[LLDB][MIPS] Handle PIC calling convention for MIPS64

[bhushan]

Summary

The PIC calling convention for MIPS requires that on entry to a function, register r25 (t9) holds the address of the function’s entry point.
This patch sets up register r25 with the address of function to be called in PrepareTrivialCall().

[clayborg]

Looks good.

[bhushan]

Closed by commit http://reviews.llvm.org/rL257441

source/Plugins/ABI/SysV-mips64/ABISysV_mips64.cpp

if (log)
         log->Printf("16-byte aligning SP: 0x%" PRIx64 " to 0x%" PRIx64, (uint64_t)sp, (uint64_t)(sp & ~0xfull));
 
     sp &= ~(0xfull); // 16-byte alignment
 
     Error error;
     const RegisterInfo *pc_reg_info = reg_ctx->GetRegisterInfo (eRegisterKindGeneric, LLDB_REGNUM_GENERIC_PC);
     const RegisterInfo *sp_reg_info = reg_ctx->GetRegisterInfo (eRegisterKindGeneric, LLDB_REGNUM_GENERIC_SP);
     const RegisterInfo *ra_reg_info = reg_ctx->GetRegisterInfo (eRegisterKindGeneric, LLDB_REGNUM_GENERIC_RA);
+    const RegisterInfo *r25_info = reg_ctx->GetRegisterInfoByName("r25", 0);
 
     if (log)
     log->Printf("Writing SP: 0x%" PRIx64, (uint64_t)sp);
 
     // Set "sp" to the requested value
     if (!reg_ctx->WriteRegisterFromUnsigned (sp_reg_info, sp))
         return false;
 
     if (log)
     log->Printf("Writing RA: 0x%" PRIx64, (uint64_t)return_addr);
 
     // Set "ra" to the return address
     if (!reg_ctx->WriteRegisterFromUnsigned (ra_reg_info, return_addr))
         return false;
 
     if (log)
         log->Printf("Writing PC: 0x%" PRIx64, (uint64_t)func_addr);
 
     // Set pc to the address of the called function.
     if (!reg_ctx->WriteRegisterFromUnsigned (pc_reg_info, func_addr))
         return false;
 
+    if (log)
+        log->Printf("Writing r25: 0x%" PRIx64, (uint64_t)func_addr);
+
+    // All callers of position independent functions must place the address of the called function in t9 (r25)
+    if (!reg_ctx->WriteRegisterFromUnsigned (r25_info, func_addr))
+        return false;
+
     return true;
 }
 
 bool
 ABISysV_mips64::GetArgumentValues (Thread &thread, ValueList &values) const
 {
     return false;
 }