Thoughts about D

[Iain Buclaw]

On 1 December 2017 at 04:23, Walter Bright via Digitalmars-d
<digitalmars-d at puremagic.com> 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
> -------------------------------------------
>

Wouldn't it be more optimal if dmd instead emitted the following?

---------------------------------

__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:           je     $+3Ah               ;  <--- Using `je` instead of `jne`
0011:           mov     RSP,RBP
0014:           pop     RBP
0015:           ret
--- start of inserted assert failure code ---
...
> --- end of inserted assert failure code ---
-------------------------------------------