[llvm] r255691 - [LoopVectorizer] Refine loop vectorizer's register usage calculator by ignoring specific instructions.

[Hans Wennborg via llvm-commits]

On Tue, Feb 16, 2016 at 11:06 AM, Hans Wennborg <hans at chromium.org> wrote:
> On Mon, Feb 15, 2016 at 12:28 PM, Dimitry Andric <dimitry at andric.com> wrote:
>> On 15 Dec 2015, at 23:45, Cong Hou via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>>>
>>> Author: conghou
>>> Date: Tue Dec 15 16:45:09 2015
>>> New Revision: 255691
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=255691&view=rev
>>> Log:
>>> [LoopVectorizer] Refine loop vectorizer's register usage calculator by ignoring specific instructions.
>>>
>>> (This is the third attempt to check in this patch, and the first two are r255454
>>> and r255460. The once failed test file reg-usage.ll is now moved to
>>> test/Transform/LoopVectorize/X86 directory with target datalayout and target
>>> triple indicated.)
>>>
>>> LoopVectorizationCostModel::calculateRegisterUsage() is used to estimate the
>>> register usage for specific VFs. However, it takes into account many
>>> instructions that won't be vectorized, such as induction variables,
>>> GetElementPtr instruction, etc.. This makes the loop vectorizer too conservative
>>> when choosing VF. In this patch, the induction variables that won't be
>>> vectorized plus GetElementPtr instruction will be added to ValuesToIgnore set
>>> so that their register usage won't be considered any more.
>>>
>>>
>>> Differential revision: http://reviews.llvm.org/D15177
>>
>> This appears to have introduced a use-after-free, see https://llvm.org/bugs/show_bug.cgi?id=26509 for the details and a valgrind log.  Is there any way we can fix this one before 3.8 release, or revert it otherwise?
>
> Cong, do you think this is something that's easy to fix or should we
> revert in the branch to be safe?

Cong: ping?