On Thursday, 8 February 2018 at 10:44:37 UTC, Mike Parker wrote:
On Thursday, 8 February 2018 at 10:09:12 UTC, Timothee Cour
wrote:
I'm curious whether scope guards add any cost over the naive
way, eg:
```
void fun(){
...
scope(success) {bar;}
...
}
```
vs:
```
void fun(){
...
if(foo1){
bar; // add this before each return
return;
}
...
bar;
return;
}
```
For scope(success) and scope(failure), the naive way would
anyway
involve try/catch statements but what about scope(exit)? Does
the
zero-cost exception model (zero cost being for non-thrown
exceptions)
guarantee us that scope(success) has 0 overhead over naive way?
Scope guards are lowered to the equivalent try/catch/finally
blocks anyway.
Yes, let's look at it:
```
scope(failure) printf("%s", "error".ptr);
printf("%s", "All good.".ptr);
```
1) Lowered AST
https://run.dlang.io/is/PmRfkb
---
import object;
import core.stdc.stdio;
void main()
{
try
{
printf("%s", "All good.");
}
catch(Throwable __o2)
{
printf("%s", "error");
throw __o2;
}
return 0;
}
---
2) Assembly
(reduced to the interesting bits - see
https://run.dlang.io/is/Cafgja for the full ASM)
---
call printf@PLT32
jmp short L69
lea RSP,0FFFFFFF0h[RBP]
mov -8[RBP],EDX
mov RDI,RAX
call __dmd_begin_catch@PLT32
mov -010h[RBP],RAX
mov EAX,-8[RBP]
cmp EAX,1
je L3F
jmp short L62
L3F: lea RSI,FLAT:.rodata[00h][RIP]
lea RDI,FLAT:.rodata[00h][RIP]
xor EAX,EAX
call printf@PLT32
mov RDI,-010h[RBP]
call _d_throwdwarf@PLT32
mov RSP,RBP
pop RBP
ret
---
