Re: How to defeat the optimizer in GDC

[Mike Franklin via D.gnu]

On Monday, 20 May 2019 at 08:21:19 UTC, Iain Buclaw wrote:

Looks like you've done a typo to me. Memory should be a 
clobber, not an input operand.


Yes, that too.

Re: How to defeat the optimizer in GDC

[Mike Franklin via D.gnu]

On Monday, 20 May 2019 at 08:11:19 UTC, Mike Franklin wrote:

But I can't get GDC to do the same:  
https://explore.dgnu.org/z/quCjhU


Is this currently possible in GDC?


Gah!! Ignore that.  `version (GNU)`, not `version(GDC)`.

This works:

void use(void* p)
{
version(LDC)
{
import ldc.llvmasm;
 __asm("", "r,~{memory}", p);
}
version(GNU)
{
asm { "" : : "r" p : "memory"; };
}
}

Re: How to defeat the optimizer in GDC

[Iain Buclaw via D.gnu]

On Monday, 20 May 2019 at 08:11:19 UTC, Mike Franklin wrote:
I'm trying to benchmark some code, but the optimizer is 
basically removing all of it, so I'm benchmarking nothing.


I'd like to do something like what Chandler Carruth does here 
to defeat the optimizer:  
https://www.youtube.com/watch?v=nXaxk27zwlk=youtu.be=2446


Here presents the following inline asm function to tell the 
optimizer that `p` is being used (at least that's how I 
understand it):

```
void escape(void* p)
{
asm volatile("" : : "g"(p) : memory);
}
```

I tried to do the same thing in D with this function:
```
void use(void* p)
{
version(LDC)
{
import ldc.llvmasm;
 __asm("", "r,~{memory}", p);
}
version(GDC)
{
asm { "" : : "g"(p), "memory"; }
}
}
```

The LDC version seems to work fine:  
https://d.godbolt.org/z/qbg54J


But I can't get GDC to do the same:  
https://explore.dgnu.org/z/quCjhU


Is this currently possible in GDC?



Looks like you've done a typo to me. Memory should be a clobber, 
not an input operand.


--
Iain

How to defeat the optimizer in GDC

[Mike Franklin via D.gnu]

I'm trying to benchmark some code, but the optimizer is basically 
removing all of it, so I'm benchmarking nothing.


I'd like to do something like what Chandler Carruth does here to 
defeat the optimizer:  
https://www.youtube.com/watch?v=nXaxk27zwlk=youtu.be=2446


Here presents the following inline asm function to tell the 
optimizer that `p` is being used (at least that's how I 
understand it):

```
void escape(void* p)
{
asm volatile("" : : "g"(p) : memory);
}
```

I tried to do the same thing in D with this function:
```
void use(void* p)
{
version(LDC)
{
import ldc.llvmasm;
 __asm("", "r,~{memory}", p);
}
version(GDC)
{
asm { "" : : "g"(p), "memory"; }
}
}
```

The LDC version seems to work fine:  
https://d.godbolt.org/z/qbg54J


But I can't get GDC to do the same:  
https://explore.dgnu.org/z/quCjhU


Is this currently possible in GDC?

Mike