[clang] [DebugInfo] Init DwarfVersion of MCOptions like the other. (PR #146666)

[Liu Ke via cfe-commits]

================
@@ -51,6 +51,16 @@
 // RUN: not %clang -target powerpc64-ibm-aix-xcoff -gdwarf-5 -S -emit-llvm -o - %s 2>&1| \
 // RUN:   FileCheck %s --check-prefix=UNSUPPORTED-VER5
 
+// Check what version of dwarf is used to emit debug info when compiling ir with clang.
+// RUN: %clang -target x86_64-linux-gnu -g -S -emit-llvm -o - %s \
+// RUN:   | %clang -target x86_64-linux-gnu -gdwarf-4 -x ir -c -o - - \
+// RUN:   | llvm-dwarfdump -v - \
+// RUN:   | FileCheck %s --check-prefix=SINGLE-4
+// RUN: %clang -target x86_64-linux-gnu -gdwarf-4 -S -emit-llvm -o - %s \
+// RUN:   | %clang -target x86_64-linux-gnu -g -x ir -c -o - - \
+// RUN:   | llvm-dwarfdump -v - \
+// RUN:   | FileCheck %s --check-prefix=SINGLE-5
----------------
Sockke wrote:

> Is this actually the behavior we want? Normally, I'd expect bitcode files to already have a version specified; when clang generates LLVM IR, it generates the DWARF version as metadata. This patch explicitly overrides that version in favor of whatever is specified on the command-line when the bitcode is lowered to asm.

``` c++
DwarfDebug::DwarfDebug(AsmPrinter *A)
...
  unsigned DwarfVersionNumber = Asm->TM.Options.MCOptions.DwarfVersion;
  unsigned DwarfVersion = DwarfVersionNumber ? DwarfVersionNumber
                                    : MMI->getModule()->getDwarfVersion();
...
  Dwarf64 &=
      ((Asm->TM.Options.MCOptions.Dwarf64 || MMI->getModule()->isDwarf64()) &&
       TT.isOSBinFormatELF()) ||
      TT.isOSBinFormatXCOFF();
```
The effective value of DwarfVersion was designed to let MCOptions override the metadata, including  Dwarf64. However, unlike other options, DwarfVersion was not initialized by CodeGenOpts.

> Overriding the version won't really work right, anyway: clang generates different debug info depending on the version it's targeting.

Sorry, could you please explain in detail why it's not working properly?

https://github.com/llvm/llvm-project/pull/146666