[NFC][CLANG] Fix Static Code Analysis Concerns

Reported by Static Analyzer Tool, Coverity:

  Bad bit shift operation
  The operation may have an undefined behavior or yield an unexpected result.

  In <unnamed>::SVEEmitter::encodeFlag(unsigned long long, llvm::StringRef): A bit shift operation has a shift amount which is too large or has a negative value.

    // Returns the SVETypeFlags for a given value and mask.
    uint64_t encodeFlag(uint64_t V, StringRef MaskName) const {
      auto It = FlagTypes.find(MaskName);
     	//Condition It != llvm::StringMap<unsigned long long, llvm::MallocAllocator>::const_iterator const(this->FlagTypes.end()), taking true branch.
      if (It != FlagTypes.end()) {
        uint64_t Mask = It->getValue();
        //return_constant: Function call llvm::countr_zero(Mask) may return 64.
        //assignment: Assigning: Shift = llvm::countr_zero(Mask). The value of Shift is now 64.
        unsigned Shift = llvm::countr_zero(Mask);

       //Bad bit shift operation (BAD_SHIFT)
       //large_shift: In expression V << Shift, left shifting by more than 63 bits has undefined behavior. The shift amount, Shift, is 64.
        return (V << Shift) & Mask;
      }
      llvm_unreachable("Unsupported flag");
    }

Asserting Mask != 0 will not suffice to silence Coverity. While Coverity can specifically observe that countr_zero might return 0 (because TrailingZerosCounter<T, 8>::count() has a return 64 statement), It seems like Coverity can not determine that the function can't return 65 or higher. Coverity is reporting is that the shift might overflow,
so that is what should be guarded.
assert(Shift < 64 && "Mask value produced an invalid shift value");

Reviewed By: tahonermann, sdesmalen, erichkeane

Differential Revision: https://reviews.llvm.org/D150140

diff --git a/clang/utils/TableGen/SveEmitter.cpp b/clang/utils/TableGen/SveEmitter.cpp
index d5d3f5f..d7f1e5a 100644 (file)
--- a/clang/utils/TableGen/SveEmitter.cpp
+++ b/clang/utils/TableGen/SveEmitter.cpp
@@ -299,6 +299,7 @@ public:
     if (It != FlagTypes.end()) {
       uint64_t Mask = It->getValue();
       unsigned Shift = llvm::countr_zero(Mask);
+      assert(Shift < 64 && "Mask value produced an invalid shift value");
       return (V << Shift) & Mask;
     }
     llvm_unreachable("Unsupported flag");