[PATCH] Support the assume_aligned function attribute

[Hal Finkel]

----- Original Message -----
> From: "Aaron Ballman" <aaron.ballman at gmail.com>
> To: reviews+D4601+public+521bc0d2736e59d3 at reviews.llvm.org
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "Richard Smith" <richard at metafoo.co.uk>, "llvm cfe" <cfe-commits at cs.uiuc.edu>
> Sent: Monday, July 21, 2014 11:22:01 AM
> Subject: Re: [PATCH] Support the assume_aligned function attribute
> 
> 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.

No, I put them in specifically because the common handler did not. I noticed because when I was writing the regression tests, if I passed too few parameters (specifically none), the later call to getArg would assert.

 -Hal

> 
> >
> > ================
> > 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.
> 
> ~Aaron
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory