nim-stew/stew/ptrops.nim

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# stew
# Copyright 2018-2019 Status Research & Development GmbH
# Licensed under either of
#
# * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
# * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
#
# at your option. This file may not be copied, modified, or distributed except according to those terms.
# Pointer operations and helpers - generally, dereferencing pointers that have
# been offset is unsafe and many of the operations herein have undefined
# and platform-specific behavior in corner cases.
# Due to poor codegen and subsequent lack of inlining, many of these operations
# are templates even where they could be func.
# ByteAddress in std lib is signed - this leads to issues with overflow checking
# when address is on boundary.
type
MemAddress* = distinct uint
template toMemAddress*(p: pointer): MemAddress = cast[MemAddress](p)
template toPointer*(p: MemAddress): pointer = cast[pointer](p)
template toPtr*(p: MemAddress, T: type): ptr T = cast[ptr T](p)
template offset*(p: MemAddress, bytes: int): MemAddress =
## Offset a memory address by a number of bytes. Behavior is undefined on
## overflow.
# Actual behavior is wrapping, but this may be revised in the future to enable
# better optimizations
{.checks: off.}
MemAddress(uint(p) + cast[uint](bytes))
template offset*(p: pointer, bytes: int): pointer =
## Offset a memory address by a number of bytes. Behavior is undefined on
## overflow.
# Actual behavior is wrapping, but this may be revised in the future to enable
# better optimizations
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mixin toMemAddress, offset, toPointer
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p.toMemAddress().offset(bytes).toPointer()
template offset*[T](p: ptr T, count: int): ptr T =
## Offset a pointer to T by count elements. Behavior is undefined on
## overflow.
# Actual behavior is wrapping, but this may be revised in the future to enable
# better optimizations.
# We turn off checking here - too large counts is UB
{.checks: off.}
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mixin toMemAddress, offset, toPtr
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let bytes = count * sizeof(T)
p.toMemAddress().offset(bytes).toPtr(type p[])
template distance*(a, b: MemAddress): int =
cast[int](cast[uint](b) - cast[uint](a))
template distance*(a, b: pointer): int =
# Number of bytes between a and b - undefined behavior when difference exceeds
# what can be represented in an int
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mixin toMemAddress
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a.toMemAddress().distance(b.toMemAddress())
template distance*[T](a, b: ptr T): int =
# Number of elements between a and b - undefined behavior when difference
# exceeds what can be represented in an int
{.checks: off.}
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mixin toMemAddress, distance
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a.toMemAddress().distance(b.toMemAddress()) div sizeof(T)
proc `<`*(a, b: MemAddress): bool =
cast[uint](a) < cast[uint](b)