From: Jeff Law <law@redhat.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: gcc-patches@gcc.gnu.org, dje.gcc@gmail.com
Subject: Re: [PATCH 4/5] shrink-wrap: Shrink-wrapping for separate components
Date: Mon, 10 Oct 2016 21:21:00 -0000 [thread overview]
Message-ID: <db1c033f-ed38-c165-2e3e-489383a92e49@redhat.com> (raw)
In-Reply-To: <20160930103456.GC14933@gate.crashing.org>
On 09/30/2016 04:34 AM, Segher Boessenkool wrote:
> [ whoops, message too big, resending with the attachment compressed ]
>
> On Tue, Sep 27, 2016 at 03:14:51PM -0600, Jeff Law wrote:
>> With transposition issue addressed, the only blocker I see are some
>> simple testcases we can add to the suite. They don't have to be real
>> extensive. And one motivating example for the list archives, ideally
>> the glibc malloc case.
>
> And here is the malloc testcase.
>
> A very important (for performance) function is _int_malloc, which starts
> with
[ ... ]
THanks. What I think is important to note with this example is the bits
that were pushed into the path with the sysmalloc/alloc_perturb calls.
That's an unlikely path.
We have to extrapolate a bit from the assembly provided. In the not
separately shrink-wrapped version, we have a full prologue of stores and
two instances of a full epilogue (though only one ever executes) provided.
With separate shrink wrapping the (presumably) very cold path where we
error has virtually no prologue/epilogue. That's probably a nop from a
performance standpoint.
More interesting is the path where we call sysmalloc/alloc_perturb, it's
a cold path, but not as cold as the error path. We save/restore 4 regs
in that case. Rather than a full prologue/epilogue. So there's clearly
a savings there, though again, via the expect it's a cold path.
Where we have to extrapolate is the hot path. Presumably on the hot
path we're saving/restoring ~4 fewer registers. I haven't verified
that, but that is kindof the whole point here.
Thanks,
Jeff
next prev parent reply other threads:[~2016-10-10 21:21 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-09-23 8:22 [PATCH v3 0/5] Separate shrink-wrapping Segher Boessenkool
2016-09-23 8:22 ` [PATCH 1/5] separate shrink-wrap: New command-line flag, status flag, hooks, and doc Segher Boessenkool
2016-09-26 17:02 ` Jeff Law
2016-09-23 8:22 ` [PATCH 2/5] dce: Don't dead-code delete separately wrapped restores Segher Boessenkool
2016-09-26 16:55 ` Jeff Law
2016-09-23 8:23 ` [PATCH 3/5] regrename: Don't rename restores Segher Boessenkool
2016-09-26 16:44 ` Jeff Law
2016-09-23 8:33 ` [PATCH 4/5] shrink-wrap: Shrink-wrapping for separate components Segher Boessenkool
2016-09-27 21:25 ` Jeff Law
2016-09-28 9:26 ` Segher Boessenkool
2016-09-28 16:36 ` Jeff Law
2016-09-30 10:14 ` Segher Boessenkool
[not found] ` <20160930102908.GB14933@gate.crashing.org>
2016-09-30 10:52 ` Segher Boessenkool
2016-10-10 21:21 ` Jeff Law [this message]
2016-10-10 22:23 ` Segher Boessenkool
2016-09-23 8:44 ` [PATCH 5/5] rs6000: Separate shrink-wrapping Segher Boessenkool
2016-09-26 16:39 ` Jeff Law
2016-09-26 18:16 ` David Edelsohn
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