Unverified Commit 6f01cb91 authored by Justin Lebar's avatar Justin Lebar
Browse files

Handle `select` in programUndefinedIfPoison. 

If both the true and false operands of a `select` are poison, then the `select`
is poison.

Differential Revision: https://reviews.llvm.org/D149427
parent 8df59132

llvm/lib/Analysis/ValueTracking.cpp

+10 −1
Original line number Diff line number Diff line
@@ -6707,7 +6707,7 @@ bool llvm::isGuaranteedNotToBePoison(const Value *V, AssumptionCache *AC, 
  return ::isGuaranteedNotToBeUndefOrPoison(V, AC, CtxI, DT, Depth, true);

}



/// Return true if undefined behavior would provable be executed on the path to

/// Return true if undefined behavior would provably be executed on the path to

/// OnPathTo if Root produced a posion result.  Note that this doesn't say

/// anything about whether OnPathTo is actually executed or whether Root is

/// actually poison.  This can be used to assess whether a new use of Root can
@@ -7041,6 +7041,15 @@ static bool programUndefinedIfUndefOrPoison(const Value *V, 
          break;

        }

      }



      // Special handling for select, which returns poison if its operand 0 is

      // poison (handled in the loop above) *or* if both its true/false operands

      // are poison (handled here).

      if (I.getOpcode() == Instruction::Select &&

          YieldsPoison.count(I.getOperand(1)) &&

          YieldsPoison.count(I.getOperand(2))) {

        YieldsPoison.insert(&I);

      }

    }



    BB = BB->getSingleSuccessor();

llvm/unittests/Analysis/ValueTrackingTest.cpp

+14 −0
Original line number Diff line number Diff line
@@ -982,6 +982,20 @@ TEST_F(ValueTrackingTest, programUndefinedIfPoison) { 
  EXPECT_EQ(programUndefinedIfPoison(A), true);

}



TEST_F(ValueTrackingTest, programUndefinedIfPoisonSelect) {

  parseAssembly("declare i32 @any_num()"

                "define void @test(i1 %Cond) {\n"

                "  %A = call i32 @any_num()\n"

                "  %B = add i32 %A, 1\n"

                "  %C = select i1 %Cond, i32 %A, i32 %B\n"

                "  udiv i32 1, %C"

                "  ret void\n"

                "}\n");

  // If A is poison, B is also poison, and therefore C is poison regardless of

  // the value of %Cond.

  EXPECT_EQ(programUndefinedIfPoison(A), true);

}



TEST_F(ValueTrackingTest, programUndefinedIfUndefOrPoison) {

  parseAssembly("declare i32 @any_num()"

                "define void @test(i32 %mask) {\n"