[InstSimplify] Handle nsw shl -> poison patterns

[RKSimon]

Summary

Pulled out from D90479 - this recognises invalid nsw shl patterns with signbit changes that result in poison.

[lebedev.ri]

Looks good to me.

llvm/lib/Analysis/InstructionSimplify.cpp

1266–1274

NFC, please precommit

1299–1300

llvm/lib/Analysis/InstructionSimplify.cpp

static bool isPoisonShift(Value *Amount, const SimplifyQuery &Q) {
   }
 
   return false;
 }
 
 /// Given operands for an Shl, LShr or AShr, see if we can fold the result.
 /// If not, this returns null.
 static Value *SimplifyShift(Instruction::BinaryOps Opcode, Value *Op0,
-                            Value *Op1, const SimplifyQuery &Q, unsigned MaxRecurse) {
+                            Value *Op1, bool IsNSW, const SimplifyQuery &Q,
+                            unsigned MaxRecurse) {
   if (Constant *C = foldOrCommuteConstant(Opcode, Op0, Op1, Q))
     return C;
 
   // 0 shift by X -> 0
   if (match(Op0, m_Zero()))
     return Constant::getNullValue(Op0->getType());
 
   // X shift by 0 -> X
static Value *SimplifyShift(Instruction::BinaryOps Opcode, Value *Op0,
   // If the operation is with the result of a phi instruction, check whether
   // operating on all incoming values of the phi always yields the same value.
   if (isa<PHINode>(Op0) || isa<PHINode>(Op1))
     if (Value *V = ThreadBinOpOverPHI(Opcode, Op0, Op1, Q, MaxRecurse))
       return V;
 
   // If any bits in the shift amount make that value greater than or equal to
   // the number of bits in the type, the shift is undefined.
   KnownBits KnownAmt = computeKnownBits(Op1, Q.DL, 0, Q.AC, Q.CxtI, Q.DT);
   if (KnownAmt.getMinValue().uge(KnownAmt.getBitWidth()))
     return PoisonValue::get(Op0->getType());
 
   // If all valid bits in the shift amount are known zero, the first operand is
   // unchanged.
   unsigned NumValidShiftBits = Log2_32_Ceil(KnownAmt.getBitWidth());
   if (KnownAmt.countMinTrailingZeros() >= NumValidShiftBits)
     return Op0;
 
+  // Check for nsw shl leading to a poison value.
+  if (IsNSW) {
+    assert(Opcode == Instruction::Shl && "Expected shl for nsw instruction");
+    KnownBits KnownVal = computeKnownBits(Op0, Q.DL, 0, Q.AC, Q.CxtI, Q.DT);
+    KnownBits KnownShl = KnownBits::shl(KnownVal, KnownAmt);
+
+    if (KnownVal.Zero.isSignBitSet())
+      KnownShl.Zero.setSignBit();
+    if (KnownVal.One.isSignBitSet())
+      KnownShl.One.setSignBit();
+
+    if (KnownShl.hasConflict())
+      return PoisonValue::get(Op0->getType());
+  }
+
   return nullptr;
 }
 
 /// Given operands for an Shl, LShr or AShr, see if we can
 /// fold the result.  If not, this returns null.
 static Value *SimplifyRightShift(Instruction::BinaryOps Opcode, Value *Op0,
                                  Value *Op1, bool isExact, const SimplifyQuery &Q,
                                  unsigned MaxRecurse) {
-  if (Value *V = SimplifyShift(Opcode, Op0, Op1, Q, MaxRecurse))
+  if (Value *V =
+          SimplifyShift(Opcode, Op0, Op1, /*IsNSW*/ false, Q, MaxRecurse))
     return V;
 
   // X >> X -> 0
   if (Op0 == Op1)
     return Constant::getNullValue(Op0->getType());
 
   // undef >> X -> 0
   // undef >> X -> undef (if it's exact)
static Value *SimplifyRightShift(Instruction::BinaryOps Opcode, Value *Op0,
 
   return nullptr;
 }
 
 /// Given operands for an Shl, see if we can fold the result.
 /// If not, this returns null.
 static Value *SimplifyShlInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
                               const SimplifyQuery &Q, unsigned MaxRecurse) {
-  if (Value *V = SimplifyShift(Instruction::Shl, Op0, Op1, Q, MaxRecurse))
+  if (Value *V =
+          SimplifyShift(Instruction::Shl, Op0, Op1, isNSW, Q, MaxRecurse))
     return V;
 
   // undef << X -> 0
   // undef << X -> undef if (if it's NSW/NUW)
   if (Q.isUndefValue(Op0))
     return isNSW || isNUW ? Op0 : Constant::getNullValue(Op0->getType());
 
   // (X >> A) << A -> X

llvm/test/Transforms/InstCombine/known-signbit-shift.ll

;
   %cmp = icmp slt i32 %shift, 0
   ret i1 %cmp
 }
 
 ; If sign bit is a known zero, it cannot be a known one.
 ; This test should not crash opt. The shift produces poison.
 define i32 @test_no_sign_bit_conflict1(i1 %b) {
 ; CHECK-LABEL: @test_no_sign_bit_conflict1(
-; CHECK-NEXT:    ret i32 undef
+; CHECK-NEXT:    ret i32 poison
 ;
   %sel = select i1 %b, i32 8193, i32 8192
   %mul = shl nsw i32 %sel, 18
   ret i32 %mul
 }
 
 ; If sign bit is a known one, it cannot be a known zero.
 ; This test should not crash opt. The shift produces poison.
 define i32 @test_no_sign_bit_conflict2(i1 %b) {
 ; CHECK-LABEL: @test_no_sign_bit_conflict2(
-; CHECK-NEXT:    ret i32 undef
+; CHECK-NEXT:    ret i32 poison
 ;
   %sel = select i1 %b, i32 -8193, i32 -8194
   %mul = shl nsw i32 %sel, 18
   ret i32 %mul
 }

llvm/test/Transforms/InstSimplify/icmp-constant.ll

;
   ret <2 x i1> %cmp
 }
 
 ; PR34838 - https://bugs.llvm.org/show_bug.cgi?id=34838
 ; The shift is known to create poison, so we can simplify the cmp.
 
 define i1 @ne_shl_by_constant_produces_poison(i8 %x) {
 ; CHECK-LABEL: @ne_shl_by_constant_produces_poison(
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    ret i1 poison
 ;
   %zx = zext i8 %x to i16      ; zx  = 0x00xx
   %xor = xor i16 %zx, 32767    ; xor = 0x7fyy
   %sub = sub nsw i16 %zx, %xor ; sub = 0x80zz  (the top bit is known one)
   %poison = shl nsw i16 %sub, 2    ; oops! this shl can't be nsw; that's POISON
   %cmp = icmp ne i16 %poison, 1
   ret i1 %cmp
 }
 
 define i1 @eq_shl_by_constant_produces_poison(i8 %x) {
 ; CHECK-LABEL: @eq_shl_by_constant_produces_poison(
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 poison
 ;
   %clear_high_bit = and i8 %x, 127                 ; 0x7f
   %set_next_high_bits = or i8 %clear_high_bit, 112 ; 0x70
   %poison = shl nsw i8 %set_next_high_bits, 3
   %cmp = icmp eq i8 %poison, 15
   ret i1 %cmp
 }