[go] runtime: make nelems a constant in sizespecializedmalloc
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Michael Matloob (Gerrit)
May 17, 2026, 2:39:59 PM (2 days ago) May 17
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Michael Matloob (Gerrit)
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Michael Pratt (Gerrit)
May 18, 2026, 3:47:24 PM (16 hours ago) May 18
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File src/runtime/malloc_stubs.go
Line 442, Patchset 22 (Latest):
const nbytes = npages_ * 8192
const heapBitsReserve = (1 - noscanint_) * nbytes / int(goarch.PtrSize) / 8
const gcInlineMarkBitsReserve = int(unsafe.Sizeof(spanInlineMarkBits{})) // same on 32 and 64-bit platforms
var nelemsGreenTea = uintptr(nbytes-gcInlineMarkBitsReserve-heapBitsReserve) / elemsizeMichael Pratt . unresolved
Where did this come from? The calculation seems quite different from the base nextFreeFastTiny.
Line 446, Patchset 22 (Latest):
var nelemsNoGreenTea = uintptr(nbytes-heapBitsReserve) / elemsizeMichael Pratt . resolved
No need to rip it out now, but IMO don't even try to support sizespecializedmalloc for !greentea. Basically no one uses that path.
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Michael Matloob (Gerrit)
May 18, 2026, 4:00:01 PM (16 hours ago) May 18
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Michael Matloob (Gerrit)
May 18, 2026, 4:29:52 PM (16 hours ago) May 18
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File src/runtime/malloc_stubs.go
Line 442, Patchset 22:
const nbytes = npages_ * 8192
const heapBitsReserve = (1 - noscanint_) * nbytes / int(goarch.PtrSize) / 8
const gcInlineMarkBitsReserve = int(unsafe.Sizeof(spanInlineMarkBits{})) // same on 32 and 64-bit platforms
var nelemsGreenTea = uintptr(nbytes-gcInlineMarkBitsReserve-heapBitsReserve) / elemsizeMichael Pratt . unresolved
Where did this come from? The calculation seems quite different from the base nextFreeFastTiny.
Michael Matloob
The original code that does the calculation is in https://cs.opensource.google/go/go/+/master:src/runtime/mheap.go;l=1457;drc=d247ed00e498e9717fb7c80d126bee5a8afdb4e8:
The calculation in the green tea case is:
```
s.nelems = uint16((nbytes - reserve) / s.elemsize)
```
and reserve has two components (note that gcUsersSpanInlineMarkBits(s.elemsize) and heapBitsInSpan(s.elemsize) are always true in the size specialized case- even in the tiny case we're using a span of size 16 bytes and we only specialize in cases where heapBitsInSpan is true) They are unsafe.Sizeof(spanInlineMarkBits) (always) and nbytes/goarch.PtrSize/8 (only in the scan case)
so reserve = unsafe.Sizeof(spanInlineMarkBits) + (1-noscanint_)*(nbytes/goarch.PtrSize/8)
and
nelemsGreenTea = uint16(nbytes-gcInlineMarkBitsReserve-heapBitsReserve) / elemsize
------
It's equivalent in the tiny case(though spelled a bit differently):
- For tiny (and in fact all specialized classes) npages_ = 1 so it doesn't need to appear in the calculation. So nbytes is the same between them.
- heapBitsReserve is also 0 for the tiny case becasue noscanint=1.
- in the tiny calculation, we inlined gcInlineMarkBitsReserve. (i left it as an individual factor so that i could add the comment because it wasn't obvious to me that it would be the same size on 32 and 64 bit)
- elemsize_ becomes elemsize to make things a bit easier when we add the slow path cases where it will be a variable and not a constant. But it's still a constant in the specialized fast path cases when it's inlined
- So in the tiny case:
``` var nelemsGreenTea = uintptr(nbytes-gcInlineMarkBitsReserve-heapBitsReserve) / elemsize // new calculation
= uintptr(nbytes-int(unsafe.Sizeof(spanInlineMarkBits{})-0) / elemsize_ // substitute gcInlineMarkBitsReserve=int(unsafe.Sizeof(spanInlineMarkBits{}), heapBitsReserve=0, elemsize = elemsize_
= uint16(nbytes-unsafe.Sizeof(spanInlineMarkBits{}))/elemsize_ // (changing type conversions since sizeof span inline markbits fits into an int and nelems fits into a uint16)
```
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Michael Matloob (Gerrit)
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Michael Matloob (Gerrit)
May 18, 2026, 4:31:11 PM (16 hours ago) May 18
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File src/runtime/malloc_stubs.go
Line 446, Patchset 22:
var nelemsNoGreenTea = uintptr(nbytes-heapBitsReserve) / elemsizeMichael Pratt . resolved
No need to rip it out now, but IMO don't even try to support sizespecializedmalloc for !greentea. Basically no one uses that path.
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Michael Matloob (Gerrit)
May 18, 2026, 5:12:03 PM (15 hours ago) May 18
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Michael Matloob (Gerrit)
May 18, 2026, 10:07:22 PM (10 hours ago) May 18
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Michael Matloob (Gerrit)
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Michael Matloob (Gerrit)
7:58 AM (11 minutes ago) 7:58 AM
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File-level comment, Patchset 26 (Latest):
Michael Matloob . resolved
The logic is still right but somehow this is making performance worse. I'll take this out of review for now.
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