[cfe-commits] [PATCH] Improve -Wtautological-constant-out-of-range-compare

[Richard Trieu]

On Mon, Nov 12, 2012 at 6:35 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> On Mon, Nov 12, 2012 at 5:31 PM, Richard Trieu <rtrieu at google.com> wrote:
> > I uploaded a new patch.
> >
> > On Mon, Nov 12, 2012 at 11:22 AM, Eli Friedman <eli.friedman at gmail.com>
> > wrote:
> >>
> >> Comments below.
> >>
> >> On Mon, Nov 12, 2012 at 9:46 AM, Richard Trieu <rtrieu at google.com>
> wrote:
> >> > Make -Wtautological-constant-out-of-range-compare checking take into
> >> > account types and conversion between types.  The old version merely
> checked
> >> > the bit widths, which allowed failed to catch a few cases, while
> warning on
> >> > other safe comparisons.
> >> >
> >> > http://llvm-reviews.chandlerc.com/D113
> >> >
> >> > Files:
> >> >   test/Analysis/additive-folding.cpp
> >> >   test/SemaCXX/compare.cpp
> >> >   test/SemaCXX/warn-enum-compare.cpp
> >> >   lib/Sema/SemaChecking.cpp
> >>
> >> Index: test/SemaCXX/warn-enum-compare.cpp
> >> ===================================================================
> >> --- test/SemaCXX/warn-enum-compare.cpp
> >> +++ test/SemaCXX/warn-enum-compare.cpp
> >> @@ -39,8 +39,8 @@
> >>    while (b == c);
> >>    while (B1 == name1::B2);
> >>    while (B2 == name2::B1);
> >> -  while (x == AnonAA); // expected-warning {{comparison of constant
> >> 42 with expression of type 'Foo' is always false}}
> >> -  while (AnonBB == y); // expected-warning {{comparison of constant
> >> 45 with expression of type 'Bar' is always false}}
> >> +  while (x == AnonAA);
> >> +  while (AnonBB == y);
> >>    while (AnonAA == AnonAB);
> >>    while (AnonAB == AnonBA);
> >>    while (AnonBB == AnonAA);
> >>
> >> Why are you changing this warning?
> >
> > See discussion below at IntRange.
> >>
> >>
> >>    while (AnonBB == AnonAA);
> >> Index: lib/Sema/SemaChecking.cpp
> >> ===================================================================
> >> --- lib/Sema/SemaChecking.cpp
> >> +++ lib/Sema/SemaChecking.cpp
> >> @@ -4328,38 +4328,91 @@
> >>                                           Expr *Constant, Expr *Other,
> >>                                           llvm::APSInt Value,
> >>                                           bool RhsConstant) {
> >> +  // 0 values are handled later by CheckTrivialUnsignedComparison().
> >> +  if (Value == 0)
> >> +    return;
> >> +
> >>    BinaryOperatorKind op = E->getOpcode();
> >>    QualType OtherT = Other->getType();
> >>    QualType ConstantT = Constant->getType();
> >> +  QualType CommonT = E->getLHS()->getType();
> >>    if (S.Context.hasSameUnqualifiedType(OtherT, ConstantT))
> >>      return;
> >>    assert((OtherT->isIntegerType() && ConstantT->isIntegerType())
> >>           && "comparison with non-integer type");
> >>    // FIXME. handle cases for signedness to catch (signed char)N == 200
> >>
> >> Does this patch fix this FIXME?
> >
> > Yes.  FIXME removed, added test case.
> >>
> >>
> >> -  IntRange OtherRange = IntRange::forValueOfType(S.Context, OtherT);
> >> -  IntRange LitRange = GetValueRange(S.Context, Value,
> >> Value.getBitWidth());
> >> -  if (OtherRange.Width >= LitRange.Width)
> >> +
> >> +  bool ConstantSigned = ConstantT->isSignedIntegerType();
> >> +  bool OtherSigned = OtherT->isSignedIntegerType();
> >> +  bool CommonSigned = CommonT->isSignedIntegerType();
> >>
> >> Why are you getting rid of the IntRange usage?  Granted, we didn't
> >> really have any good testcases, but it does add power to the analysis
> >> in some cases.
> >
> >  For my use, IntRange didn't add any value.  Everything I needed, I could
> > just get from the APSInt.  The only difference was what width I used for
> > Other.  IntRange does truncate the enum width so that different values
> will
> > be considered out of range when comparing against enums.  I factored out
> the
> > width into a variable.  The current code will remove the existing
> warnings
> > in warn-enum-compare.cpp.  The code in the comments would have preserved
> > them.
>
> It does add value in cases other than enums; it can recognize
> bitfields and knows some tricks with binary "&", logical operators in
> C, etc.
>
> IntRange doesn't appear to take those special cases into consideration.
 From looking at the code, it appears to special case enum, otherwise it
falls back to ASTContext::getIntWdith().


> Again, if you want to change our enum-related warnings, please do it
> in a separate patch.
>
> Will do.

>
> I would like to see more tests for the various cases in this code.
> Please make sure there's a test covering every branch you introduced.
>

Also looking into it.

>
> -Eli
>
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