[ConstantFolding] Fold freeze if it is never undef or poison

[aqjune]

Summary

This is a simple patch that adds constant folding for freeze
instruction.

IIUC, it isn't needed to update ConstantFold.cpp because there is no freeze
constexpr.

[nikic]

LGTM

llvm/lib/Analysis/ConstantFolding.cpp

1073

I'd move this below the llvm_unreachable case for compares.

[aqjune]

Move the added check below compares

llvm/lib/Analysis/ConstantFolding.cpp

Constant *ConstantFoldInstOperandsImpl(const Value *InstOrCE, unsigned Opcode,
   }
 
   if (auto *CE = dyn_cast<ConstantExpr>(InstOrCE))
     return CE->getWithOperands(Ops);
 
   switch (Opcode) {
   default: return nullptr;
   case Instruction::ICmp:
   case Instruction::FCmp: llvm_unreachable("Invalid for compares");
+  case Instruction::Freeze:
+    return isGuaranteedNotToBeUndefOrPoison(Ops[0]) ? Ops[0] : nullptr;
   case Instruction::Call:
     if (auto *F = dyn_cast<Function>(Ops.back())) {
       const auto *Call = cast<CallBase>(InstOrCE);
       if (canConstantFoldCallTo(Call, F))
         return ConstantFoldCall(Call, F, Ops.slice(0, Ops.size() - 1), TLI);
     }
     return nullptr;
   case Instruction::Select:

llvm/test/Analysis/ConstantFolding/freeze.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -constprop -S | FileCheck %s
 
 @g = external global i16, align 1
 @g2 = external global i16, align 1
 
 define i64 @ptrdiff1() {
 ; CHECK-LABEL: @ptrdiff1(
-; CHECK-NEXT:    [[R:%.*]] = freeze i64 sub (i64 ptrtoint (i16* @g to i64), i64 ptrtoint (i16* @g2 to i64))
-; CHECK-NEXT:    ret i64 [[R]]
+; CHECK-NEXT:    ret i64 sub (i64 ptrtoint (i16* @g to i64), i64 ptrtoint (i16* @g2 to i64))
 ;
   %i = ptrtoint i16* @g to i64
   %i2 = ptrtoint i16* @g2 to i64
   %diff = sub i64 %i, %i2
   %r = freeze i64 %diff
   ret i64 %r
 }
 
 define i64 @ptrdiff2() {
 ; CHECK-LABEL: @ptrdiff2(
-; CHECK-NEXT:    [[R:%.*]] = freeze i64 -2
-; CHECK-NEXT:    ret i64 [[R]]
+; CHECK-NEXT:    ret i64 -2
 ;
   %i = ptrtoint i16* @g to i64
   %gep = getelementptr i16, i16* @g, i64 1
   %i2 = ptrtoint i16* %gep to i64
   %diff = sub i64 %i, %i2
   %r = freeze i64 %diff
   ret i64 %r
 }