Reapply [X86] Add a new LOW32_ADDR_ACCESS_RBP register class.

This reapplies commit r268796, with a fix for the setting of the inline asm
constraints. I.e., "mark" LOW32_ADDR_ACCESS_RBP as a GR variant, so that the
regular processing of the GR operands (setting of the subregisters) happens.

Original commit log:
[X86] Add a new LOW32_ADDR_ACCESS_RBP register class.

ABIs like NaCl uses 32-bit addresses but have 64-bit frame.
The new register class reflects those constraints when choosing a
register class for a address access.

llvm-svn: 268955

Author: Quentin Colombet

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -30209,6 +30209,7 @@ static bool isGRClass(const TargetRegisterClass &RC) {
   case X86::GR64_NOSPRegClassID:
   case X86::GR64_NOREX_NOSPRegClassID:
   case X86::LOW32_ADDR_ACCESSRegClassID:
+  case X86::LOW32_ADDR_ACCESS_RBPRegClassID:
     return true;
   default:
     return false;

llvm/lib/Target/X86/X86RegisterInfo.cpp

@@ -166,7 +166,15 @@ X86RegisterInfo::getPointerRegClass(const MachineFunction &MF,
     // we can still use 64-bit register as long as we know the high bits
     // are zeros.
     // Reflect that in the returned register class.
-    return Is64Bit ? &X86::LOW32_ADDR_ACCESSRegClass : &X86::GR32RegClass;
+    if (Is64Bit) {
+      // When the target also allows 64-bit frame pointer and we do have a
+      // frame, this is fine to use it for the address accesses as well.
+      const X86FrameLowering *TFI = getFrameLowering(MF);
+      return TFI->hasFP(MF) && TFI->Uses64BitFramePtr
+                 ? &X86::LOW32_ADDR_ACCESS_RBPRegClass
+                 : &X86::LOW32_ADDR_ACCESSRegClass;
+    }
+    return &X86::GR32RegClass;
   case 1: // Normal GPRs except the stack pointer (for encoding reasons).
     if (Subtarget.isTarget64BitLP64())
       return &X86::GR64_NOSPRegClass;

llvm/lib/Target/X86/X86RegisterInfo.td

@@ -427,6 +427,13 @@ def GR64_NOREX_NOSP : RegisterClass<"X86", [i64], 64,
 // which we do not have right now.
 def LOW32_ADDR_ACCESS : RegisterClass<"X86", [i32], 64, (add GR32, RIP)>;
 
+// When RBP is used as a base pointer in a 32-bit addresses environement,
+// this is also safe to use the full register to access addresses.
+// Since RBP will never be spilled, stick to a 32 alignment to save
+// on memory consumption.
+def LOW32_ADDR_ACCESS_RBP : RegisterClass<"X86", [i32], 32,
+                                          (add LOW32_ADDR_ACCESS, RBP)>;
+
 // A class to support the 'A' assembler constraint: EAX then EDX.
 def GR32_AD : RegisterClass<"X86", [i32], 32, (add EAX, EDX)>;
 

llvm/test/CodeGen/X86/x86-64-stack-and-frame-ptr.ll

@@ -1,6 +1,6 @@
-; RUN: llc -mtriple=x86_64-pc-linux < %s | FileCheck %s
-; RUN: llc -mtriple=x86_64-pc-linux-gnux32 < %s | FileCheck -check-prefix=X32ABI %s
-; RUN: llc -mtriple=x86_64-pc-nacl < %s | FileCheck -check-prefix=NACL %s
+; RUN: llc -verify-machineinstrs -mtriple=x86_64-pc-linux < %s | FileCheck %s
+; RUN: llc -verify-machineinstrs -mtriple=x86_64-pc-linux-gnux32 < %s | FileCheck -check-prefix=X32ABI %s
+; RUN: llc -verify-machineinstrs -mtriple=x86_64-pc-nacl < %s | FileCheck -check-prefix=NACL %s
 
 ; x32 uses %esp, %ebp as stack and frame pointers