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* [Bug tree-optimization/45021] New: Redundant prefetches for the vectorized loop
@ 2010-07-21 17:46 changpeng dot fang at amd dot com
2010-07-21 18:27 ` [Bug tree-optimization/45021] " changpeng dot fang at amd dot com
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: changpeng dot fang at amd dot com @ 2010-07-21 17:46 UTC (permalink / raw)
To: gcc-bugs
For the following test case, prefetches will be inserted for both the load and
store of a[i] if the loop is vectorized:
float a[1024], b[1024];
void foo(int beta)
{
int i;
for(i=0; i<1024; i++)
a[i] = a[i] + beta * b[i];
}
with gcc -O3 -fprefetch-loop-arrays -march=amdfam10 -S, a piece of the assembly
is:
movaps (%rcx), %xmm0
addl $4, %edi
prefetcht0 (%rdx)
prefetcht0 240(%rcx)
prefetchw (%rdx)
leaq 64(%rax), %rsi
mulps %xmm1, %xmm0
If we don't vectorize the loop, we only generate prefetch for the load a[i]:
addl $16, %eax
salq $2, %rcx
mulss %xmm1, %xmm0
prefetcht0 a+92(%rcx)
prefetcht0 b+92(%rcx)
movl %esi, %ecx
--
Summary: Redundant prefetches for the vectorized loop
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: changpeng dot fang at amd dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45021
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45021] Redundant prefetches for the vectorized loop
2010-07-21 17:46 [Bug tree-optimization/45021] New: Redundant prefetches for the vectorized loop changpeng dot fang at amd dot com
@ 2010-07-21 18:27 ` changpeng dot fang at amd dot com
2010-07-24 20:32 ` [Bug tree-optimization/45021] Redundant prefetches for some loops (vectorizer produced ones too) pinskia at gcc dot gnu dot org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: changpeng dot fang at amd dot com @ 2010-07-21 18:27 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from changpeng dot fang at amd dot com 2010-07-21 18:26 -------
The direct reason is that prefetching could not differentiate the base
addresses
of the vectorized load and store (of a[i]):
*vect_pa.6_24
*vect_pa.19_37
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45021
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45021] Redundant prefetches for some loops (vectorizer produced ones too)
2010-07-21 17:46 [Bug tree-optimization/45021] New: Redundant prefetches for the vectorized loop changpeng dot fang at amd dot com
2010-07-21 18:27 ` [Bug tree-optimization/45021] " changpeng dot fang at amd dot com
@ 2010-07-24 20:32 ` pinskia at gcc dot gnu dot org
2010-07-24 20:42 ` rakdver at kam dot mff dot cuni dot cz
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-07-24 20:32 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from pinskia at gcc dot gnu dot org 2010-07-24 20:32 -------
(In reply to comment #1)
> The direct reason is that prefetching could not differentiate the base
> addresses
> of the vectorized load and store (of a[i]):
> *vect_pa.6_24
> *vect_pa.19_37
Here is a testcase which shows the same issue without the vectorizer (compile
-O2 -fprefetch-loop-arrays -march=amdfam10 -fno-tree-ccp -fno-tree-vrp
-fno-tree-dominator-opts):
float *f();
float aa[1024];
float bb[1024];
void foo(int beta)
{
int i;
float *a = aa, *a1 = aa, *b = bb;
for(i=0; i<1024; i++)
{
*a = *a1 + beta * *b;
a++; a1++; b++;
}
}
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Ever Confirmed|0 |1
Keywords| |missed-optimization
Last reconfirmed|0000-00-00 00:00:00 |2010-07-24 20:32:24
date| |
Summary|Redundant prefetches for the|Redundant prefetches for
|vectorized loop |some loops (vectorizer
| |produced ones too)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45021
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45021] Redundant prefetches for some loops (vectorizer produced ones too)
2010-07-21 17:46 [Bug tree-optimization/45021] New: Redundant prefetches for the vectorized loop changpeng dot fang at amd dot com
2010-07-21 18:27 ` [Bug tree-optimization/45021] " changpeng dot fang at amd dot com
2010-07-24 20:32 ` [Bug tree-optimization/45021] Redundant prefetches for some loops (vectorizer produced ones too) pinskia at gcc dot gnu dot org
@ 2010-07-24 20:42 ` rakdver at kam dot mff dot cuni dot cz
2010-07-28 18:23 ` changpeng dot fang at amd dot com
2010-07-28 18:28 ` changpeng dot fang at amd dot com
4 siblings, 0 replies; 6+ messages in thread
From: rakdver at kam dot mff dot cuni dot cz @ 2010-07-24 20:42 UTC (permalink / raw)
To: gcc-bugs
------- Comment #3 from rakdver at kam dot mff dot cuni dot cz 2010-07-24 20:41 -------
Subject: Re: Redundant prefetches for some
loops (vectorizer produced ones too)
> > The direct reason is that prefetching could not differentiate the base
> > addresses
> > of the vectorized load and store (of a[i]):
> > *vect_pa.6_24
> > *vect_pa.19_37
>
> Here is a testcase which shows the same issue without the vectorizer (compile
> -O2 -fprefetch-loop-arrays -march=amdfam10 -fno-tree-ccp -fno-tree-vrp
> -fno-tree-dominator-opts):
> float *f();
> float aa[1024];
> float bb[1024];
> void foo(int beta)
> {
> int i;
> float *a = aa, *a1 = aa, *b = bb;
> for(i=0; i<1024; i++)
> {
> *a = *a1 + beta * *b;
> a++; a1++; b++;
> }
> }
I am not sure that this issue should be addressed in the prefetching pass; as
this
example shows, we already have three other passes that deal with it ordinarily.
Perhaps adjusting the vectorizer code generation or scheduling copy propagation
after vectorizer would be better.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45021
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45021] Redundant prefetches for some loops (vectorizer produced ones too)
2010-07-21 17:46 [Bug tree-optimization/45021] New: Redundant prefetches for the vectorized loop changpeng dot fang at amd dot com
` (2 preceding siblings ...)
2010-07-24 20:42 ` rakdver at kam dot mff dot cuni dot cz
@ 2010-07-28 18:23 ` changpeng dot fang at amd dot com
2010-07-28 18:28 ` changpeng dot fang at amd dot com
4 siblings, 0 replies; 6+ messages in thread
From: changpeng dot fang at amd dot com @ 2010-07-28 18:23 UTC (permalink / raw)
To: gcc-bugs
------- Comment #4 from changpeng dot fang at amd dot com 2010-07-28 18:22 -------
Andrew's example is exactly what the prefetch sees for the test case (in the
bug description). Unfortunately, the prefetch pass could not recognize that
vect_pa.6_24 and vect_pa.20_38 are exactly the same address:
<bb 2>:
pretmp.2_18 = (float) beta_4(D);
vect_pa.9_22 = (vector(4) float *) &a;
vect_pa.6_23 = vect_pa.9_22;
vect_cst_.12_27 = {pretmp.2_18, pretmp.2_18, pretmp.2_18, pretmp.2_18};
vect_pb.16_29 = (vector(4) float *) &b;
vect_pb.13_30 = vect_pb.16_29;
vect_pa.23_36 = (vector(4) float *) &a;
vect_pa.20_37 = vect_pa.23_36;
<bb 3>:
# vect_pa.6_24 = PHI <vect_pa.6_25(4), vect_pa.6_23(2)>
# vect_pb.13_31 = PHI <vect_pb.13_32(4), vect_pb.13_30(2)>
# vect_pa.20_38 = PHI <vect_pa.20_39(4), vect_pa.20_37(2)>
# ivtmp.24_40 = PHI <ivtmp.24_41(4), 0(2)>
vect_var_.10_26 = *vect_pa.6_24;
vect_var_.11_28 = vect_cst_.12_27;
vect_var_.17_33 = *vect_pb.13_31;
vect_var_.18_34 = vect_var_.11_28 * vect_var_.17_33;
vect_var_.19_35 = vect_var_.10_26 + vect_var_.18_34;
*vect_pa.20_38 = vect_var_.19_35;
vect_pa.6_25 = vect_pa.6_24 + 16;
vect_pb.13_32 = vect_pb.13_31 + 16;
vect_pa.20_39 = vect_pa.20_38 + 16;
ivtmp.24_41 = ivtmp.24_40 + 1;
if (ivtmp.24_41 < 256)
goto <bb 4>;
else
goto <bb 5>;
<bb 4>:
goto <bb 3>;
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45021
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45021] Redundant prefetches for some loops (vectorizer produced ones too)
2010-07-21 17:46 [Bug tree-optimization/45021] New: Redundant prefetches for the vectorized loop changpeng dot fang at amd dot com
` (3 preceding siblings ...)
2010-07-28 18:23 ` changpeng dot fang at amd dot com
@ 2010-07-28 18:28 ` changpeng dot fang at amd dot com
4 siblings, 0 replies; 6+ messages in thread
From: changpeng dot fang at amd dot com @ 2010-07-28 18:28 UTC (permalink / raw)
To: gcc-bugs
------- Comment #5 from changpeng dot fang at amd dot com 2010-07-28 18:28 -------
Thing is a little complicate if we change the code to:
a[i] = a[i+1] + beta * b[i];
The prefetch pass want to group a[i] and a[i+1], i.e. they have
the same base address with an offset of 4 bytes.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45021
^ permalink raw reply [flat|nested] 6+ messages in thread
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2010-07-21 17:46 [Bug tree-optimization/45021] New: Redundant prefetches for the vectorized loop changpeng dot fang at amd dot com
2010-07-21 18:27 ` [Bug tree-optimization/45021] " changpeng dot fang at amd dot com
2010-07-24 20:32 ` [Bug tree-optimization/45021] Redundant prefetches for some loops (vectorizer produced ones too) pinskia at gcc dot gnu dot org
2010-07-24 20:42 ` rakdver at kam dot mff dot cuni dot cz
2010-07-28 18:23 ` changpeng dot fang at amd dot com
2010-07-28 18:28 ` changpeng dot fang at amd dot com
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