[PATCH] Support the assume_aligned function attribute

[Richard Smith]

On Mon, Jul 21, 2014 at 9:22 AM, Aaron Ballman <aaron.ballman at gmail.com>
wrote:

> On Sun, Jul 20, 2014 at 7:25 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> > Somewhat unrelated to this patch in particular, but we should add a
> UBSan check to catch failed assumptions.
> >
> > ================
> > Comment at: include/clang/Basic/Attr.td:867
> > @@ +866,3 @@
> > +                             "ExpectedFunctionOrMethod">;
> > +  let Args = [IntArgument<"Alignment">, DefaultIntArgument<"Offset",
> 0>];
> > +  let Documentation = [Undocumented];
> > ----------------
> > These should probably both be `ExprArgument`s, in order to support
> dependent alignment/offset expressions in templates.
> >
> > ================
> > Comment at: include/clang/Basic/Attr.td:868
> > @@ +867,3 @@
> > +  let Args = [IntArgument<"Alignment">, DefaultIntArgument<"Offset",
> 0>];
> > +  let Documentation = [Undocumented];
> > +}
> > ----------------
> > Please add documentation.
> >
> > ================
> > Comment at: lib/CodeGen/CGExpr.cpp:3352
> > @@ +3351,3 @@
> > +
> > +  if (Ret.isScalar() && TargetDecl)
> > +    if (const AssumeAlignedAttr *AA =
> > ----------------
> > Please add parens around this `if`.
> >
> > ================
> > Comment at: lib/Sema/SemaDeclAttr.cpp:1219-1227
> > @@ +1218,11 @@
> > +
> > +  if (Attr.getNumArgs() > 2) {
> > +    S.Diag(Attr.getLoc(), diag::err_attribute_too_many_arguments)
> > +      << Attr.getName() << 2;
> > +    return;
> > +  } else if (Attr.getNumArgs() < 1) {
> > +    S.Diag(Attr.getLoc(), diag::err_attribute_too_few_arguments)
> > +      << Attr.getName() << 1;
> > +    return;
> > +  }
> > +
> > ----------------
> > I think there's a helper function for this sequence of
> checks/diagnostics. If not, there should be.
>
> They aren't required -- the common handler takes care of that stuff.
>
> >
> > ================
> > Comment at: lib/Sema/SemaDeclAttr.cpp:1232-1235
> > @@ +1231,6 @@
> > +    return;
> > +  if (Attr.getNumArgs() > 1 && !checkUInt32Argument(S, Attr,
> > +
>  Attr.getArgAsExpr(1),
> > +                                                    Offset, 2))
> > +    return;
> > +
> > ----------------
> > Do you really need to check that the `Offset` fits into an `uint32_t`?
> Truncating the offset to the width of `Alignment` seems correct in all
> cases.
>
> This doesn't check that it fits into a uint32_t. It checks that it's a
> valid integer constant expression and errs if it's not. It then gets
> you the value as a uint32_t.


Huh. That's a bug in the existing function, then; it should be checking for
truncation.
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