Thoughts about D

[Nicholas Wilson]

On Friday, 1 December 2017 at 03:23:23 UTC, Walter Bright wrote:
> On 11/30/2017 3:51 PM, Nicholas Wilson wrote:
>> On Thursday, 30 November 2017 at 18:18:41 UTC, Jonathan M 
>> Davis wrote:
>>> But I have a hard time believing that the cost of assertions 
>>> relates to constructing an AssertError unless the compiler is 
>>> inlining a bunch of stuff at the assertion site. If that's 
>>> what's happening, then it would increase the code size around 
>>> assertions and potentially affect performance.
>>>
>>> - Jonathan M Davis
>> 
>> Indeed, if DMD is not marking the conditional call to 
>> _d_assert (or whatever it is) 'cold' and the call itself 
>> `pragma(inline, false)` then it needs to be changed to do so.
>
> Instead of speculation, let's look at what actually happens:
>
> ---------------------------------
>   void test(int i) {
>     assert(i, "message");
>   }
> ---------------------------------
> dmd -c -m64 -O test
> obj2asm -x test.obj
> ---------------------------------
>
> __a6_746573742e64:
>         db      074h,065h,073h,074h,02eh,064h,000h      ;test.d.
> __a7_6d657373616765:
>         db      06dh,065h,073h,073h,061h,067h,065h,000h 
> ;message.
>
> _D4test4testFiZv:
> 0000:           push    RBP
> 0001:           mov     RBP,RSP
> 0004:           sub     RSP,040h
> 0008:           mov     010h[RBP],ECX
> 000b:           cmp     dword ptr 010h[RBP],0
> 000f:           jne     $+3Ah
> --- start of inserted assert failure code ---
> 0011:           mov     R8D,5                     // line number
> 0017:           lea     RAX,FLAT:_BSS[00h][RIP]
> 001e:           mov     -018h[RBP],RAX            // 
> filename.ptr
> 0022:           mov     qword ptr -020h[RBP],6    // 
> filename.length
> 002a:           lea     RDX,-020h[RBP]            // &filename[]
> 002e:           lea     RCX,FLAT:_BSS[00h][RIP]
> 0035:           mov     -8[RBP],RCX               // msg.ptr
> 0039:           mov     qword ptr -010h[RBP],7    // msg.length
> 0041:           lea     RCX,-010h[RBP]            // &msg[]
> 0045:           call      L0
> --- end of inserted assert failure code ---
> 004a:           mov     RSP,RBP
> 004d:           pop     RBP
> 004e:           ret
> -------------------------------------------
>
> 26 bytes of inserted Bloaty McBloatface code and 15 bytes of 
> data. My proposal:
>
> _D4test4testFiZv:
> 0000:           push    RBP
> 0001:           mov     RBP,RSP
> 0004:           sub     RSP,040h
> 0008:           mov     010h[RBP],ECX
> 000b:           cmp     dword ptr 010h[RBP],0
> 000f:           jne     $+01h
> 0011:           hlt                                // look ma, 
> 1 byte!
> 0012:           mov     RSP,RBP
> 0015:           pop     RBP
> 0016:           ret
>
> 1 byte of inserted code, and the data strings are gone as well.

I see you are concerned with the total size, which I understand. 
I think we misunderstood each other.

What I meant in terms of icache pollution is with the 'cold' is 
instead of generating:

if(!cond)
     _d_assert(__FILE__, __LINE__,message);
//rest of code

it should actually generate,

if (!cond)
     goto failed;
//rest of code

failed:
      _d_assert(__FILE__, __LINE__,message);//call is cold & out 
of line. no icache pollution

I'm not sure that it does that given the triviality of the 
example, but it looks like it doesn't.