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D.gnu - How to defeat the optimizer in GDC

reply Mike Franklin <slavo5150 yahoo.com> writes:
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&feature=youtu.be&t=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
May 20 2019
next sibling parent reply Iain Buclaw <ibuclaw gdcproject.org> writes:
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&feature=youtu.be&t=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
May 20 2019
parent Mike Franklin <slavo5150 yahoo.com> writes:
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.
May 20 2019
prev sibling parent Mike Franklin <slavo5150 yahoo.com> writes:
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"; };
     }
}
May 20 2019