optimizing StgPtr allocate (Capability *cap, W_ n)

[Bulat Ziganshin]

Hello Mikolaj,

Wednesday, October 15, 2014, 2:07:35 AM, you wrote:

i'm sorry, it's the same


> Hi Bulat!

> Is this exactly the same email you posted to
> glasgow-haskell-users@ previously (I'm asking to know
> if I need to reread)? It was already forwarded here by none
> other but SPJ. :)

> Welcome,
> Mikolaj


> On Wed, Oct 15, 2014 at 12:01 AM, Bulat Ziganshin
> <bulat.ziganshin at gmail.com> wrote:
>> Hello,
>>
>> i'm looking a the https://github.com/ghc/ghc/blob/23bb90460d7c963ee617d250fa0a33c6ac7bbc53/rts/sm/Storage.c#L680
>>
>> if i correctly understand, it's speed-critical routine?
>>
>> i think that it may be improved in this way:
>>
>> StgPtr allocate (Capability *cap, W_ n)
>> {
>>     bdescr *bd;
>>     StgPtr p;
>>
>>     TICK_ALLOC_HEAP_NOCTR(WDS(n));
>>     CCS_ALLOC(cap->r.rCCCS,n);
>>
>> /// here starts new improved code:
>>
>>     bd = cap->r.rCurrentAlloc;
>>     if (bd == NULL || bd->free + n > bd->end) {
>>         if (n >= LARGE_OBJECT_THRESHOLD/sizeof(W_)) {
>>             ....
>>         }
>>         if (bd->free + n <= bd->start + BLOCK_SIZE_W)
>>             bd->end = min (bd->start + BLOCK_SIZE_W, bd->free + LARGE_OBJECT_THRESHOLD)
>>             goto usual_alloc;
>>         }
>>         ....
>>     }
>>
>> /// and here it stops
>>
>> usual_alloc:
>>     p = bd->free;
>>     bd->free += n;
>>
>>     IF_DEBUG(sanity, ASSERT(*((StgWord8*)p) == 0xaa));
>>     return p;
>> }
>>
>>
>> i  think  it's  obvious - we consolidate two if's on the crirical path
>> into the single one plus avoid one ADD by keeping highly-useful bd->end pointer
>>
>> further   improvements   may   include   removing  bd==NULL  check  by
>> initializing bd->free=bd->end=NULL   and   moving   entire   "if" body
>> into   separate   slow_allocate()  procedure  marked  "noinline"  with
>> allocate() probably marked as forceinline:
>>
>> StgPtr allocate (Capability *cap, W_ n)
>> {
>>     bdescr *bd;
>>     StgPtr p;
>>
>>     TICK_ALLOC_HEAP_NOCTR(WDS(n));
>>     CCS_ALLOC(cap->r.rCCCS,n);
>>
>>     bd = cap->r.rCurrentAlloc;
>>     if (bd->free + n > bd->end)
>>         return slow_allocate(cap,n);
>>
>>     p = bd->free;
>>     bd->free += n;
>>
>>     IF_DEBUG(sanity, ASSERT(*((StgWord8*)p) == 0xaa));
>>     return p;
>> }
>>
>> this  change  will  greatly simplify optimizer's work. according to my
>> experience   current  C++  compilers  are  weak  on  optimizing  large
>> functions with complex execution paths and such transformations really
>> improve the generated code
>>
>> --
>> Best regards,
>>  Bulat                          mailto:Bulat.Ziganshin at gmail.com
>>
>> _______________________________________________
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>> ghc-devs at haskell.org
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>>



-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com