r333044 - Use zeroinitializer for (trailing zero portion of) large array initializers

[Hans Wennborg via cfe-commits]

We're hitting asserts after this in Chromium. I've filed PR37560 and
reverted in r333067.

On Wed, May 23, 2018 at 2:09 AM, Richard Smith via cfe-commits
<cfe-commits at lists.llvm.org> wrote:
> Author: rsmith
> Date: Tue May 22 17:09:29 2018
> New Revision: 333044
>
> URL: http://llvm.org/viewvc/llvm-project?rev=333044&view=rev
> Log:
> Use zeroinitializer for (trailing zero portion of) large array initializers
> more reliably.
>
> Clang has two different ways it emits array constants (from InitListExprs and
> from APValues), and both had some ability to emit zeroinitializer, but neither
> was able to catch all cases where we could use zeroinitializer reliably. In
> particular, emitting from an APValue would fail to notice if all the explicit
> array elements happened to be zero. In addition, for large arrays where only an
> initial portion has an explicit initializer, we would emit the complete
> initializer (which could be huge) rather than emitting only the non-zero
> portion. With this change, when the element would have a suffix of more than 8
> zero elements, we emit the array constant as a packed struct of its initial
> portion followed by a zeroinitializer constant for the trailing zero portion.
>
> In passing, I found a bug where SemaInit would sometimes walk the entire array
> when checking an initializer that only covers the first few elements; that's
> fixed here to unblock testing of the rest.
>
> Differential Revision: https://reviews.llvm.org/D47166
>
> Modified:
>     cfe/trunk/lib/CodeGen/CGExprConstant.cpp
>     cfe/trunk/lib/Sema/SemaInit.cpp
>     cfe/trunk/test/CodeGen/init.c
>     cfe/trunk/test/CodeGenCXX/cxx11-initializer-aggregate.cpp
>     cfe/trunk/test/SemaCXX/aggregate-initialization.cpp