[Driver] Default riscv*- triples to -fdebug-default-version=4

This adds a RISC-V special case to ToolChain::GetDefaultDwarfVersion,
affecting Linux/Haiku/RISCVToolChain.

DWARF v5 .debug_loclists/.debug_rnglists's
DW_LLE_offset_pair/DW_RLE_offset_pair entry kinds utilitize `.uleb128 A-B`
directives where A and B reference local labels in code sections.
When A and B are separated by a RISC-V linker-relaxable instruction,
A-B is incorrectly folded without a relocation, causing incorrect debug
information.

```
void ext(void);
int foo(int x) {ext(); return 0;}
// DW_AT_location [DW_FORM_loclistx] of a DW_TAG_formal_parameter references a DW_LLE_offset_pair that can be incorrect after linker relaxation.

int ext(void);
void foo() { {
  int ret = ext();
  if (__builtin_expect(ret, 0))
    ext();
} }
// DW_AT_ranges [DW_FORM_rnglistx] of a DW_TAG_lexical_block references a DW_RLE_offset_pair that can be incorrect after linker relaxation.
```

D157657 will implement R_RISCV_SET_ULEB128/R_RISCV_SUB_ULEB128
relocations, fixing the issue, but the relocation is only supported by
bleeding-edge binutils 2.41 and not by lld/ELF yet.

The goal is to make the emitted DWARF correct after linking.
Many users don't care about the default DWARF version, but a linker
error will be unacceptable. Let's just downgrade the default DWARF
version, before binutils>=2.41 is more widely available.

An alternative compatibility option is to add a toggle to DwarfDebug.cpp,
but that doesn't seem like a good idea.

Reviewed By: asb, kito-cheng

Differential Revision: https://reviews.llvm.org/D157663

(cherry picked from commit bbc0f99)
(with a release note)

clang/docs/ReleaseNotes.rst

@@ -962,6 +962,10 @@ RISC-V Support
   * Added intrinsics for reinterpret cast between vector boolean and vector
     integer ``m1`` value
   * Removed the ``vread_csr`` and ``vwrite_csr`` intrinsics
+- Default ``-fdebug-dwarf-version=`` is downgraded to 4 to work around
+  incorrect DWARF related to ULEB128 and linker compatibility before
+  ``R_RISCV_SET_ULEB128`` becomes more widely supported.
+  (`D157663 <https://reviews.llvm.org/D157663>`_).
 
 CUDA/HIP Language Changes
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Driver/ToolChain.h

@@ -561,7 +561,7 @@ class ToolChain {
 
   // Return the DWARF version to emit, in the absence of arguments
   // to the contrary.
-  virtual unsigned GetDefaultDwarfVersion() const { return 5; }
+  virtual unsigned GetDefaultDwarfVersion() const;
 
   // Some toolchains may have different restrictions on the DWARF version and
   // may need to adjust it. E.g. NVPTX may need to enforce DWARF2 even when host

clang/lib/Driver/ToolChain.cpp

@@ -427,6 +427,12 @@ ToolChain::getDefaultUnwindTableLevel(const ArgList &Args) const {
   return UnwindTableLevel::None;
 }
 
+unsigned ToolChain::GetDefaultDwarfVersion() const {
+  // TODO: Remove the RISC-V special case when R_RISCV_SET_ULEB128 linker
+  // support becomes more widely available.
+  return getTriple().isRISCV() ? 4 : 5;
+}
+
 Tool *ToolChain::getClang() const {
   if (!Clang)
     Clang.reset(new tools::Clang(*this, useIntegratedBackend()));

clang/test/Driver/clang-g-opts.c

@@ -37,3 +37,8 @@
 
 // CHECK-WITH-G-STANDALONE: "-debug-info-kind=standalone"
 // CHECK-WITH-G-STANDALONE: "-dwarf-version=2"
+
+/// TODO: Special case before R_RISCV_SET_ULEB128 linker support becomes more widely available.
+// RUN: %clang -### -S %s -g --target=riscv64-linux-gnu 2>&1 | FileCheck --check-prefix=VERSION4 %s
+// RUN: %clang -### -S %s -g --target=riscv64-unknown-elf 2>&1 | FileCheck --check-prefix=VERSION4 %s
+// VERSION4: "-dwarf-version=4"