[clang][RISCV] Enable -fasynchronous-unwind-tables by default on Linux

[kito-cheng]

Summary

This could improve user experience for stack unwinding, and also this is
enabled by default by X86 and AArch64 and RISC-V GCC.

[luismarques]

From Kito's comment in D144174: https://github.com/gcc-mirror/gcc/commit/3cd08f7168c196d7a481b9ed9f4289fd1f14eea8

[MaskRay]

LG since GCC made a similar change and this matches several major architectures on Linux/other ELF based operating systems.

(At heart I think -fasynchronous-unwind-tables is not a good default. I agree with https://blogs.gnome.org/chergert/2022/12/31/frame-pointers-and-other-practical-near-term-solutions/ which has said:
"Again, I don’t think any of us like frame pointers, just that they are generally lower effort today while also being reasonably reliable.")

[asb]

Funnily enough, @jobnoorman and I were just discussion something related to this, how -fexceptions doesn't seem to enable unwind tables as you'd expect. This change hides that issue in the common case you're targeting Linux, but I've filed https://github.com/llvm/llvm-project/issues/61216 to track it. Just thought I'd advertise here because reviewers of this patch have likely looked through these code paths recently and might have a view on this bug.

clang/lib/Driver/ToolChains/Gnu.cpp

 ToolChain::UnwindTableLevel
 Generic_GCC::getDefaultUnwindTableLevel(const ArgList &Args) const {
   switch (getArch()) {
   case llvm::Triple::aarch64:
   case llvm::Triple::ppc:
   case llvm::Triple::ppcle:
   case llvm::Triple::ppc64:
   case llvm::Triple::ppc64le:
+  case llvm::Triple::riscv32:
+  case llvm::Triple::riscv64:
   case llvm::Triple::x86:
   case llvm::Triple::x86_64:
     return UnwindTableLevel::Asynchronous;
   default:
     return UnwindTableLevel::None;
   }
 }
 

clang/test/Driver/riscv-features.c

 // DEFAULT: "-target-feature" "-save-restore"
 // DEFAULT-NOT: "-target-feature" "+save-restore"
 
 // RUN: %clang --target=riscv32-linux -### %s -fsyntax-only 2>&1 \
 // RUN:   | FileCheck %s -check-prefix=DEFAULT-LINUX
 // RUN: %clang --target=riscv64-linux -### %s -fsyntax-only 2>&1 \
 // RUN:   | FileCheck %s -check-prefix=DEFAULT-LINUX
 
-// DEFAULT-LINUX: "-target-feature" "+m"
+// DEFAULT-LINUX: "-funwind-tables=2"
+// DEFAULT-LINUX-SAME: "-target-feature" "+m"
 // DEFAULT-LINUX-SAME: "-target-feature" "+a"
 // DEFAULT-LINUX-SAME: "-target-feature" "+f"
 // DEFAULT-LINUX-SAME: "-target-feature" "+d"
 // DEFAULT-LINUX-SAME: "-target-feature" "+c"
 
 // RUN: not %clang -cc1 -triple riscv64-unknown-elf -target-feature +e 2>&1 | FileCheck %s -check-prefix=RV64-WITH-E
 
 // RV64-WITH-E: error: invalid feature combination: standard user-level extension 'e' requires 'rv32'
 
 // RUN: not %clang -c --target=riscv64-linux-gnu -gsplit-dwarf %s 2>&1 | FileCheck %s --check-prefix=ERR-SPLIT-DWARF
 // RUN: not %clang -c --target=riscv64 -gsplit-dwarf=single %s 2>&1 | FileCheck %s --check-prefix=ERR-SPLIT-DWARF
 // RUN: %clang -### -c --target=riscv64 -mno-relax -g -gsplit-dwarf %s 2>&1 | FileCheck %s --check-prefix=SPLIT-DWARF
 
 // ERR-SPLIT-DWARF: error: -gsplit-dwarf{{.*}} is unsupported with RISC-V linker relaxation (-mrelax)
 // SPLIT-DWARF:     "-split-dwarf-file"