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Jingyue Wu

2014-Apr-19 05:02 UTC

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

Hi,

We wrote an optimization that eliminates common sub-expressions in a group
of similar GEPs for the NVPTX backend. It speeds up some of our benchmarks
by up to 20%, which convinces us to try to upstream it. Here's a brief
description of why we wrote this optimization, what we did, and how we did
it.

Loops in CUDA programs are often extensively unrolled by programmers and
compilers, leading to many similar
GEPs for array accesses.

e.g., a 2-level loop like

__shared__ float a[32][32];
unroll for (int i = 0; i < 2; ++i) {
  unroll for (int j = 0; j < 2; ++j) {
    ...
    ... = a[threadIdx.x + i][threadIdx.y + j];
    ...
  }
}

will be unrolled to:

gep a, 0, tid.x, tid.y; load
gep a, 0, tid.x, tid.y + 1; load
gep a, 0, tid.x + 1, tid.y; load
gep a, 0, tid.x + 1, tid.y + 1; load

The NVPTX backend currently doesn't handle many similar multi-dimensional
GEPs
well enough. It emits PTX code that literally computes the pointer address
of
each GEP, wasting tons of registers. e.g., it emits the following PTX for
the
first load and similar PTX for other loads.

mov.u32         %r1, %tid.x;
mov.u32         %r2, %tid.y;
mul.wide.u32    %rl2, %r1, 128;
mov.u64         %rl3, a;
add.s64         %rl4, %rl3, %rl2;
mul.wide.u32    %rl5, %r2, 4;
add.s64         %rl6, %rl4, %rl5;
ld.shared.f32   %f1, [%rl6];

The resultant register pressure causes up to 20% slowdown on some of our
benchmarks.

To reduce register pressure, the optimization implemented in this patch
merges
the common subexpression in a group of GEPs, saving many registers used for
pointer arithmetics. It works by splitting each GEP into a variadic base
and a
constant offset. The variadic base can be computed once and reused by
multiple
GEPs, and the constant offsets can be nicely folded into NVPTX's base+offset
addressing mode without using any extra register. e.g., we transform the
four
GEPs and four loads in the above example conceptually into:

base = gep a, 0, x, y
load base
laod base + 1  * sizeof(float)
load base + 32 * sizeof(float)
load base + 33 * sizeof(float)

The resultant PTX code will look like:

mov.u32         %r1, %tid.x;
mov.u32         %r2, %tid.y;
mul.wide.u32    %rl2, %r1, 128;
mov.u64         %rl3, a;
add.s64         %rl4, %rl3, %rl2;
mul.wide.u32    %rl5, %r2, 4;
add.s64         %rl6, %rl4, %rl5;
ld.shared.f32   %f1, [%rl6]; // so far the same as unoptimized PTX
*ld.shared.f32   %f2, [%rl6+4]; // much better*
*ld.shared.f32   %f3, [%rl6+128]; // much better*
*ld.shared.f32   %f4, [%rl6+132]; // much better*

which uses much fewer registers than the unoptimized PTX.

I am attaching a proof-of-concept patch. It fully implements our idea and
contains a contrived test case to demonstrate how it works. It also
discusses why our implementation is safe in terms that the optimization
won't cause new undefined behavior. There's more work that needs to be
done, e.g., adding more tests. If this idea sounds good to you, we will
improve the patch and send it out for code review.

Thanks,
Jingyue
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Hal Finkel

2014-Apr-19 13:38 UTC

head link

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

Jingyue,

I can't speak for the NVPTX backend, but I think this looks useful as an
(optional) target-independent pass. A few thoughts:

 - Running GVN tends to be pretty expensive; have you tried EarlyCSE instead?
(When I was working on the BB vectorizer I was first using GVN for cleanup
afterward, someone suggested trying EarlyCSE instead, the performance slowdown
was a bit less than 1% on average, but the runtime impact was much less).

 - Are you doing this on the IR level, instead of in DAGCombine, because you
want the cross-block combining from GVN? Or some other reason (or both)?

 - To make this target independent, I think you just need to insert some calls
to TLI.isLegalAddressingMode (or equivalently, TTI.isLegalAddressingMode) just
to make sure that the offsets you're creating are legal on the target. This
will essentially be a noop for NVPTX, but will matter for other targets.

Thanks for posting this,
Hal

----- Original Message -----> From: "Jingyue Wu" <jingyue at google.com>
> To: llvmdev at cs.uiuc.edu, "Eli Bendersky" <eliben at
google.com>, "Justin Holewinski" <jholewinski at nvidia.com>,
"Justin
> Holewinski" <justin.holewinski at gmail.com>
> Sent: Saturday, April 19, 2014 12:02:28 AM
> Subject: [LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of
similar GEPs
> 
> 
> 
> 
> Hi,
> 
> 
> We wrote an optimization that eliminates common sub-expressions in a
> group of similar GEPs for the NVPTX backend. It speeds up some of
> our benchmarks by up to 20%, which convinces us to try to upstream
> it. Here's a brief description of why we wrote this optimization,
> what we did, and how we did it.
> 
> 
> Loops in CUDA programs are often extensively unrolled by programmers
> and compilers, leading to many similar
> GEPs for array accesses.
> 
> 
> e.g., a 2-level loop like
> 
> 
> __shared__ float a[32][32];
> unroll for (int i = 0; i < 2; ++i) {
> unroll for (int j = 0; j < 2; ++j) {
> ...
> ... = a[threadIdx.x + i][threadIdx.y + j];
> ...
> }
> }
> 
> 
> will be unrolled to:
> 
> 
> gep a, 0, tid.x, tid.y; load
> gep a, 0, tid.x, tid.y + 1; load
> gep a, 0, tid.x + 1, tid.y; load
> gep a, 0, tid.x + 1, tid.y + 1; load
> 
> 
> The NVPTX backend currently doesn't handle many similar
> multi-dimensional GEPs
> well enough. It emits PTX code that literally computes the pointer
> address of
> each GEP, wasting tons of registers. e.g., it emits the following PTX
> for the
> first load and similar PTX for other loads.
> 
> 
> mov.u32 %r1, %tid.x;
> mov.u32 %r2, %tid.y;
> mul.wide.u32 %rl2, %r1, 128;
> mov.u64 %rl3, a;
> add.s64 %rl4, %rl3, %rl2;
> mul.wide.u32 %rl5, %r2, 4;
> add.s64 %rl6, %rl4, %rl5;
> ld.shared.f32 %f1, [%rl6];
> 
> 
> The resultant register pressure causes up to 20% slowdown on some of
> our
> benchmarks.
> 
> 
> To reduce register pressure, the optimization implemented in this
> patch merges
> the common subexpression in a group of GEPs, saving many registers
> used for
> pointer arithmetics. It works by splitting each GEP into a variadic
> base and a
> constant offset. The variadic base can be computed once and reused by
> multiple
> GEPs, and the constant offsets can be nicely folded into NVPTX's
> base+offset
> addressing mode without using any extra register. e.g., we transform
> the four
> GEPs and four loads in the above example conceptually into:
> 
> 
> base = gep a, 0, x, y
> load base
> laod base + 1 * sizeof(float)
> load base + 32 * sizeof(float)
> load base + 33 * sizeof(float)
> 
> 
> The resultant PTX code will look like:
> 
> 
> mov.u32 %r1, %tid.x;
> mov.u32 %r2, %tid.y;
> mul.wide.u32 %rl2, %r1, 128;
> mov.u64 %rl3, a;
> add.s64 %rl4, %rl3, %rl2;
> mul.wide.u32 %rl5, %r2, 4;
> add.s64 %rl6, %rl4, %rl5;
> ld.shared.f32 %f1, [%rl6]; // so far the same as unoptimized PTX
> ld.shared.f32 %f2, [%rl6+4]; // much better
> ld.shared.f32 %f3, [%rl6+128]; // much better
> ld.shared.f32 %f4, [%rl6+132]; // much better
> 
> 
> which uses much fewer registers than the unoptimized PTX.
> 
> 
> I am attaching a proof-of-concept patch. It fully implements our idea
> and contains a contrived test case to demonstrate how it works. It
> also discusses why our implementation is safe in terms that the
> optimization won't cause new undefined behavior. There's more work
> that needs to be done, e.g., adding more tests. If this idea sounds
> good to you, we will improve the patch and send it out for code
> review.
> 
> 
> 
> Thanks,
> Jingyue
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

Justin Holewinski

2014-Apr-19 14:34 UTC

head link

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

This looks great!  I'm a bit surprised the existing IR optimizers do not
handle this.  I agree with Hal that this should be made target-independent.
 I don't see anything here that would be specific to NVPTX.  Do you have
any performance data for open-source benchmarks?


On Sat, Apr 19, 2014 at 9:38 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> Jingyue,
>
> I can't speak for the NVPTX backend, but I think this looks useful as
an
> (optional) target-independent pass. A few thoughts:
>
>  - Running GVN tends to be pretty expensive; have you tried EarlyCSE
> instead? (When I was working on the BB vectorizer I was first using GVN for
> cleanup afterward, someone suggested trying EarlyCSE instead, the
> performance slowdown was a bit less than 1% on average, but the runtime
> impact was much less).
>
>  - Are you doing this on the IR level, instead of in DAGCombine, because
> you want the cross-block combining from GVN? Or some other reason (or
both)?
>
>  - To make this target independent, I think you just need to insert some
> calls to TLI.isLegalAddressingMode (or equivalently,
> TTI.isLegalAddressingMode) just to make sure that the offsets you're
> creating are legal on the target. This will essentially be a noop for
> NVPTX, but will matter for other targets.
>
> Thanks for posting this,
> Hal
>
> ----- Original Message -----
> > From: "Jingyue Wu" <jingyue at google.com>
> > To: llvmdev at cs.uiuc.edu, "Eli Bendersky" <eliben at
google.com>, "Justin
> Holewinski" <jholewinski at nvidia.com>, "Justin
> > Holewinski" <justin.holewinski at gmail.com>
> > Sent: Saturday, April 19, 2014 12:02:28 AM
> > Subject: [LLVMdev] [NVPTX] Eliminate common sub-expressions in a group
> of     similar GEPs
> >
> >
> >
> >
> > Hi,
> >
> >
> > We wrote an optimization that eliminates common sub-expressions in a
> > group of similar GEPs for the NVPTX backend. It speeds up some of
> > our benchmarks by up to 20%, which convinces us to try to upstream
> > it. Here's a brief description of why we wrote this optimization,
> > what we did, and how we did it.
> >
> >
> > Loops in CUDA programs are often extensively unrolled by programmers
> > and compilers, leading to many similar
> > GEPs for array accesses.
> >
> >
> > e.g., a 2-level loop like
> >
> >
> > __shared__ float a[32][32];
> > unroll for (int i = 0; i < 2; ++i) {
> > unroll for (int j = 0; j < 2; ++j) {
> > ...
> > ... = a[threadIdx.x + i][threadIdx.y + j];
> > ...
> > }
> > }
> >
> >
> > will be unrolled to:
> >
> >
> > gep a, 0, tid.x, tid.y; load
> > gep a, 0, tid.x, tid.y + 1; load
> > gep a, 0, tid.x + 1, tid.y; load
> > gep a, 0, tid.x + 1, tid.y + 1; load
> >
> >
> > The NVPTX backend currently doesn't handle many similar
> > multi-dimensional GEPs
> > well enough. It emits PTX code that literally computes the pointer
> > address of
> > each GEP, wasting tons of registers. e.g., it emits the following PTX
> > for the
> > first load and similar PTX for other loads.
> >
> >
> > mov.u32 %r1, %tid.x;
> > mov.u32 %r2, %tid.y;
> > mul.wide.u32 %rl2, %r1, 128;
> > mov.u64 %rl3, a;
> > add.s64 %rl4, %rl3, %rl2;
> > mul.wide.u32 %rl5, %r2, 4;
> > add.s64 %rl6, %rl4, %rl5;
> > ld.shared.f32 %f1, [%rl6];
> >
> >
> > The resultant register pressure causes up to 20% slowdown on some of
> > our
> > benchmarks.
> >
> >
> > To reduce register pressure, the optimization implemented in this
> > patch merges
> > the common subexpression in a group of GEPs, saving many registers
> > used for
> > pointer arithmetics. It works by splitting each GEP into a variadic
> > base and a
> > constant offset. The variadic base can be computed once and reused by
> > multiple
> > GEPs, and the constant offsets can be nicely folded into NVPTX's
> > base+offset
> > addressing mode without using any extra register. e.g., we transform
> > the four
> > GEPs and four loads in the above example conceptually into:
> >
> >
> > base = gep a, 0, x, y
> > load base
> > laod base + 1 * sizeof(float)
> > load base + 32 * sizeof(float)
> > load base + 33 * sizeof(float)
> >
> >
> > The resultant PTX code will look like:
> >
> >
> > mov.u32 %r1, %tid.x;
> > mov.u32 %r2, %tid.y;
> > mul.wide.u32 %rl2, %r1, 128;
> > mov.u64 %rl3, a;
> > add.s64 %rl4, %rl3, %rl2;
> > mul.wide.u32 %rl5, %r2, 4;
> > add.s64 %rl6, %rl4, %rl5;
> > ld.shared.f32 %f1, [%rl6]; // so far the same as unoptimized PTX
> > ld.shared.f32 %f2, [%rl6+4]; // much better
> > ld.shared.f32 %f3, [%rl6+128]; // much better
> > ld.shared.f32 %f4, [%rl6+132]; // much better
> >
> >
> > which uses much fewer registers than the unoptimized PTX.
> >
> >
> > I am attaching a proof-of-concept patch. It fully implements our idea
> > and contains a contrived test case to demonstrate how it works. It
> > also discusses why our implementation is safe in terms that the
> > optimization won't cause new undefined behavior. There's more
work
> > that needs to be done, e.g., adding more tests. If this idea sounds
> > good to you, we will improve the patch and send it out for code
> > review.
> >
> >
> >
> > Thanks,
> > Jingyue
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>


-- 

Thanks,

Justin Holewinski
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Jingyue Wu

2014-Apr-21 02:34 UTC

head link

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

Hi Hal,

Thanks for your comments! I'm inlining my responses below.

Jingyue


On Sat, Apr 19, 2014 at 6:38 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> Jingyue,
>
> I can't speak for the NVPTX backend, but I think this looks useful as
an
> (optional) target-independent pass. A few thoughts:
>
>  - Running GVN tends to be pretty expensive; have you tried EarlyCSE
> instead? (When I was working on the BB vectorizer I was first using GVN for
> cleanup afterward, someone suggested trying EarlyCSE instead, the
> performance slowdown was a bit less than 1% on average, but the runtime
> impact was much less).
>
EarlyCSE surprisingly generates code that runs 10% slower than GVN on one
of our benchmarks. However, after looking into the ll and ptx, we probably
shouldn't blame EarlyCSE for the slowdown. The ptx generated using EarlyCSE
is only slightly different from that generated by GVN, but ptxas amplifies
the seemingly unharmful difference to much more register usage. We need to
further investigate this issue.

I can change the code to use EarlyCSE by default, and leave a flag to run
GVN.

>
>  - Are you doing this on the IR level, instead of in DAGCombine, because
> you want the cross-block combining from GVN? Or some other reason (or
both)?
>
Cross-block combining from GVN is one of the reasons. The second reason is
we observed a GEP and its index are sometimes defined in different BBs. The
third reason is we also observed a GEP and the load/store that uses it are
sometimes in different BBs. Although CodeGenPrepare attempts to sink a GEP
to the same BB as its load/store user, it doesn't sink GEPs with two
variadic indices (e.g., gep %a, 0, %i, %j) because it "smartly"
recognizes
ptx doesn't support any addressing mode that can fold this GEP (This
addressing mode issue is worth another thread, and one of my colleagues is
working on that).

>
>  - To make this target independent, I think you just need to insert some
> calls to TLI.isLegalAddressingMode (or equivalently,
> TTI.isLegalAddressingMode) just to make sure that the offsets you're
> creating are legal on the target. This will essentially be a noop for
> NVPTX, but will matter for other targets.
>
Ack'ed

>
> Thanks for posting this,
> Hal
>
> ----- Original Message -----
> > From: "Jingyue Wu" <jingyue at google.com>
> > To: llvmdev at cs.uiuc.edu, "Eli Bendersky" <eliben at
google.com>, "Justin
> Holewinski" <jholewinski at nvidia.com>, "Justin
> > Holewinski" <justin.holewinski at gmail.com>
> > Sent: Saturday, April 19, 2014 12:02:28 AM
> > Subject: [LLVMdev] [NVPTX] Eliminate common sub-expressions in a group
> of     similar GEPs
> >
> >
> >
> >
> > Hi,
> >
> >
> > We wrote an optimization that eliminates common sub-expressions in a
> > group of similar GEPs for the NVPTX backend. It speeds up some of
> > our benchmarks by up to 20%, which convinces us to try to upstream
> > it. Here's a brief description of why we wrote this optimization,
> > what we did, and how we did it.
> >
> >
> > Loops in CUDA programs are often extensively unrolled by programmers
> > and compilers, leading to many similar
> > GEPs for array accesses.
> >
> >
> > e.g., a 2-level loop like
> >
> >
> > __shared__ float a[32][32];
> > unroll for (int i = 0; i < 2; ++i) {
> > unroll for (int j = 0; j < 2; ++j) {
> > ...
> > ... = a[threadIdx.x + i][threadIdx.y + j];
> > ...
> > }
> > }
> >
> >
> > will be unrolled to:
> >
> >
> > gep a, 0, tid.x, tid.y; load
> > gep a, 0, tid.x, tid.y + 1; load
> > gep a, 0, tid.x + 1, tid.y; load
> > gep a, 0, tid.x + 1, tid.y + 1; load
> >
> >
> > The NVPTX backend currently doesn't handle many similar
> > multi-dimensional GEPs
> > well enough. It emits PTX code that literally computes the pointer
> > address of
> > each GEP, wasting tons of registers. e.g., it emits the following PTX
> > for the
> > first load and similar PTX for other loads.
> >
> >
> > mov.u32 %r1, %tid.x;
> > mov.u32 %r2, %tid.y;
> > mul.wide.u32 %rl2, %r1, 128;
> > mov.u64 %rl3, a;
> > add.s64 %rl4, %rl3, %rl2;
> > mul.wide.u32 %rl5, %r2, 4;
> > add.s64 %rl6, %rl4, %rl5;
> > ld.shared.f32 %f1, [%rl6];
> >
> >
> > The resultant register pressure causes up to 20% slowdown on some of
> > our
> > benchmarks.
> >
> >
> > To reduce register pressure, the optimization implemented in this
> > patch merges
> > the common subexpression in a group of GEPs, saving many registers
> > used for
> > pointer arithmetics. It works by splitting each GEP into a variadic
> > base and a
> > constant offset. The variadic base can be computed once and reused by
> > multiple
> > GEPs, and the constant offsets can be nicely folded into NVPTX's
> > base+offset
> > addressing mode without using any extra register. e.g., we transform
> > the four
> > GEPs and four loads in the above example conceptually into:
> >
> >
> > base = gep a, 0, x, y
> > load base
> > laod base + 1 * sizeof(float)
> > load base + 32 * sizeof(float)
> > load base + 33 * sizeof(float)
> >
> >
> > The resultant PTX code will look like:
> >
> >
> > mov.u32 %r1, %tid.x;
> > mov.u32 %r2, %tid.y;
> > mul.wide.u32 %rl2, %r1, 128;
> > mov.u64 %rl3, a;
> > add.s64 %rl4, %rl3, %rl2;
> > mul.wide.u32 %rl5, %r2, 4;
> > add.s64 %rl6, %rl4, %rl5;
> > ld.shared.f32 %f1, [%rl6]; // so far the same as unoptimized PTX
> > ld.shared.f32 %f2, [%rl6+4]; // much better
> > ld.shared.f32 %f3, [%rl6+128]; // much better
> > ld.shared.f32 %f4, [%rl6+132]; // much better
> >
> >
> > which uses much fewer registers than the unoptimized PTX.
> >
> >
> > I am attaching a proof-of-concept patch. It fully implements our idea
> > and contains a contrived test case to demonstrate how it works. It
> > also discusses why our implementation is safe in terms that the
> > optimization won't cause new undefined behavior. There's more
work
> > that needs to be done, e.g., adding more tests. If this idea sounds
> > good to you, we will improve the patch and send it out for code
> > review.
> >
> >
> >
> > Thanks,
> > Jingyue
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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Jingyue Wu

2014-Apr-21 20:15 UTC

head link

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

Hal,

Would you put this pass in lib/CodeGen or lib/Transforms? lib/CodeGen
sounds too low-level; lib/Transforms doesn't seem to encourage using
TargetLowering or TargetTransformInfo.

Jingyue


On Sat, Apr 19, 2014 at 6:38 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> Jingyue,
>
> I can't speak for the NVPTX backend, but I think this looks useful as
an
> (optional) target-independent pass. A few thoughts:
>
>  - Running GVN tends to be pretty expensive; have you tried EarlyCSE
> instead? (When I was working on the BB vectorizer I was first using GVN for
> cleanup afterward, someone suggested trying EarlyCSE instead, the
> performance slowdown was a bit less than 1% on average, but the runtime
> impact was much less).
>
>  - Are you doing this on the IR level, instead of in DAGCombine, because
> you want the cross-block combining from GVN? Or some other reason (or
both)?
>
>  - To make this target independent, I think you just need to insert some
> calls to TLI.isLegalAddressingMode (or equivalently,
> TTI.isLegalAddressingMode) just to make sure that the offsets you're
> creating are legal on the target. This will essentially be a noop for
> NVPTX, but will matter for other targets.
>
> Thanks for posting this,
> Hal
>
> ----- Original Message -----
> > From: "Jingyue Wu" <jingyue at google.com>
> > To: llvmdev at cs.uiuc.edu, "Eli Bendersky" <eliben at
google.com>, "Justin
> Holewinski" <jholewinski at nvidia.com>, "Justin
> > Holewinski" <justin.holewinski at gmail.com>
> > Sent: Saturday, April 19, 2014 12:02:28 AM
> > Subject: [LLVMdev] [NVPTX] Eliminate common sub-expressions in a group
> of     similar GEPs
> >
> >
> >
> >
> > Hi,
> >
> >
> > We wrote an optimization that eliminates common sub-expressions in a
> > group of similar GEPs for the NVPTX backend. It speeds up some of
> > our benchmarks by up to 20%, which convinces us to try to upstream
> > it. Here's a brief description of why we wrote this optimization,
> > what we did, and how we did it.
> >
> >
> > Loops in CUDA programs are often extensively unrolled by programmers
> > and compilers, leading to many similar
> > GEPs for array accesses.
> >
> >
> > e.g., a 2-level loop like
> >
> >
> > __shared__ float a[32][32];
> > unroll for (int i = 0; i < 2; ++i) {
> > unroll for (int j = 0; j < 2; ++j) {
> > ...
> > ... = a[threadIdx.x + i][threadIdx.y + j];
> > ...
> > }
> > }
> >
> >
> > will be unrolled to:
> >
> >
> > gep a, 0, tid.x, tid.y; load
> > gep a, 0, tid.x, tid.y + 1; load
> > gep a, 0, tid.x + 1, tid.y; load
> > gep a, 0, tid.x + 1, tid.y + 1; load
> >
> >
> > The NVPTX backend currently doesn't handle many similar
> > multi-dimensional GEPs
> > well enough. It emits PTX code that literally computes the pointer
> > address of
> > each GEP, wasting tons of registers. e.g., it emits the following PTX
> > for the
> > first load and similar PTX for other loads.
> >
> >
> > mov.u32 %r1, %tid.x;
> > mov.u32 %r2, %tid.y;
> > mul.wide.u32 %rl2, %r1, 128;
> > mov.u64 %rl3, a;
> > add.s64 %rl4, %rl3, %rl2;
> > mul.wide.u32 %rl5, %r2, 4;
> > add.s64 %rl6, %rl4, %rl5;
> > ld.shared.f32 %f1, [%rl6];
> >
> >
> > The resultant register pressure causes up to 20% slowdown on some of
> > our
> > benchmarks.
> >
> >
> > To reduce register pressure, the optimization implemented in this
> > patch merges
> > the common subexpression in a group of GEPs, saving many registers
> > used for
> > pointer arithmetics. It works by splitting each GEP into a variadic
> > base and a
> > constant offset. The variadic base can be computed once and reused by
> > multiple
> > GEPs, and the constant offsets can be nicely folded into NVPTX's
> > base+offset
> > addressing mode without using any extra register. e.g., we transform
> > the four
> > GEPs and four loads in the above example conceptually into:
> >
> >
> > base = gep a, 0, x, y
> > load base
> > laod base + 1 * sizeof(float)
> > load base + 32 * sizeof(float)
> > load base + 33 * sizeof(float)
> >
> >
> > The resultant PTX code will look like:
> >
> >
> > mov.u32 %r1, %tid.x;
> > mov.u32 %r2, %tid.y;
> > mul.wide.u32 %rl2, %r1, 128;
> > mov.u64 %rl3, a;
> > add.s64 %rl4, %rl3, %rl2;
> > mul.wide.u32 %rl5, %r2, 4;
> > add.s64 %rl6, %rl4, %rl5;
> > ld.shared.f32 %f1, [%rl6]; // so far the same as unoptimized PTX
> > ld.shared.f32 %f2, [%rl6+4]; // much better
> > ld.shared.f32 %f3, [%rl6+128]; // much better
> > ld.shared.f32 %f4, [%rl6+132]; // much better
> >
> >
> > which uses much fewer registers than the unoptimized PTX.
> >
> >
> > I am attaching a proof-of-concept patch. It fully implements our idea
> > and contains a contrived test case to demonstrate how it works. It
> > also discusses why our implementation is safe in terms that the
> > optimization won't cause new undefined behavior. There's more
work
> > that needs to be done, e.g., adding more tests. If this idea sounds
> > good to you, we will improve the patch and send it out for code
> > review.
> >
> >
> >
> > Thanks,
> > Jingyue
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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llvm dev - Apr 2014 - [LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs

[LLVMdev] [NVPTX] Eliminate common sub-expressions in a group of similar GEPs