https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103429
--- Comment #2 from Edward Rosten <ed at edwardrosten dot com> ---
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):
test edi, 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;
}
