[X86] Remove MWAITX_SAVE_EBX pseudo instruction. Always save/restore the full %rbx register even in gnux32.

ebx/rbx only needs to be saved when 64-bit registers are supported
anyway. It should be fine to save/restore the whole rbx register
even in gnux32 where the base is technically just ebx.

This matches what we do for cmpxchg16b where rbx is saved/restored
regardless of gnux32.

Author: topperc

llvm/lib/Target/X86/X86ExpandPseudo.cpp

@@ -442,7 +442,6 @@ bool X86ExpandPseudo::ExpandMI(MachineBasicBlock &MBB,
     MBB.erase(MBBI);
     return true;
   }
-  case X86::MWAITX_SAVE_EBX:
   case X86::MWAITX_SAVE_RBX: {
     // Perform the following transformation.
     // SaveRbx = pseudomwaitx InArg, SaveRbx
@@ -458,9 +457,7 @@ bool X86ExpandPseudo::ExpandMI(MachineBasicBlock &MBB,
     BuildMI(MBB, MBBI, DL, TII->get(X86::MWAITXrrr));
     // Finally, restore the value of RBX.
     Register SaveRbx = MBBI->getOperand(2).getReg();
-    unsigned BasePointer = Opcode == X86::MWAITX_SAVE_EBX ? X86::EBX : X86::RBX;
-    TII->copyPhysReg(MBB, MBBI, DL, BasePointer, SaveRbx,
-                     /*SrcIsKill*/ true);
+    TII->copyPhysReg(MBB, MBBI, DL, X86::RBX, SaveRbx, /*SrcIsKill*/ true);
     // Delete the pseudo.
     MBBI->eraseFromParent();
     return true;

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -33793,7 +33793,7 @@ X86TargetLowering::EmitInstrWithCustomInserter(MachineInstr &MI,
     Register BasePtr = TRI->getBaseRegister();
     bool IsRBX = (BasePtr == X86::RBX || BasePtr == X86::EBX);
     // If no need to save the base pointer, we generate MWAITXrrr,
-    // else we generate pseudo MWAITX_SAVE_RBX/EBX.
+    // else we generate pseudo MWAITX_SAVE_RBX.
     if (!IsRBX || !TRI->hasBasePointer(*MF)) {
       BuildMI(*BB, MI, DL, TII->get(TargetOpcode::COPY), X86::ECX)
           .addReg(MI.getOperand(0).getReg());
@@ -33812,17 +33812,15 @@ X86TargetLowering::EmitInstrWithCustomInserter(MachineInstr &MI,
           .addReg(MI.getOperand(0).getReg());
       BuildMI(*BB, MI, DL, TII->get(TargetOpcode::COPY), X86::EAX)
           .addReg(MI.getOperand(1).getReg());
-      const TargetRegisterClass *RegClass =
-          BasePtr == X86::EBX ? &X86::GR32RegClass : &X86::GR64RegClass;
-      // Save RBX (or EBX) into a virtual register.
-      Register SaveRBX = MF->getRegInfo().createVirtualRegister(RegClass);
+      assert(Subtarget.is64Bit() && "Expected 64-bit mode!");
+      // Save RBX into a virtual register.
+      Register SaveRBX =
+          MF->getRegInfo().createVirtualRegister(&X86::GR64RegClass);
       BuildMI(*BB, MI, DL, TII->get(TargetOpcode::COPY), SaveRBX)
-          .addReg(BasePtr);
+          .addReg(X86::RBX);
       // Generate mwaitx pseudo.
-      unsigned Opcode =
-          BasePtr == X86::RBX ? X86::MWAITX_SAVE_RBX : X86::MWAITX_SAVE_EBX;
-      Register Dst = MF->getRegInfo().createVirtualRegister(RegClass);
-      BuildMI(*BB, MI, DL, TII->get(Opcode))
+      Register Dst = MF->getRegInfo().createVirtualRegister(&X86::GR64RegClass);
+      BuildMI(*BB, MI, DL, TII->get(X86::MWAITX_SAVE_RBX))
           .addDef(Dst) // Destination tied in with SaveRBX.
           .addReg(MI.getOperand(2).getReg()) // input value of EBX.
           .addUse(SaveRBX);                  // Save of base pointer.

llvm/lib/Target/X86/X86InstrCompiler.td

@@ -896,19 +896,9 @@ def LCMPXCHG16B_SAVE_RBX :
                                                     GR64:$rbx_save))]>;
 }
 
-// This pseudo must be used when the frame uses RBX as
+// This pseudo must be used when the frame uses RBX/EBX as
 // the base pointer.
-// cf comment for LCMPXCHG8B_SAVE_EBX.
-let Defs = [EBX], Uses = [ECX, EAX, EBX],
-    Predicates = [HasMWAITX], SchedRW = [WriteSystem],
-    isCodeGenOnly = 1, isPseudo = 1, Constraints = "$ebx_save = $dst" in {
-def MWAITX_SAVE_EBX :
-    I<0, Pseudo, (outs GR32:$dst),
-      (ins GR32:$ebx_input, GR32:$ebx_save),
-      "mwaitx",
-      []>;
-}
-// Same as MWAITX_SAVE_EBX but for the case where RBX is the base pointer.
+// cf comment for LCMPXCHG16B_SAVE_RBX.
 let Defs = [EBX], Uses = [ECX, EAX, EBX],
     Predicates = [HasMWAITX], SchedRW = [WriteSystem],
     isCodeGenOnly = 1, isPseudo = 1, Constraints = "$rbx_save = $dst" in {

llvm/test/CodeGen/X86/base-pointer-and-mwaitx.ll

@@ -42,12 +42,12 @@ entry:
 ; USE_BASE_32: movl %ecx, %eax
 ; USE_BASE_32: movl %edx, %ecx
 ; Save base pointer.
-; USE_BASE_32: movl %ebx, [[SAVE_ebx:%e(di|si)]]
+; USE_BASE_32: movq %rbx, [[SAVE_rbx:%r(di|si)]]
 ; Set mwaitx ebx argument.
 ; USE_BASE_32: movl %r8d, %ebx
 ; USE_BASE_32-NEXT: mwaitx
 ; Restore base pointer.
-; USE_BASE_32-NEXT: movl [[SAVE_ebx]], %ebx
+; USE_BASE_32-NEXT: movq [[SAVE_rbx]], %rbx
 
 ; Pass mwaitx 3 arguments in eax, ecx, ebx
 ; NO_BASE_64: movl %r8d, %ebx
@@ -111,12 +111,12 @@ if.end:
 ; USE_BASE_32: movl %esi, %eax
 ; USE_BASE_32: movl %edi, %ecx
 ; Save base pointer.
-; USE_BASE_32: movl %ebx, [[SAVE_ebx:%e(di|si)]]
+; USE_BASE_32: movq %rbx, [[SAVE_rbx:%r(di|si)]]
 ; Set mwaitx ebx argument.
 ; USE_BASE_32: movl %edx, %ebx
 ; USE_BASE_32-NEXT: mwaitx
 ; Restore base pointer.
-; USE_BASE_32-NEXT: movl [[SAVE_ebx]], %ebx
+; USE_BASE_32-NEXT: movq [[SAVE_rbx]], %rbx
 
 ; Pass mwaitx 3 arguments in eax, ecx, ebx
 ; NO_BASE_64: movl %edx, %ebx
@@ -179,12 +179,12 @@ if.end:
 ; USE_BASE_32: movl %esi, %eax
 ; USE_BASE_32: movl %edi, %ecx
 ; Save base pointer.
-; USE_BASE_32: movl %ebx, [[SAVE_ebx:%e(di|si)]]
+; USE_BASE_32: movq %rbx, [[SAVE_rbx:%r(di|si)]]
 ; Set mwaitx ebx argument.
 ; USE_BASE_32: movl %edx, %ebx
 ; USE_BASE_32-NEXT: mwaitx
 ; Restore base pointer.
-; USE_BASE_32-NEXT: movl [[SAVE_ebx]], %ebx
+; USE_BASE_32-NEXT: movq [[SAVE_rbx]], %rbx
 
 ; Pass mwaitx 3 arguments in eax, ecx, ebx
 ; NO_BASE_64: movl %edx, %ebx