
+## debug info corresponding to the directives in the source.
+## Note gcc accepts ".file 1" after a label, although not after an opcode.
+## If no other directives appear, gcc emits no debug info at all.
+
+# RUN: llvm-mc -g -triple i386-unknown-unknown -filetype=obj %s -o %t
+# RUN: llvm-dwarfdump -debug-info -debug-line %t | FileCheck %s
+
+foo:
+        .file 1 "a.c"
+        .loc 1 1 1
+        nop
+
+# CHECK: .debug_info
+## gcc does generate a DW_TAG_compile_unit in this case, with or without
+## -g on the command line, but we do not.
+# CHECK-EMPTY:
+# CHECK_NEXT: .debug_line
+# CHECK: file_names[ 1]:
+# CHECK-NEXT: name: "a.c"
+# CHECK-NEXT: dir_index: 0
+# CHECK: 0x{{0+}}0 1 1 1 0 0 is_stmt
+# CHECK: 0x{{0+}}1 1 1 1 0 0 is_stmt end_sequence