[PATCH] Support the assume_aligned function attribute

[Aaron Ballman]

On Mon, Jul 21, 2014 at 4:36 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> 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.

I'd agree with that and can add the functionality to do it (it makes
sense). I was only pointing out that it wasn't the (original) purpose
of the function.

~Aaron