ARM: When re-creating a branch via InsertBranch, preserve CPSR flags.

[pcc]

Summary

In particular, this preserves the kill flag, which allows the Thumb2 cbn?z
optimization to be applied in cases where a branch has been re-created after
the live variables analysis pass, e.g. by the machine block placement pass.

This appears to be low risk; a number of other targets seem to already be
doing something similar, e.g. AArch64, PowerPC.

[rengolin]

LGTM, Thanks!

lib/Target/ARM/ARMBaseInstrInfo.cpp

int BccOpc = !AFI->isThumbFunction()
     ? ARM::Bcc : (AFI->isThumb2Function() ? ARM::t2Bcc : ARM::tBcc);
   bool isThumb = AFI->isThumbFunction() || AFI->isThumb2Function();
 
   // Shouldn't be a fall through.
   assert(TBB && "InsertBranch must not be told to insert a fallthrough");
   assert((Cond.size() == 2 || Cond.size() == 0) &&
          "ARM branch conditions have two components!");
 
+  // For conditional branches, we use addOperand to preserve CPSR flags.
+
   if (!FBB) {
     if (Cond.empty()) { // Unconditional branch?
       if (isThumb)
         BuildMI(&MBB, DL, get(BOpc)).addMBB(TBB).addImm(ARMCC::AL).addReg(0);
       else
         BuildMI(&MBB, DL, get(BOpc)).addMBB(TBB);
     } else
       BuildMI(&MBB, DL, get(BccOpc)).addMBB(TBB)
-        .addImm(Cond[0].getImm()).addReg(Cond[1].getReg());
+        .addImm(Cond[0].getImm()).addOperand(Cond[1]);
     return 1;
   }
 
   // Two-way conditional branch.
   BuildMI(&MBB, DL, get(BccOpc)).addMBB(TBB)
-    .addImm(Cond[0].getImm()).addReg(Cond[1].getReg());
+    .addImm(Cond[0].getImm()).addOperand(Cond[1]);
   if (isThumb)
     BuildMI(&MBB, DL, get(BOpc)).addMBB(FBB).addImm(ARMCC::AL).addReg(0);
   else
     BuildMI(&MBB, DL, get(BOpc)).addMBB(FBB);
   return 2;
 }
 
 bool ARMBaseInstrInfo::

test/CodeGen/ARM/sjlj-prepare-critical-edge.ll

 ; %do.body.i.i.i.
 
 ; CHECK-LABEL: __Z4foo1c:
 ; CHECK: blx __Znwm
 ; CHECK: {{.*}}@ %entry.do.body.i.i.i_crit_edge
 ; CHECK: str r0, [sp, [[OFFSET:#[0-9]+]]]
 ; CHECK: ldr [[R0:r[0-9]+]], [sp, [[OFFSET]]]
 ; CHECK: {{.*}}@ %do.body.i.i.i
-; CHECK: cmp [[R0]], #0
+; CHECK: cbz [[R0]]
 
 %"class.std::__1::basic_string" = type { %"class.std::__1::__compressed_pair" }
 %"class.std::__1::__compressed_pair" = type { %"class.std::__1::__libcpp_compressed_pair_imp" }
 %"class.std::__1::__libcpp_compressed_pair_imp" = type { %"struct.std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__rep" }
 %"struct.std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__rep" = type { %union.anon }
 %union.anon = type { %"struct.std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__long" }
 %"struct.std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__long" = type { i32, i32, i8* }
 

test/CodeGen/Thumb2/v8_IT_5.ll

 ; RUN: llc < %s -mtriple=thumbv8 -arm-atomic-cfg-tidy=0 | FileCheck %s
 ; RUN: llc < %s -mtriple=thumbv7 -arm-atomic-cfg-tidy=0 -arm-restrict-it | FileCheck %s
 ; CHECK: it ne
 ; CHECK-NEXT: cmpne
 ; CHECK-NEXT: bne [[JUMPTARGET:.LBB[0-9]+_[0-9]+]]
-; CHECK: cmp
-; CHECK-NEXT: beq
+; CHECK: cbz
 ; CHECK-NEXT: %if.else163
 ; CHECK-NEXT: mov.w
 ; CHECK-NEXT: b
 ; CHECK-NEXT: %if.else145
 ; CHECK-NEXT: mov.w
 ; CHECK: [[JUMPTARGET]]:{{.*}}%if.else173
 
 %struct.hc = type { i32, i32, i32, i32 }