[ABI] Rewrite RegisterIsCalleeSaved

[davide]

Summary

The goal of this patch is twofold:
First, it removes a wrong comment (at least, not correctly describing what the function does).
Then, it rewrites the function to use a stringswitch where the registers are enumerated explicitly instead of being computed programmatically. Other than being much shorter, it's much easier to read (and given the ABI won't change anytime soon, I don't think there's need to generalize).
While here, I added an assert that the register name is always empty, as the previous implementation of the function assumed so.

Do we want to add unittests for this to make sure the thing don't regress? I think it's a good thing to to anyway, but I wanted to reach consensus (and probably can be done in a follow-up commit).

[davide]

source/Plugins/ABI/SysV-x86_64/ABISysV_x86_64.cpp

1916 ↗

(On Diff #113675)

Currently this uses two cases as the underlying StringSwitch implementation allows up to 10 arguments. I have plans to switch it to use variadic templates so that this code can be rewritten to use a single case.

[compnerd]

Nice cleanup :-)

source/Plugins/ABI/SysV-x86_64/ABISysV_x86_64.cpp

1918 ↗

(On Diff #113675)

Would be nice to reoder these to rbp, ebp, rbx, ebx. It is easier to see that you are looking at the 32-bit and 64-bit register names that way due to the association.

lldb/trunk/source/Plugins/ABI/SysV-x86_64/ABISysV_x86_64.cpp

 //===-- ABISysV_x86_64.cpp --------------------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "ABISysV_x86_64.h"
 
 // C Includes
 // C++ Includes
 // Other libraries and framework includes
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/StringSwitch.h"
 #include "llvm/ADT/Triple.h"
 
 // Project includes
 #include "lldb/Core/Module.h"
 #include "lldb/Core/PluginManager.h"
 #include "lldb/Core/RegisterValue.h"
 #include "lldb/Core/Value.h"
 #include "lldb/Core/ValueObjectConstResult.h"

 // (or "x86-64(tm) Architecture Processor Supplement" in earlier revisions)
 // (this doc is also commonly referred to as the x86-64/AMD64 psABI)
 // Edited by Michael Matz, Jan Hubicka, Andreas Jaeger, and Mark Mitchell
 // current version is 0.99.6 released 2012-07-02 at
 // http://refspecs.linuxfoundation.org/elf/x86-64-abi-0.99.pdf
 // It's being revised & updated at https://github.com/hjl-tools/x86-psABI/
 
 bool ABISysV_x86_64::RegisterIsCalleeSaved(const RegisterInfo *reg_info) {
-  if (reg_info) {
-    // Preserved registers are :
-    //    rbx, rsp, rbp, r12, r13, r14, r15
-    //    mxcsr (partially preserved)
-    //    x87 control word
-
-    const char *name = reg_info->name;
-    if (name[0] == 'r') {
-      switch (name[1]) {
-      case '1': // r12, r13, r14, r15
-        if (name[2] >= '2' && name[2] <= '5')
-          return name[3] == '\0';
-        break;
-
-      default:
-        break;
-      }
-    }
-
-    // Accept shorter-variant versions, rbx/ebx, rip/ eip, etc.
-    if (name[0] == 'r' || name[0] == 'e') {
-      switch (name[1]) {
-      case 'b': // rbp, rbx
-        if (name[2] == 'p' || name[2] == 'x')
-          return name[3] == '\0';
-        break;
-
-      case 'i': // rip
-        if (name[2] == 'p')
-          return name[3] == '\0';
-        break;
-
-      case 's': // rsp
-        if (name[2] == 'p')
-          return name[3] == '\0';
-        break;
-      }
-    }
-    if (name[0] == 's' && name[1] == 'p' && name[2] == '\0') // sp
-      return true;
-    if (name[0] == 'f' && name[1] == 'p' && name[2] == '\0') // fp
-      return true;
-    if (name[0] == 'p' && name[1] == 'c' && name[2] == '\0') // pc
-      return true;
-  }
+  if (!reg_info)
     return false;
+  assert(reg_info->name != nullptr && "unnamed register?");
+  std::string Name = std::string(reg_info->name);
+  bool IsCalleeSaved =
+      llvm::StringSwitch<bool>(Name)
+          .Cases("r12", "r13", "r14", "r15", "rbp", "ebp", "rbx", "ebx", true)
+          .Cases("rip", "eip", "rsp", "esp", "sp", "fp", "pc", true)
+          .Default(false);
+  return IsCalleeSaved;
 }
 
 void ABISysV_x86_64::Initialize() {
   PluginManager::RegisterPlugin(
       GetPluginNameStatic(), "System V ABI for x86_64 targets", CreateInstance);
 }
 
 void ABISysV_x86_64::Terminate() {