[X86] Don't copy kill flag when expanding LCMPXCHG16B_SAVE_RBX

[craig.topper]

Summary

The expansion code creates a copy to RBX before the real LCMPXCHG16B.
It's possible this copy uses a register that is also used by the
real LCMPXCHG16B. If we set the kill flag on the use in the copy,
then we'll fail the machine verifier on the use on the LCMPXCHG16B.

Here's an example of expansion that breaks the verifier

$rbx = MOV64rr killed $rcx
LCMPXCHG16B killed renamable $r8, 1, $noreg, 0, $noreg, implicit-def $rax, implicit-def $rdx, implicit-def $eflags, implicit $rax, implicit $rbx, implicit $rcx, implicit $rdx
$rbx = MOV64rr killed $r9

[qcolombet]

Nice catch!

LGTM.

llvm/lib/Target/X86/X86ExpandPseudo.cpp

case X86::LCMPXCHG16B_SAVE_RBX: {
     // RBX = InArg
     // actualcmpxchg Addr
     // RBX = SaveRbx
     const MachineOperand &InArg = MBBI->getOperand(6);
     Register SaveRbx = MBBI->getOperand(7).getReg();
 
     // Copy the input argument of the pseudo into the argument of the
     // actual instruction.
-    TII->copyPhysReg(MBB, MBBI, DL, X86::RBX, InArg.getReg(), InArg.isKill());
+    // NOTE: We don't copy the kill flag since the input might be the same reg
+    // as one of the other operands of LCMPXCHG16B.
+    TII->copyPhysReg(MBB, MBBI, DL, X86::RBX, InArg.getReg(), false);
     // Create the actual instruction.
     MachineInstr *NewInstr = BuildMI(MBB, MBBI, DL, TII->get(X86::LCMPXCHG16B));
     // Copy the operands related to the address.
     for (unsigned Idx = 1; Idx < 6; ++Idx)
       NewInstr->addOperand(MBBI->getOperand(Idx));
     // Finally, restore the value of RBX.
     TII->copyPhysReg(MBB, MBBI, DL, X86::RBX, SaveRbx,
                      /*SrcIsKill*/ true);

llvm/test/CodeGen/X86/pr42064.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc < %s -mtriple=x86_64-pc-windows-msvc19.11.0 -mattr=+avx,+cx16 | FileCheck %s
+; RUN: llc < %s -verify-machineinstrs -mtriple=x86_64-pc-windows-msvc19.11.0 -mattr=+avx,+cx16 | FileCheck %s
 
 %struct.TestStruct = type { %union.Int128 }
 %union.Int128 = type { i128 }
 %struct.SomeArrays = type { %struct.SillyArray, %struct.SillyArray, %struct.SillyArray }
 %struct.SillyArray = type { i8*, i32, i32 }
 
 declare void @llvm.lifetime.start.p0i8(i64, i8*)