optimizing StgPtr allocate (Capability *cap, W_ n)
Bulat Ziganshin
bulat.ziganshin at gmail.com
Tue Oct 14 17:08:59 UTC 2014
Hello Glasgow-haskell-users,
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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