no warnings for -Wtautological-compare

[llvmbot]

Bugzilla Link	43573
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@zygoloid,@Weverything

Extended Description

clang-10 falls to emit the warning for the following:

$ cat s.c
  int main(){
    int a;
    int b = (0!=((-1)|((a = 1) == 1)));
    return 0;
  }

$ clang-10 -Wtautological-compare s.c

clang version 10.0.0 (https://github.com/llvm/llvm-project.git 49c4e58)
Target: x86_64-unknown-linux-gnu
Thread model: posix

$ gcc-9.1 -Wtautological-compare s.c

s.c:3:12: warning：bitwise comparison always evaluates to true [-Wtautological-compare]
    3 |  int b = (0!=((-1)|((c = 1) == 1)));
       |               ^~

[Weverything]

There exist a warning for:
if (0 == (5 | x)) {}
But there is not for:
if (0 == (-1 | x)) {}
if (-1 == (1 & x)) {}

That's because the warning explicitly checks for an IntegerLiteral AST node, but -1 is represented by a UnaryOperator with an IntegerLiteral sub-Expr.

The function that needs to be fixed is checkIncorrectEqualityOperator in lib/Analysis/CFG.cpp. Every check for an IntegerLiteral could be replaced by a call to the helper function IsIntegerLiteralConstantExpr. Instead of getting the value from the IntegerLiteral, a call to Expr::EvaluateAsInt would be needed. Since we're dealing with negative numbers, integer conversions also need to be tested.