[Bug tree-optimization/103429] Optimization of Auto-generated condition chain is not giving good lookup tables.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429 Richard Biener changed: What|Removed |Added Target Milestone|13.2|13.3 --- Comment #6 from Richard Biener --- GCC 13.2 is being released, retargeting bugs to GCC 13.3.
[Bug tree-optimization/103429] Optimization of Auto-generated condition chain is not giving good lookup tables.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429 Richard Biener changed: What|Removed |Added Target Milestone|13.0|13.2 --- Comment #5 from Richard Biener --- GCC 13.1 is being released, retargeting bugs to GCC 13.2.
[Bug tree-optimization/103429] Optimization of Auto-generated condition chain is not giving good lookup tables.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429 Martin Liška changed: What|Removed |Added Assignee|marxin at gcc dot gnu.org |unassigned at gcc dot gnu.org Status|ASSIGNED|NEW
[Bug tree-optimization/103429] Optimization of Auto-generated condition chain is not giving good lookup tables.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429 Martin Liška changed: What|Removed |Added Target Milestone|--- |13.0
[Bug tree-optimization/103429] Optimization of Auto-generated condition chain is not giving good lookup tables.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429 Andrew Pinski changed: What|Removed |Added Severity|normal |enhancement
[Bug tree-optimization/103429] Optimization of Auto-generated condition chain is not giving good lookup tables.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429
--- Comment #4 from Martin Liška ---
So it's very funny what's happening here. iftoswitch pass is called for all
e.g.
f_dispatch_always_inline<10>, f_dispatch_always_inline<9> and so on until
f_dispatch_always_inline<5> which is converted to switch.
And then all early passes are called for f_dispatch_always_inline<4> which
include einline and we end up with:
__attribute__((always_inline))
void f_dispatch_always_inline<4> (int i)
{
:
if (i_2(D) == 4)
goto ; [INV]
else
goto ; [INV]
:
f<4> ();
goto ; [INV]
:
switch (i_2(D)) [16.67%], case 5: [16.67%], case 6:
[16.67%], case 7: [16.67%], case 8: [16.67%], case 9: [16.67%]>
:
:
f<5> ();
goto ; [100.00%]
:
:
f<6> ();
goto ; [100.00%]
:
:
f<7> ();
goto ; [100.00%]
:
:
f<8> ();
goto ; [100.00%]
:
:
f<9> ();
:
:
return;
}
which is a mixture of if and switch statements.
So what we basically need is if-to-switch hybrid support for if-else chain
combined with switches.
[Bug tree-optimization/103429] Optimization of Auto-generated condition chain is not giving good lookup tables.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429 Martin Liška changed: What|Removed |Added Last reconfirmed||2021-11-25 Assignee|unassigned at gcc dot gnu.org |marxin at gcc dot gnu.org Ever confirmed|0 |1 Status|UNCONFIRMED |ASSIGNED --- Comment #3 from Martin Liška --- Looking into it.
[Bug tree-optimization/103429] Optimization of Auto-generated condition chain is not giving good lookup tables.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429
--- Comment #2 from Edward Rosten ---
It is doing if-to-switch, but only really with N=5, and only if force-inline is
set. I think this are two problems, one is that you need to force-inline in
order to trigger if-to-switch.
The other problem is that the if-to-switch conversion triggered only works well
for exactly 5 conditions, otherwise it uses a mix of converted and unconverted.
It doesn't appear to be doing binary tree generation, more like linear search
Here's the asm for N=7:
run(int):
ret
run_inline(int):
testedi, edi
je .L13
cmp edi, 1
je .L14
cmp edi, 6
ja .L3
mov edi, edi
jmp [QWORD PTR .L8[0+rdi*8]]
.L8:
.quad .L3
.quad .L3
.quad .L12
.quad .L11
.quad .L10
.quad .L9
.quad .L7
.L13:
jmp void f<0>()
.L7:
jmp void f<6>()
.L12:
jmp void f<2>()
.L11:
jmp void f<3>()
.L10:
jmp void f<4>()
.L9:
jmp void f<5>()
.L14:
jmp void f<1>()
.L3:
ret
Note, it's essentially doing:
if(i==0)
f<0>();
else if(i==1)
f<1>();
else if(i > 6)
return;
else switch(i){
case 0:
case 1:
return;
case 2: f<2>(); return;
case 3: f<3>(); return;
case 4: f<4>(); return;
case 5: f<5>(); return;
case 6: f<6>(); return;
}
It's not doing binary searches. For, e.g. N%5 == 1, the structure is more like:
if(i==0)
f<0>();
else if(i > 5){
if(i-5 > 4){
if(i-11>4){
if(i-16 > 4){
// and so on, linearly
}
else switch(i-16){
//...
}
}
else switch(i-11){
//...
}
}
else switch(i-6){
//...
}
}
else switch(i){
case 0:
return;
case 1: f<1>(); return;
case 2: f<2>(); return;
case 3: f<3>(); return;
case 4: f<4>(); return;
case 5: f<5>(); return;
}
[Bug tree-optimization/103429] Optimization of Auto-generated condition chain is not giving good lookup tables.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429 Richard Biener changed: What|Removed |Added Component|c++ |tree-optimization CC||marxin at gcc dot gnu.org --- Comment #1 from Richard Biener --- Sounds like if-to-switch / switch conversion could help. Note that GCC converts large switches into binary if trees in some cases as well.
