[Driver] Error for -gsplit-dwarf with RISC-V linker relaxation

-gsplit-dwarf produces a .dwo file which will not be processed by the linker. If
.dwo files contain relocations, they will not be resolved. Therefore the
practice is that .dwo files do not contain relocations.

Address ranges and location description need to use forms/entry kinds indexing
into .debug_addr (DW_FORM_addrx/DW_RLE_startx_endx/etc), which is currently not
implemented.

There is a difficult-to-read MC error with -gsplit-dwarf with RISC-V for both -mrelax and -mno-relax.
```
% clang --target=riscv64-linux-gnu -g -gsplit-dwarf -c a.c
error: A dwo section may not contain relocations
```

We expect to fix -mno-relax soon, so report a driver error for -mrelax for now.

Link: llvm/llvm-project#56642

Reviewed By: compnerd, kito-cheng

Differential Revision: https://reviews.llvm.org/D130190

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -699,4 +699,7 @@ def err_drv_invalid_empty_dxil_validator_version : Error<
 def warn_drv_sarif_format_unstable : Warning<
   "diagnostic formatting in SARIF mode is currently unstable">,
   InGroup<DiagGroup<"sarif-format-unstable">>;
+
+def err_drv_riscv_unsupported_with_linker_relaxation : Error<
+  "%0 is unsupported with RISC-V linker relaxation (-mrelax)">;
 }

clang/lib/Driver/ToolChains/Arch/RISCV.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "RISCV.h"
+#include "../Clang.h"
 #include "ToolChains/CommonArgs.h"
 #include "clang/Basic/CharInfo.h"
 #include "clang/Driver/Driver.h"
@@ -156,10 +157,17 @@ void riscv::getRISCVTargetFeatures(const Driver &D, const llvm::Triple &Triple,
     Features.push_back("+reserve-x31");
 
   // -mrelax is default, unless -mno-relax is specified.
-  if (Args.hasFlag(options::OPT_mrelax, options::OPT_mno_relax, true))
+  if (Args.hasFlag(options::OPT_mrelax, options::OPT_mno_relax, true)) {
     Features.push_back("+relax");
-  else
+    // -gsplit-dwarf -mrelax requires DW_AT_high_pc/DW_AT_ranges/... indexing
+    // into .debug_addr, which is currently not implemented.
+    Arg *A;
+    if (getDebugFissionKind(D, Args, A) != DwarfFissionKind::None)
+      D.Diag(clang::diag::err_drv_riscv_unsupported_with_linker_relaxation)
+          << A->getAsString(Args);
+  } else {
     Features.push_back("-relax");
+  }
 
   if (Arg *A = Args.getLastArg(options::OPT_mabi_EQ)) {
     bool IsPureCapability = isCheriPurecapABIName(A->getValue());

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4078,9 +4078,7 @@ static void RenderDiagnosticsOptions(const Driver &D, const ArgList &Args,
                      options::OPT_fno_spell_checking);
 }
 
-enum class DwarfFissionKind { None, Split, Single };
-
-static DwarfFissionKind getDebugFissionKind(const Driver &D,
+DwarfFissionKind tools::getDebugFissionKind(const Driver &D,
                                             const ArgList &Args, Arg *&Arg) {
   Arg = Args.getLastArg(options::OPT_gsplit_dwarf, options::OPT_gsplit_dwarf_EQ,
                         options::OPT_gno_split_dwarf);

clang/lib/Driver/ToolChains/Clang.h

@@ -202,6 +202,12 @@ class LLVM_LIBRARY_VISIBILITY LinkerWrapper final : public Tool {
                     const char *LinkingOutput) const override;
 };
 
+enum class DwarfFissionKind { None, Split, Single };
+
+DwarfFissionKind getDebugFissionKind(const Driver &D,
+                                     const llvm::opt::ArgList &Args,
+                                     llvm::opt::Arg *&Arg);
+
 } // end namespace tools
 
 } // end namespace driver

clang/test/Driver/riscv-features.c

@@ -46,3 +46,10 @@
 // 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"