msize_allocheaders.go

Documentation: runtime

     1  // Copyright 2009 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  //go:build goexperiment.allocheaders
     6  
     7  // Malloc small size classes.
     8  //
     9  // See malloc.go for overview.
    10  // See also mksizeclasses.go for how we decide what size classes to use.
    11  
    12  package runtime
    13  
    14  // Returns size of the memory block that mallocgc will allocate if you ask for the size,
    15  // minus any inline space for metadata.
    16  func roundupsize(size uintptr, noscan bool) (reqSize uintptr) {
    17  	reqSize = size
    18  	if reqSize <= maxSmallSize-mallocHeaderSize {
    19  		// Small object.
    20  		if !noscan && reqSize > minSizeForMallocHeader { // !noscan && !heapBitsInSpan(reqSize)
    21  			reqSize += mallocHeaderSize
    22  		}
    23  		// (reqSize - size) is either mallocHeaderSize or 0. We need to subtract mallocHeaderSize
    24  		// from the result if we have one, since mallocgc will add it back in.
    25  		if reqSize <= smallSizeMax-8 {
    26  			return uintptr(class_to_size[size_to_class8[divRoundUp(reqSize, smallSizeDiv)]]) - (reqSize - size)
    27  		}
    28  		return uintptr(class_to_size[size_to_class128[divRoundUp(reqSize-smallSizeMax, largeSizeDiv)]]) - (reqSize - size)
    29  	}
    30  	// Large object. Align reqSize up to the next page. Check for overflow.
    31  	reqSize += pageSize - 1
    32  	if reqSize < size {
    33  		return size
    34  	}
    35  	return reqSize &^ (pageSize - 1)
    36  }
    37
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