lldb/ObjectFile,Disassembler: read some state from the executable

Add support to inspect the ELF headers for RISCV targets to determine if
RVC or RVE are enabled and the floating point support to enable.  As per
the RISCV specification, d implies f, q implies d implies f, which gives
us the cascading effect that is used to enable the features when setting
up the disassembler.  With this change, it is now possible to attach the
debugger to a remote process and be able to disassemble the instruction
stream.

~~~
$ bin/lldb tmp/reduced
(lldb) target create "reduced"
Current executable set to '/tmp/reduced' (riscv64).
(lldb) gdb-remote localhost:1234
(lldb) Process 5737 stopped
* thread #1, name = 'reduced', stop reason = signal SIGTRAP
    frame #0: 0x0000003ff7fe1b20
->  0x3ff7fe1b20: mv     a0, sp
    0x3ff7fe1b22: jal    1936
    0x3ff7fe1b26: mv     s0, a0
    0x3ff7fe1b28: auipc  a0, 27
~~~

lldb/include/lldb/Utility/ArchSpec.h

@@ -92,6 +92,17 @@ class ArchSpec {
     eARM_abi_hard_float = 0x00000400
   };
 
+  enum RISCVeflags {
+    eRISCV_rvc              = 0x00000001, /// RVC, +c
+    eRISCV_float_abi_soft   = 0x00000000, /// soft float
+    eRISCV_float_abi_single = 0x00000002, /// single precision floating point, +f
+    eRISCV_float_abi_double = 0x00000004, /// double precision floating point, +d
+    eRISCV_float_abi_quad   = 0x00000006, /// quad precision floating point, +q
+    eRISCV_float_abi_mask   = 0x00000006,
+    eRISCV_rve              = 0x00000008, /// RVE, +e
+    eRISCV_tso              = 0x00000010, /// RVTSO (total store ordering)
+  };
+
   enum RISCVSubType {
     eRISCVSubType_unknown,
     eRISCVSubType_riscv32,

lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp

@@ -1187,6 +1187,24 @@ DisassemblerLLVMC::DisassemblerLLVMC(const ArchSpec &arch,
       cpu = "apple-latest";
   }
 
+  if (triple.isRISCV()) {
+    uint32_t arch_flags = arch.GetFlags();
+    if (arch_flags & ArchSpec::eRISCV_rvc)
+      features_str += "+c,";
+    if (arch_flags & ArchSpec::eRISCV_rve)
+      features_str += "+e,";
+    if ((arch_flags & ArchSpec::eRISCV_float_abi_single) ==
+        ArchSpec::eRISCV_float_abi_single)
+      features_str += "+f,";
+    if ((arch_flags & ArchSpec::eRISCV_float_abi_double) ==
+        ArchSpec::eRISCV_float_abi_double)
+      features_str += "+f,+d,";
+    if ((arch_flags & ArchSpec::eRISCV_float_abi_quad) ==
+        ArchSpec::eRISCV_float_abi_quad)
+      features_str += "+f,+d,+q,";
+    // FIXME: how do we detect features such as `+a`, `+m`?
+  }
+
   // We use m_disasm_up.get() to tell whether we are valid or not, so if this
   // isn't good for some reason, we won't be valid and FindPlugin will fail and
   // we won't get used.

lldb/source/Plugins/ObjectFile/ELF/ObjectFileELF.cpp

@@ -1364,6 +1364,28 @@ size_t ObjectFileELF::GetSectionHeaderInfo(SectionHeaderColl &section_headers,
       arch_spec.SetFlags(ArchSpec::eARM_abi_hard_float);
   }
 
+  if (arch_spec.GetMachine() == llvm::Triple::riscv32 ||
+      arch_spec.GetMachine() == llvm::Triple::riscv64) {
+    uint32_t flags = arch_spec.GetFlags();
+
+    if (header.e_flags & llvm::ELF::EF_RISCV_RVC)
+      flags |= ArchSpec::eRISCV_rvc;
+    if (header.e_flags & llvm::ELF::EF_RISCV_RVE)
+      flags |= ArchSpec::eRISCV_rve;
+
+    if ((header.e_flags & llvm::ELF::EF_RISCV_FLOAT_ABI_SINGLE) ==
+        llvm::ELF::EF_RISCV_FLOAT_ABI_SINGLE)
+      flags |= ArchSpec::eRISCV_float_abi_single;
+    else if ((header.e_flags & llvm::ELF::EF_RISCV_FLOAT_ABI_DOUBLE) ==
+             llvm::ELF::EF_RISCV_FLOAT_ABI_DOUBLE)
+      flags |= ArchSpec::eRISCV_float_abi_double;
+    else if ((header.e_flags & llvm::ELF::EF_RISCV_FLOAT_ABI_QUAD) ==
+             llvm::ELF::EF_RISCV_FLOAT_ABI_QUAD)
+      flags |= ArchSpec::eRISCV_float_abi_quad;
+
+    arch_spec.SetFlags(flags);
+  }
+
   // If there are no section headers we are done.
   if (header.e_shnum == 0)
     return 0;