llvm dev - Aug 2016 - RFC: We should stop merging allocas in the inliner

On Thu, Aug 4, 2016 at 9:44 AM Joerg Sonnenberger via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On Mon, Aug 01, 2016 at 04:47:49AM +0000, Chandler Carruth via llvm-dev
> wrote:
> > Chris added alloca merging in the inliner a looooong time ago, 2009.
The
> > reason he added it was because at the time we didn't do stack
coloring
> and
> > without it we had serious stack size problems in LLVM.
>
> Do we have any way to hunt for stack size regressions? I've been hit by
> some serious bugs in this area in GCC in the past and I would prefer to
> not repeat that experience with LLVM.
>
My technique was to log the stack size and build a bunch of code, and then
analyze the logs before and after. It seemed really effective though?

I also have some tests (sadly internal) that check stacksize is under some
bound and those tend to serve is good indicators that something has gone
badly wrong. For example, without the Clang tricks at -O0, we couldn't get
their tests to pass.
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> On Aug 4, 2016, at 9:48 AM, Chandler Carruth via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> On Thu, Aug 4, 2016 at 9:44 AM Joerg Sonnenberger via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> On Mon, Aug 01, 2016 at 04:47:49AM +0000, Chandler Carruth via llvm-dev
wrote:
> > Chris added alloca merging in the inliner a looooong time ago, 2009.
The
> > reason he added it was because at the time we didn't do stack
coloring and
> > without it we had serious stack size problems in LLVM.
> 
> Do we have any way to hunt for stack size regressions? I've been hit by
> some serious bugs in this area in GCC in the past and I would prefer to
> not repeat that experience with LLVM.
> 
> My technique was to log the stack size and build a bunch of code, and then
analyze the logs before and after. It seemed really effective though?
> 
> I also have some tests (sadly internal) that check stacksize is under some
bound and those tend to serve is good indicators that something has gone badly
wrong. For example, without the Clang tricks at -O0, we couldn't get their
tests to pass.
Are these “clang tricks” some indications of weaknesses in LLVM on this aspect?

— 
Mehdi

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On Thu, Aug 4, 2016 at 9:56 AM Mehdi Amini via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> On Aug 4, 2016, at 9:48 AM, Chandler Carruth via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> On Thu, Aug 4, 2016 at 9:44 AM Joerg Sonnenberger via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On Mon, Aug 01, 2016 at 04:47:49AM +0000, Chandler Carruth via llvm-dev
>> wrote:
>> > Chris added alloca merging in the inliner a looooong time ago,
2009. The
>> > reason he added it was because at the time we didn't do stack
coloring
>> and
>> > without it we had serious stack size problems in LLVM.
>>
>> Do we have any way to hunt for stack size regressions? I've been
hit by
>> some serious bugs in this area in GCC in the past and I would prefer to
>> not repeat that experience with LLVM.
>>
>
> My technique was to log the stack size and build a bunch of code, and then
> analyze the logs before and after. It seemed really effective though?
>
> I also have some tests (sadly internal) that check stacksize is under some
> bound and those tend to serve is good indicators that something has gone
> badly wrong. For example, without the Clang tricks at -O0, we couldn't
get
> their tests to pass.
>
>
> Are these “clang tricks” some indications of weaknesses in LLVM on this
> aspect?
>
Depends on how you look at it. They're only enabled at O0. It's
essentially
the lowest compile time cost way to re-use stack space in *highly*
repeatative functions (like where you have macros stamping out 1000s of
temporary std::string objects). Clang just keeps a freelist around of
allocas and re-uses them.

We could always turn on some form of stack coloring at O0, but when we
looked at it that would have been much harder to have match the compile
time profile, and also harder to make survive debug info etc.

Within Clang there are somewhat more obvious repeated patterns that we can
target for re-use.

At least, this is my memory from the discussion with Richard Smith when he
implemented this. =] He may want to provide more details or correct my
memory of things.

>
> —
> Mehdi
>
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