Mark CR0 Live if PPCInstrInfo::optimizeCompareInstr Creates a Use of CR0

[tjablin]

Summary

Resolve Bug 27046 (https://llvm.org/bugs/show_bug.cgi?id=27046). The PPCInstrInfo::optimizeCompareInstr function could create a new use of CR0, even if CR0 were previously dead. This patch marks CR0 live if a use of CR0 is created.

[kbarton]

LGTM

[cycheng]

Committed r266040
(On behalf of Tom)

lib/Target/PowerPC/PPCInstrInfo.cpp

bool PPCInstrInfo::optimizeCompareInstr(MachineInstr *CmpInstr,
   // record form, then set the kill flag on the CR.
   CmpInstr->eraseFromParent();
 
   MachineBasicBlock::iterator MII = MI;
   BuildMI(*MI->getParent(), std::next(MII), MI->getDebugLoc(),
           get(TargetOpcode::COPY), CRReg)
     .addReg(PPC::CR0, MIOpC != NewOpC ? RegState::Kill : 0);
 
+  // Even if CR0 register were dead before, it is alive now since the
+  // instruction we just built uses it.
+  MI->clearRegisterDeads(PPC::CR0);
+  
   if (MIOpC != NewOpC) {
     // We need to be careful here: we're replacing one instruction with
     // another, and we need to make sure that we get all of the right
     // implicit uses and defs. On the other hand, the caller may be holding
     // an iterator to this instruction, and so we can't delete it (this is
     // specifically the case if this is the instruction directly after the
     // compare).
 

test/CodeGen/PowerPC/opt-cmp-inst-cr0-live.ll

+; RUN: llc -print-before=peephole-opts -print-after=peephole-opts -o /dev/null 2>&1 < %s | FileCheck %s
+
+define signext i32 @fn1(i32 %baz) {
+  %1 = mul nsw i32 %baz, 208
+  %2 = zext i32 %1 to i64
+  %3 = shl i64 %2, 48
+  %4 = ashr exact i64 %3, 48
+; CHECK: ANDIo8 {{[^,]+}}, 65520, %CR0<imp-def,dead>;
+; CHECK: CMPLDI
+; CHECK: BCC
+
+; CHECK: ANDIo8 {{[^,]+}}, 65520, %CR0<imp-def>;
+; CHECK: COPY %CR0
+; CHECK: BCC
+  %5 = icmp eq i64 %4, 0
+  br i1 %5, label %foo, label %bar
+
+foo:
+  ret i32 1
+
+bar:
+  ret i32 0}
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