[cfe-commits] r162545 - in /cfe/trunk: lib/Analysis/CFG.cpp test/Sema/warn-unreachable.c

[Chris Lattner]

On Aug 24, 2012, at 12:42 AM, Ted Kremenek <kremenek at apple.com> wrote:

> Author: kremenek
> Date: Fri Aug 24 02:42:09 2012
> New Revision: 162545
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=162545&view=rev
> Log:
> Teach CFG that 'if (x & 0)' and 'if (x * 0)' is an unfeasible branch.

Why not teach the general front-end constant folding logic (in lib/AST/ExprConstant.cpp) to handle this identity, and then just use it?

-Chris

> 
> Fixes <rdar://problem/11005770>.
> 
> Modified:
>    cfe/trunk/lib/Analysis/CFG.cpp
>    cfe/trunk/test/Sema/warn-unreachable.c
> 
> Modified: cfe/trunk/lib/Analysis/CFG.cpp
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Analysis/CFG.cpp?rev=162545&r1=162544&r2=162545&view=diff
> ==============================================================================
> --- cfe/trunk/lib/Analysis/CFG.cpp (original)
> +++ cfe/trunk/lib/Analysis/CFG.cpp Fri Aug 24 02:42:09 2012
> @@ -467,6 +467,30 @@
>         CachedBoolEvals[S] = Result; // update or insert
>         return Result;
>       }
> +      else {
> +        switch (Bop->getOpcode()) {
> +          default: break;
> +          // For 'x & 0' and 'x * 0', we can determine that
> +          // the value is always false.
> +          case BO_Mul:
> +          case BO_And: {
> +            // If either operand is zero, we know the value
> +            // must be false.
> +            llvm::APSInt IntVal;
> +            if (Bop->getLHS()->EvaluateAsInt(IntVal, *Context)) {
> +              if (IntVal.getBoolValue() == false) {
> +                return TryResult(false);
> +              }
> +            }
> +            if (Bop->getRHS()->EvaluateAsInt(IntVal, *Context)) {
> +              if (IntVal.getBoolValue() == false) {
> +                return TryResult(false);
> +              }
> +            }
> +          }
> +          break;
> +        }
> +      }
>     }
> 
>     return evaluateAsBooleanConditionNoCache(S);
> 
> Modified: cfe/trunk/test/Sema/warn-unreachable.c
> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Sema/warn-unreachable.c?rev=162545&r1=162544&r2=162545&view=diff
> ==============================================================================
> --- cfe/trunk/test/Sema/warn-unreachable.c (original)
> +++ cfe/trunk/test/Sema/warn-unreachable.c Fri Aug 24 02:42:09 2012
> @@ -132,3 +132,12 @@
>         s[i] = 0;
> }
> 
> +// Test case for <rdar://problem/11005770>.  We should treat code guarded
> +// by 'x & 0' and 'x * 0' as unreachable.
> +void calledFun();
> +void test_mul_and_zero(int x) {
> +  if (x & 0) calledFun(); // expected-warning {{will never be executed}}
> +  if (0 & x) calledFun(); // expected-warning {{will never be executed}}
> +  if (x * 0) calledFun(); // expected-warning {{will never be executed}}
> +  if (0 * x) calledFun(); // expected-warning {{will never be executed}}
> +}
> 
> 
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