[llvm] r220138 - [InstCombine] Do an about-face on how LLVM canonicalizes (cast (load

[Hans Wennborg]

On Tue, Nov 25, 2014 at 9:48 AM, Hans Wennborg <hans at chromium.org> wrote:
> On Tue, Nov 25, 2014 at 2:13 AM, Chandler Carruth <chandlerc at gmail.com> wrote:
>>
>> On Thu, Nov 20, 2014 at 11:19 AM, Hans Wennborg <hans at chromium.org> wrote:
>>>
>>> > The only fallout I've found so far from this change was SROA and I have
>>> > fixed it to not be impeded by the different type of load. If you find
>>> > more places where this change causes optimizations not to fire, those
>>> > too are likely bugs where we are assuming that the type of pointers is
>>> > "significant" for optimization purposes.
>>>
>>> This caused a 97 k binary size regression in Chromium. I'll try to
>>> produce some diffs of how the code changed.
>>
>>
>> What ever came of this? I know you were side-tracked on LVI, but I don't
>> want to miss this.
>
> Hopefully that side track is done now :)
>
>> I've also landed the other half of this in r222748. This may have (sadly)
>> papered over the issue, or exacerbated the issue, depending on the nature of
>> the problem. Feel free to hit me up with examples of IR diff...
>
> I'll try to figure out what the status is today.

Chrome is still bigger with ToT Clang than our previously used version
:( I'm looking at one object file that regressed at r220138, and that
is still unchanged on ToT.

Preprocessed source attached, build command is:

  clang++ -fstack-protector --param=ssp-buffer-size=4
-fno-strict-aliasing -fvisibility=hidden -fPIC -pthread -m64
-march=x86-64 -O2 -fno-ident -fdata-sections -ffunction-sections
-funwind-tables -fno-exceptions -fno-rtti -fno-threadsafe-statics
-fvisibility-inlines-hidden -w -std=gnu++11 -c
/tmp/ash.resize_shadow_controller.ii -o a.o

Sizes before and after your change, and at ToT:

   text    data     bss     dec     hex filename
   2626     184       0    2810     afa /tmp/220137.o
   2853     184       0    3037     bdd /tmp/220138.o
   2853     184       0    3037     bdd /tmp/222768.o

I'll see if I can figure out anything from the diffs.

 - Hans
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