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* [Bug tree-optimization/104950] New: GCC does not emit branchless code
@ 2022-03-16 8:51 vincenzo.innocente at cern dot ch
2022-03-16 9:08 ` [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other pinskia at gcc dot gnu.org
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: vincenzo.innocente at cern dot ch @ 2022-03-16 8:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104950
Bug ID: 104950
Summary: GCC does not emit branchless code
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vincenzo.innocente at cern dot ch
Target Milestone: ---
In this example GCC fails to emit branchless code while CLANG does.
In the actual application, measurements shows slow down up to a factor 2.
I managed to force branchless (-DBL) but the code is pretty unfriendly
godbolt link (GCC, clang, GCC -DBL
https://godbolt.org/z/KWY1rjhhY
and here inlined
include <vector>
const float defaultBaseResponse = 0.5;
class DForest {
public:
//based on FastForest::evaluate() and BDTree::parseTree()
DForest() {
}
float evaluate(const float* features) const;
std::vector<int> rootIndices_;
//"node" layout: cut, index, left, right
struct Node{
float v; int i,l,r;
constexpr int eval(float const * f) const {
#ifdef BL
auto m = f[i] > v;
return *((&l) + int(m));
#else
return f[i] > v ? r : l;
#endif
}
};
std::vector<Node> nodes_;
std::vector<float> responses_;
std::vector<float> baseResponses_;
};
float DForest::evaluate(const float* features) const{
float sum{defaultBaseResponse + baseResponses_[0]};
for(int index : rootIndices_){
do {
index = nodes_[index].eval(features);
} while (index>0);
sum += responses_[-index];
}
return sum;
}
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other
2022-03-16 8:51 [Bug tree-optimization/104950] New: GCC does not emit branchless code vincenzo.innocente at cern dot ch
@ 2022-03-16 9:08 ` pinskia at gcc dot gnu.org
2022-03-16 9:18 ` rguenth at gcc dot gnu.org
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-03-16 9:08 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104950
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2022-03-16
Summary|GCC does not emit |GCC does not emit
|branchless code |branchless code for load
| |next to each other
Ever confirmed|0 |1
Severity|normal |enhancement
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, reduced testcase:
struct Node{
int i, l,r;
};
int eval(int t, struct Node *a) {
return t == 0 ? a->r : a->l;
}
Note on aarch64, if we remove the i field, then ifcvt on the RTL level is able
to catch it but it does not do it for x86_64 (maybe a cost issue).
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other
2022-03-16 8:51 [Bug tree-optimization/104950] New: GCC does not emit branchless code vincenzo.innocente at cern dot ch
2022-03-16 9:08 ` [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other pinskia at gcc dot gnu.org
@ 2022-03-16 9:18 ` rguenth at gcc dot gnu.org
2022-03-16 9:21 ` rguenth at gcc dot gnu.org
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: rguenth at gcc dot gnu.org @ 2022-03-16 9:18 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104950
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> Confirmed, reduced testcase:
> struct Node{
> int i, l,r;
> };
> int eval(int t, struct Node *a) {
> return t == 0 ? a->r : a->l;
> }
>
> Note on aarch64, if we remove the i field, then ifcvt on the RTL level is
> able to catch it but it does not do it for x86_64 (maybe a cost issue).
I think it's for the fear of one of a->r / a->l trapping while the other is
not. With
struct Node{
int i, l,r;
};
int eval(int t, struct Node *a) {
int r = a->r;
int l = a->l;
return t == 0 ? r : l;
}
and using -fno-tree-sink we get branchless code on x86_64.
I think we can use TBAA to argue that *a should be valid to dereference here.
What Andrew shows with removing 'i' is likely struct Node then having
large enough alignment guarantees(?) or a bug in the aarch64 backend.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other
2022-03-16 8:51 [Bug tree-optimization/104950] New: GCC does not emit branchless code vincenzo.innocente at cern dot ch
2022-03-16 9:08 ` [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other pinskia at gcc dot gnu.org
2022-03-16 9:18 ` rguenth at gcc dot gnu.org
@ 2022-03-16 9:21 ` rguenth at gcc dot gnu.org
2022-03-16 9:27 ` crazylht at gmail dot com
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: rguenth at gcc dot gnu.org @ 2022-03-16 9:21 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104950
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ah, on aarch64 we get
cmp w0, 0
add x0, x1, 4
csel x0, x0, x1, eq
ldr w0, [x0]
so we do not load from the possibly trapping mem. With the testcase I provided
and -fno-tree-sink on x86_64 we get
movl 4(%rsi), %eax
testl %edi, %edi
cmove 8(%rsi), %eax
which would be prone to the issue. I suppose ifcvt is oddly restricted for the
first case.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other
2022-03-16 8:51 [Bug tree-optimization/104950] New: GCC does not emit branchless code vincenzo.innocente at cern dot ch
` (2 preceding siblings ...)
2022-03-16 9:21 ` rguenth at gcc dot gnu.org
@ 2022-03-16 9:27 ` crazylht at gmail dot com
2022-03-16 9:31 ` pinskia at gcc dot gnu.org
2022-03-16 9:58 ` crazylht at gmail dot com
5 siblings, 0 replies; 7+ messages in thread
From: crazylht at gmail dot com @ 2022-03-16 9:27 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104950
Hongtao.liu <crazylht at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |crazylht at gmail dot com
--- Comment #4 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Richard Biener from comment #3)
> Ah, on aarch64 we get
>
> cmp w0, 0
> add x0, x1, 4
> csel x0, x0, x1, eq
> ldr w0, [x0]
>
> so we do not load from the possibly trapping mem. With the testcase I
> provided and -fno-tree-sink on x86_64 we get
Not for this one
float
foo (float a, float b, float *c, int i, int j)
{
return a > b ? c[i] : c[j];
}
gcc
vcomiss xmm0, xmm1
jbe .L6
movsx rsi, esi
vmovss xmm0, DWORD PTR [rdi+rsi*4]
ret
.L6:
movsx rdx, edx
vmovss xmm0, DWORD PTR [rdi+rdx*4]
ret
llvm
vucomiss xmm0, xmm1
cmovbe esi, edx
movsxd rax, esi
vmovss xmm0, dword ptr [rdi + 4*rax]
ret
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other
2022-03-16 8:51 [Bug tree-optimization/104950] New: GCC does not emit branchless code vincenzo.innocente at cern dot ch
` (3 preceding siblings ...)
2022-03-16 9:27 ` crazylht at gmail dot com
@ 2022-03-16 9:31 ` pinskia at gcc dot gnu.org
2022-03-16 9:58 ` crazylht at gmail dot com
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-03-16 9:31 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104950
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=102008
--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Hongtao.liu from comment #4)
> (In reply to Richard Biener from comment #3)
> > Ah, on aarch64 we get
> >
> > cmp w0, 0
> > add x0, x1, 4
> > csel x0, x0, x1, eq
> > ldr w0, [x0]
> >
> > so we do not load from the possibly trapping mem. With the testcase I
> > provided and -fno-tree-sink on x86_64 we get
>
> Not for this one
>
> float
> foo (float a, float b, float *c, int i, int j)
> {
> return a > b ? c[i] : c[j];
> }
That one is recorded as PR
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other
2022-03-16 8:51 [Bug tree-optimization/104950] New: GCC does not emit branchless code vincenzo.innocente at cern dot ch
` (4 preceding siblings ...)
2022-03-16 9:31 ` pinskia at gcc dot gnu.org
@ 2022-03-16 9:58 ` crazylht at gmail dot com
5 siblings, 0 replies; 7+ messages in thread
From: crazylht at gmail dot com @ 2022-03-16 9:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104950
--- Comment #6 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Andrew Pinski from comment #5)
> (In reply to Hongtao.liu from comment #4)
> > (In reply to Richard Biener from comment #3)
> > > Ah, on aarch64 we get
> > >
> > > cmp w0, 0
> > > add x0, x1, 4
> > > csel x0, x0, x1, eq
> > > ldr w0, [x0]
> > >
> > > so we do not load from the possibly trapping mem. With the testcase I
> > > provided and -fno-tree-sink on x86_64 we get
> >
> > Not for this one
> >
> > float
> > foo (float a, float b, float *c, int i, int j)
> > {
> > return a > b ? c[i] : c[j];
> > }
>
> That one is recorded as PR
Just note -fno-tree-sink works for PR102008, but not for this case.
^ permalink raw reply [flat|nested] 7+ messages in thread
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2022-03-16 8:51 [Bug tree-optimization/104950] New: GCC does not emit branchless code vincenzo.innocente at cern dot ch
2022-03-16 9:08 ` [Bug rtl-optimization/104950] GCC does not emit branchless code for load next to each other pinskia at gcc dot gnu.org
2022-03-16 9:18 ` rguenth at gcc dot gnu.org
2022-03-16 9:21 ` rguenth at gcc dot gnu.org
2022-03-16 9:27 ` crazylht at gmail dot com
2022-03-16 9:31 ` pinskia at gcc dot gnu.org
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