[llvm] 1020d16 - [InstSimplify] Handle nsw shl -> poison patterns

[Simon Pilgrim via llvm-commits]

Author: Simon Pilgrim
Date: 2021-02-23T18:26:56Z
New Revision: 1020d161565df48ec454509724e60ab8fbbdfd66

URL: https://github.com/llvm/llvm-project/commit/1020d161565df48ec454509724e60ab8fbbdfd66
DIFF: https://github.com/llvm/llvm-project/commit/1020d161565df48ec454509724e60ab8fbbdfd66.diff

LOG: [InstSimplify] Handle nsw shl -> poison patterns

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

Differential Revision: https://reviews.llvm.org/D97305

Added: 
    

Modified: 
    llvm/lib/Analysis/InstructionSimplify.cpp
    llvm/test/Transforms/InstCombine/known-signbit-shift.ll
    llvm/test/Transforms/InstSimplify/icmp-constant.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 814881da3c3a..c7c0290cbbf0 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1228,7 +1228,8 @@ static bool isPoisonShift(Value *Amount, const SimplifyQuery &Q) {
 /// 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;
 
@@ -1272,6 +1273,21 @@ static Value *SimplifyShift(Instruction::BinaryOps Opcode, Value *Op0,
   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;
 }
 
@@ -1280,7 +1296,8 @@ static Value *SimplifyShift(Instruction::BinaryOps Opcode, Value *Op0,
 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
@@ -1306,7 +1323,8 @@ static Value *SimplifyRightShift(Instruction::BinaryOps Opcode, Value *Op0,
 /// 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

diff  --git a/llvm/test/Transforms/InstCombine/known-signbit-shift.ll b/llvm/test/Transforms/InstCombine/known-signbit-shift.ll
index f189cbead6de..e758d670e3bc 100644
--- a/llvm/test/Transforms/InstCombine/known-signbit-shift.ll
+++ b/llvm/test/Transforms/InstCombine/known-signbit-shift.ll
@@ -31,7 +31,7 @@ define i1 @test_shift_negative(i32 %a, i32 %b) {
 ; 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
@@ -42,7 +42,7 @@ define i32 @test_no_sign_bit_conflict1(i1 %b) {
 ; 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

diff  --git a/llvm/test/Transforms/InstSimplify/icmp-constant.ll b/llvm/test/Transforms/InstSimplify/icmp-constant.ll
index cb3818c16df8..f8d5bdc89ceb 100644
--- a/llvm/test/Transforms/InstSimplify/icmp-constant.ll
+++ b/llvm/test/Transforms/InstSimplify/icmp-constant.ll
@@ -772,7 +772,7 @@ define <2 x i1> @add_nsw_pos_const5_splat_vec(<2 x i32> %x) {
 
 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
@@ -784,7 +784,7 @@ define i1 @ne_shl_by_constant_produces_poison(i8 %x) {
 
 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