[llvm] r243996 - Avoid passing nullptr to std::equal.

[Yaron Keren via llvm-commits]

I'm not sure how std::equal of two nullptrs should be, is this documented
or UB?

Anyhow the question is what to return for the various combinatios of
element_begin()
and Other->element_begin() being non/nullptr. What you wrote makes sense, we
probably need something like

if (!element_begin())
  return true;
else
  return std::equal(element_begin(), element_end(), Other->element_begin());

(because other cases are already handled in  the previous if)

What do you think?
Also, how this could be tested?





2015-08-12 0:04 GMT+03:00 Hans Wennborg <hans at chromium.org>:

> (now cc'ing the new llvm-commits list)
>
> On Tue, Aug 11, 2015 at 2:01 PM, Hans Wennborg <hans at chromium.org> wrote:
> > On Tue, Aug 4, 2015 at 8:57 AM, Yaron Keren <yaron.keren at gmail.com>
> wrote:
> >> Author: yrnkrn
> >> Date: Tue Aug  4 10:57:04 2015
> >> New Revision: 243996
> >>
> >> URL: http://llvm.org/viewvc/llvm-project?rev=243996&view=rev
> >> Log:
> >> Avoid passing nullptr to std::equal.
> >> As documented in the LLVM Coding Standards, indeed MSVC incorrectly
> asserts
> >> on this in Debug mode. This happens when building clang with Visual C++
> and
> >> -triple i686-pc-windows-gnu on these clang regression tests:
> >>
> >>  clang/test/CodeGen/2011-03-08-ZeroFieldUnionInitializer.c
> >>  clang/test/CodeGen/empty-union-init.c
> >
> > Should we merge this to 3.7?
> >
> >> Modified: llvm/trunk/lib/IR/Type.cpp
> >> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Type.cpp?rev=243996&r1=243995&r2=243996&view=diff
> >>
> ==============================================================================
> >> --- llvm/trunk/lib/IR/Type.cpp (original)
> >> +++ llvm/trunk/lib/IR/Type.cpp Tue Aug  4 10:57:04 2015
> >> @@ -612,7 +612,8 @@ bool StructType::isLayoutIdentical(Struc
> >>        getNumElements() != Other->getNumElements())
> >>      return false;
> >>
> >> -  return std::equal(element_begin(), element_end(),
> Other->element_begin());
> >> +  return element_begin() &&
> >> +         std::equal(element_begin(), element_end(),
> Other->element_begin());
> >
> > Actually, wouldn't std::equal return true if element_begin() and
> > element_end() are both null? It seems the new code will now return
> > false for that case. If two StructTypes both have zero elements,
> > shouldn't they be considered as having identical layout?
>
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