expand_reference.go

     1  // Copyright 2025 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package scan
     6  
     7  import (
     8  	"internal/goarch"
     9  	"internal/runtime/gc"
    10  )
    11  
    12  // ExpandReference is a reference implementation of an expander function
    13  // that translates object mark bits into a bitmap of one bit per word of
    14  // marked object, assuming the object is of the provided size class.
    15  func ExpandReference(sizeClass int, packed *gc.ObjMask, unpacked *gc.PtrMask) {
    16  	// Look up the size and derive the number of objects in a span.
    17  	// We're only concerned with small objects in single-page spans,
    18  	// and gc.PtrMask enforces this by being statically sized to
    19  	// accomodate only such spans.
    20  	size := uintptr(gc.SizeClassToSize[sizeClass])
    21  	nObj := uintptr(gc.SizeClassToNPages[sizeClass]) * gc.PageSize / size
    22  
    23  	// f is the expansion factor. For example, if our objects are of size 48,
    24  	// then each mark bit will translate into 6 (48/8 = 6) set bits in the
    25  	// pointer bitmap.
    26  	f := size / goarch.PtrSize
    27  	for i := range nObj {
    28  		// Check if the object is marked.
    29  		if packed[i/goarch.PtrBits]&(uintptr(1)<<(i%goarch.PtrBits)) == 0 {
    30  			continue
    31  		}
    32  		// Propagate that mark into the destination into one bit per the
    33  		// expansion factor f, offset to the object's offset within the span.
    34  		for j := range f {
    35  			b := i*f + j // i*f is the start bit for the object, j indexes into each corresponding word after.
    36  			unpacked[b/goarch.PtrBits] |= uintptr(1) << (b % goarch.PtrBits)
    37  		}
    38  	}
    39  }
    40
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