[PATCH] D36562: [Bitfield] Make the bitfield a separate location if it has width of legal integer type and its bit offset is naturally aligned for the type

[Wei Mi via cfe-commits]

On Tue, Aug 22, 2017 at 7:03 PM, Xinliang David Li <davidxl at google.com> wrote:
>
>
> On Tue, Aug 22, 2017 at 6:37 PM, Chandler Carruth via Phabricator
> <reviews at reviews.llvm.org> wrote:
>>
>> chandlerc added a comment.
>>
>> I'm really not a fan of the degree of complexity and subtlety that this
>> introduces into the frontend, all to allow particular backend optimizations.
>>
>> I feel like this is Clang working around a fundamental deficiency in LLVM
>> and we should instead find a way to fix this in LLVM itself.
>>
>> As has been pointed out before, user code can synthesize large integers
>> that small bit sequences are extracted from, and Clang and LLVM should
>> handle those just as well as actual bitfields.
>>
>> Can we see how far we can push the LLVM side before we add complexity to
>> Clang here? I understand that there remain challenges to LLVM's stuff, but I
>> don't think those challenges make *all* of the LLVM improvements off the
>> table, I don't think we've exhausted all ways of improving the LLVM changes
>> being proposed, and I think we should still land all of those and
>> re-evaluate how important these issues are when all of that is in place.
>
>
> The main challenge of doing  this in LLVM is that inter-procedural analysis
> (and possibly cross module) is needed (for store forwarding issues).
>
> Wei, perhaps you can provide concrete test case to illustrate the issue so
> that reviewers have a good understanding.
>
> David

Here is a runable testcase:
-------------------- 1.cc ------------------------
class A {
public:
  unsigned long f1:2;
  unsigned long f2:6;
  unsigned long f3:8;
  unsigned long f4:4;
};
A a;
unsigned long b;
unsigned long N = 1000000000;

__attribute__((noinline))
void foo() {
  a.f3 = 3;
}

__attribute__((noinline))
void goo() {
  b = a.f3;
}

int main() {
  unsigned long i;
  for (i = 0; i < N; i++) {
    foo();
    goo();
  }
}
------------------------------------------------------------
Now trunk takes about twice running time compared with trunk + this
patch. That is because trunk shrinks the store of a.f3 in foo (Done by
DagCombiner) but not shrink the load of a.f3 in goo, so store
forwarding will be blocked.

The testcases shows the potential problem of store shrinking. Before
we decide to do store shrinking, we need to know all the related loads
will be shrunk,  and that requires IPA analysis. Otherwise, when load
shrinking was blocked for some difficult case (Like the instcombine
case described in
https://www.mail-archive.com/cfe-commits@lists.llvm.org/msg65085.html),
performance regression will happen.

Wei.


>>
>>
>>
>> Repository:
>>   rL LLVM
>>
>> https://reviews.llvm.org/D36562
>>
>>
>>
>