Manjunath DN
2014-Jun-26 02:58 UTC
[LLVMdev] Contributing the Apple ARM64 compiler backend
HI James, Thanks for your reply and hints on what can be done for the Aarch64 backend optimization for llvm We have SPEC license and v8 hardware. So I will start looking into it warm regards Manjunath On Wed, Jun 25, 2014 at 8:42 PM, James Molloy <james.molloy at arm.com> wrote:> Hi Manjunath, > > At the time of writing that status we had only done our initial analysis. > This was done without real hardware and attempted to identify poor code > sequences but we were unable to quantify how much effect this would > actually > have. > > Since then we've done more analysis using Cortex-A57 and Cortex-A53 on an > internal development platform. > > For SPEC, we are between 10% and 0% behind GCC on 9 benchmarks, and 25% > ahead on one benchmark. Most benchmarks are less than 5% behind GCC. > > Because of the licencing of SPEC, I have to be quite restricted in what I > say and I can't give any numbers - sorry about that. > > We are focussing on Cortex-A57, and the things we've identified so far are: > * The CSEL instruction behaves worse than the equivalent branch structure > in at least one benchmark. In an out of order core, select-like > instructions > are going to be slower than their branched equivalent if the branch is > predictable due to CSEL having two dependencies. > > * Redundant calculations inside if conditions. We've seen: > 1. "if (a[x].b < c[y].d || a[x].e > c[y].f)" - the calculations of a[x] > and c[y] are repeated, when they are common. We've also seen similar > instances where multiple registers are used to compute very similar > addresses (such as x+0 and x+4!) and this increases register pressure. > 2. "if (a < 0 && b == c || a > 0 && b == d)" - the first comparison of > 'a' against zero is done twice, when the flag results of the first > comparison could be used for the second comparison. > > * For a loop such as "for (i = 0; i < n; ++i) > {do_something_with(&x[i]);}", GCC is using &x[i] as the loop induction > variable where LLVM uses i and performs the calculation &x[i] on every > iteration. This only creates one more add instruction but the loop we see > it > in only has 5 or so instructions. > > * The inline heuristics are way behind GCC's. If we crank the inline > threshold up to 1000, we can remove a 6.5% performance regression from one > benchmark entirely. > > * We're generating (due to SLP vectorizer and a DAG combine) loads into Q > registers when merging consecutive loads. This is bad, because there are no > callee-saved Q registers! So if the live range crosses a function call, it > will have to be immediately spilled again. This can be easily fixed by > using load-pair instructions instead. I have a patch to fix this. > > The list above is non-exhaustive and only contains things that we think may > affect multiple benchmarks or real-world code. > > I've also noticed: > * Our inline memcpy expansion pass is emitting "LDR q0, [..]; STR q0, > [..]" pairs, which is less than ideal on A53. If we switched to emitting > "LDP x0, x1, [..]; STP x0, x1, [..]", we'd get around 30% better inline > memcpy performance on A53. A57 seems to deal well with the LDR q sequence. > > I'm sorry I'm unable to provide code samples for most of the issues found > so > far - this is an artefact of them having come from SPEC. Trivial examples > do > not always show the same behaviour, and as we're still investigating we > haven't yet been able to reduce most of these to an anonymisable testcase. > > Hope this helps, but doubt it does, > > James > > > -----Original Message----- > > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] > On > > Behalf Of Manjunath N > > Sent: 24 June 2014 10:45 > > To: llvmdev at cs.uiuc.edu > > Subject: Re: [LLVMdev] Contributing the Apple ARM64 compiler backend > > > > > > > > Eric Christopher <echristo <at> gmail.com> writes: > > > > > > > > > The big pain issues I see merging from ARM64 to AArch64 are: > > > > 1. Apple have created a fairly complete scheduling model already > > for > > > > ARM64, and we'd have to merge the partial? model in AArch64 and > > theirs. > > We > > > > risk regressing performance on Apple's targets here, and we can't > > determine > > > > ourselves whether we have or not. This is not ideal. > > > > 2. Porting over the DAG-to-DAG optimizations and any other > > > > optimizations that rely on the tablegen layout will be very tricky. > > > > 3. The conditional compare pass is fairly comprehensive - we'd > > have > > to > > > > port that over or rewrite it and that would be a lot of work. > > > > 4. A very quick analysis last night indicated that ARM64 has > > > > implemented just under half of the optimizations we discovered > > opportunities > > > > for in SPEC and EEMBC. That's a fairly comprehensive number of > > > > optimizations, and they won't all be easy to port. > > Eric, > > You mention that there a quite a few optimization opportunities in SPEC > > 2000/ EEMBC. > > I am looking to optimize the Aarch64 backend. Could you please let me > know > > the big optimizations possible? > > > > > > > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > > >-- ========================================warm regards, Manjunath DN -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140626/c6548a65/attachment.html>
Sanjay Patel
2014-Jun-26 17:10 UTC
[LLVMdev] Contributing the Apple ARM64 compiler backend
>> We've also seen similar instances where multiple registers are used tocompute very similar>> addresses (such as x+0 and x+4!) and this increases register pressure.I don't have an ARM enabled build of the tools to test with, but I suspect what I'm seeing here: http://llvm.org/bugs/show_bug.cgi?id=20134 ...would also be bad on AArch64. On Wed, Jun 25, 2014 at 8:58 PM, Manjunath DN <manjunath.dn at gmail.com> wrote:> HI James, > Thanks for your reply and hints on what can be done for the Aarch64 > backend optimization for llvm > We have SPEC license and v8 hardware. So I will start looking into it > warm regards > Manjunath > > > > On Wed, Jun 25, 2014 at 8:42 PM, James Molloy <james.molloy at arm.com> > wrote: > >> Hi Manjunath, >> >> At the time of writing that status we had only done our initial analysis. >> This was done without real hardware and attempted to identify poor code >> sequences but we were unable to quantify how much effect this would >> actually >> have. >> >> Since then we've done more analysis using Cortex-A57 and Cortex-A53 on an >> internal development platform. >> >> For SPEC, we are between 10% and 0% behind GCC on 9 benchmarks, and 25% >> ahead on one benchmark. Most benchmarks are less than 5% behind GCC. >> >> Because of the licencing of SPEC, I have to be quite restricted in what I >> say and I can't give any numbers - sorry about that. >> >> We are focussing on Cortex-A57, and the things we've identified so far >> are: >> * The CSEL instruction behaves worse than the equivalent branch >> structure >> in at least one benchmark. In an out of order core, select-like >> instructions >> are going to be slower than their branched equivalent if the branch is >> predictable due to CSEL having two dependencies. >> >> * Redundant calculations inside if conditions. We've seen: >> 1. "if (a[x].b < c[y].d || a[x].e > c[y].f)" - the calculations of >> a[x] >> and c[y] are repeated, when they are common. We've also seen similar >> instances where multiple registers are used to compute very similar >> addresses (such as x+0 and x+4!) and this increases register pressure. >> 2. "if (a < 0 && b == c || a > 0 && b == d)" - the first comparison of >> 'a' against zero is done twice, when the flag results of the first >> comparison could be used for the second comparison. >> >> * For a loop such as "for (i = 0; i < n; ++i) >> {do_something_with(&x[i]);}", GCC is using &x[i] as the loop induction >> variable where LLVM uses i and performs the calculation &x[i] on every >> iteration. This only creates one more add instruction but the loop we see >> it >> in only has 5 or so instructions. >> >> * The inline heuristics are way behind GCC's. If we crank the inline >> threshold up to 1000, we can remove a 6.5% performance regression from one >> benchmark entirely. >> >> * We're generating (due to SLP vectorizer and a DAG combine) loads into >> Q >> registers when merging consecutive loads. This is bad, because there are >> no >> callee-saved Q registers! So if the live range crosses a function call, it >> will have to be immediately spilled again. This can be easily fixed by >> using load-pair instructions instead. I have a patch to fix this. >> >> The list above is non-exhaustive and only contains things that we think >> may >> affect multiple benchmarks or real-world code. >> >> I've also noticed: >> * Our inline memcpy expansion pass is emitting "LDR q0, [..]; STR q0, >> [..]" pairs, which is less than ideal on A53. If we switched to emitting >> "LDP x0, x1, [..]; STP x0, x1, [..]", we'd get around 30% better inline >> memcpy performance on A53. A57 seems to deal well with the LDR q sequence. >> >> I'm sorry I'm unable to provide code samples for most of the issues found >> so >> far - this is an artefact of them having come from SPEC. Trivial examples >> do >> not always show the same behaviour, and as we're still investigating we >> haven't yet been able to reduce most of these to an anonymisable testcase. >> >> Hope this helps, but doubt it does, >> >> James >> >> > -----Original Message----- >> > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] >> On >> > Behalf Of Manjunath N >> > Sent: 24 June 2014 10:45 >> > To: llvmdev at cs.uiuc.edu >> > Subject: Re: [LLVMdev] Contributing the Apple ARM64 compiler backend >> > >> > >> > >> > Eric Christopher <echristo <at> gmail.com> writes: >> > >> > > >> > > > The big pain issues I see merging from ARM64 to AArch64 are: >> > > > 1. Apple have created a fairly complete scheduling model >> already >> > for >> > > > ARM64, and we'd have to merge the partial? model in AArch64 and >> > theirs. >> > We >> > > > risk regressing performance on Apple's targets here, and we can't >> > determine >> > > > ourselves whether we have or not. This is not ideal. >> > > > 2. Porting over the DAG-to-DAG optimizations and any other >> > > > optimizations that rely on the tablegen layout will be very tricky. >> > > > 3. The conditional compare pass is fairly comprehensive - we'd >> > have >> > to >> > > > port that over or rewrite it and that would be a lot of work. >> > > > 4. A very quick analysis last night indicated that ARM64 has >> > > > implemented just under half of the optimizations we discovered >> > opportunities >> > > > for in SPEC and EEMBC. That's a fairly comprehensive number of >> > > > optimizations, and they won't all be easy to port. >> > Eric, >> > You mention that there a quite a few optimization opportunities in SPEC >> > 2000/ EEMBC. >> > I am looking to optimize the Aarch64 backend. Could you please let me >> know >> > the big optimizations possible? >> > >> > >> > >> > _______________________________________________ >> > LLVM Developers mailing list >> > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> >> >> >> >> > > > -- > ========================================> warm regards, > Manjunath DN > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-- Sanjay Patel RotateRight, LLC http://www.rotateright.com -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140626/481a77b2/attachment.html>
James Molloy
2014-Jun-26 17:23 UTC
[LLVMdev] Contributing the Apple ARM64 compiler backend
Hi Sanjay,
The behaviour I’m talking about I’ve actually pinned down to CodeGenPrepare not
working too well with ISA’s that don’t have a good scaled load. I have a patch
to fix it that is going through performance testing now.
Your testcase seems specific to x86 – for aarch64 we get the rather spiffy:
_Z3fooPii: // @_Z3fooPii
// BB#0: // %entry
add w8, w1, #1 // =1
add w9, w1, #2 // =2
ldr w8, [x0, w8, sxtw #2]
ldr w9, [x0, w9, sxtw #2]
add w8, w9, w8
str w8, [x0, w1, sxtw #2]
ret
The sext can be matched as part of the addressing mode for AArch64 – maybe it’s
something in codegenprepare for x86 going awry?
Cheers,
James
From: Sanjay Patel [mailto:spatel at rotateright.com]
Sent: 26 June 2014 18:11
To: Manjunath DN
Cc: James Molloy; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Contributing the Apple ARM64 compiler backend
>> We've also seen similar instances where multiple registers are used
to compute very similar
>> addresses (such as x+0 and x+4!) and this increases register pressure.
I don't have an ARM enabled build of the tools to test with, but I suspect
what I'm seeing here:
http://llvm.org/bugs/show_bug.cgi?id=20134
...would also be bad on AArch64.
On Wed, Jun 25, 2014 at 8:58 PM, Manjunath DN <manjunath.dn at gmail.com>
wrote:
HI James,
Thanks for your reply and hints on what can be done for the Aarch64 backend
optimization for llvm
We have SPEC license and v8 hardware. So I will start looking into it
warm regards
Manjunath
On Wed, Jun 25, 2014 at 8:42 PM, James Molloy <james.molloy at arm.com>
wrote:
Hi Manjunath,
At the time of writing that status we had only done our initial analysis.
This was done without real hardware and attempted to identify poor code
sequences but we were unable to quantify how much effect this would actually
have.
Since then we've done more analysis using Cortex-A57 and Cortex-A53 on an
internal development platform.
For SPEC, we are between 10% and 0% behind GCC on 9 benchmarks, and 25%
ahead on one benchmark. Most benchmarks are less than 5% behind GCC.
Because of the licencing of SPEC, I have to be quite restricted in what I
say and I can't give any numbers - sorry about that.
We are focussing on Cortex-A57, and the things we've identified so far are:
* The CSEL instruction behaves worse than the equivalent branch structure
in at least one benchmark. In an out of order core, select-like instructions
are going to be slower than their branched equivalent if the branch is
predictable due to CSEL having two dependencies.
* Redundant calculations inside if conditions. We've seen:
1. "if (a[x].b < c[y].d || a[x].e > c[y].f)" - the
calculations of a[x]
and c[y] are repeated, when they are common. We've also seen similar
instances where multiple registers are used to compute very similar
addresses (such as x+0 and x+4!) and this increases register pressure.
2. "if (a < 0 && b == c || a > 0 && b == d)"
- the first comparison of
'a' against zero is done twice, when the flag results of the first
comparison could be used for the second comparison.
* For a loop such as "for (i = 0; i < n; ++i)
{do_something_with(&x[i]);}", GCC is using &x[i] as the loop
induction
variable where LLVM uses i and performs the calculation &x[i] on every
iteration. This only creates one more add instruction but the loop we see it
in only has 5 or so instructions.
* The inline heuristics are way behind GCC's. If we crank the inline
threshold up to 1000, we can remove a 6.5% performance regression from one
benchmark entirely.
* We're generating (due to SLP vectorizer and a DAG combine) loads into Q
registers when merging consecutive loads. This is bad, because there are no
callee-saved Q registers! So if the live range crosses a function call, it
will have to be immediately spilled again. This can be easily fixed by
using load-pair instructions instead. I have a patch to fix this.
The list above is non-exhaustive and only contains things that we think may
affect multiple benchmarks or real-world code.
I've also noticed:
* Our inline memcpy expansion pass is emitting "LDR q0, [..]; STR q0,
[..]" pairs, which is less than ideal on A53. If we switched to emitting
"LDP x0, x1, [..]; STP x0, x1, [..]", we'd get around 30% better
inline
memcpy performance on A53. A57 seems to deal well with the LDR q sequence.
I'm sorry I'm unable to provide code samples for most of the issues
found so
far - this is an artefact of them having come from SPEC. Trivial examples do
not always show the same behaviour, and as we're still investigating we
haven't yet been able to reduce most of these to an anonymisable testcase.
Hope this helps, but doubt it does,
James
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu] On
> Behalf Of Manjunath N
> Sent: 24 June 2014 10:45
> To: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Contributing the Apple ARM64 compiler backend
>
>
>
> Eric Christopher <echristo <at> gmail.com> writes:
>
> >
> > > The big pain issues I see merging from ARM64 to AArch64 are:
> > > 1. Apple have created a fairly complete scheduling model
already
> for
> > > ARM64, and we'd have to merge the partial? model in AArch64
and
> theirs.
> We
> > > risk regressing performance on Apple's targets here, and we
can't
> determine
> > > ourselves whether we have or not. This is not ideal.
> > > 2. Porting over the DAG-to-DAG optimizations and any other
> > > optimizations that rely on the tablegen layout will be very
tricky.
> > > 3. The conditional compare pass is fairly comprehensive -
we'd
> have
> to
> > > port that over or rewrite it and that would be a lot of work.
> > > 4. A very quick analysis last night indicated that ARM64 has
> > > implemented just under half of the optimizations we discovered
> opportunities
> > > for in SPEC and EEMBC. That's a fairly comprehensive number
of
> > > optimizations, and they won't all be easy to port.
> Eric,
> You mention that there a quite a few optimization opportunities in SPEC
> 2000/ EEMBC.
> I am looking to optimize the Aarch64 backend. Could you please let me know
> the big optimizations possible?
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
--
========================================warm regards,
Manjunath DN
_______________________________________________
LLVM Developers mailing list
LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
--
Sanjay Patel
RotateRight, LLC
http://www.rotateright.com
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