55 lines
2.1 KiB
Nim
55 lines
2.1 KiB
Nim
# stew
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# Copyright 2018-2019 Status Research & Development GmbH
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# Licensed under either of
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#
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# * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
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# * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
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#
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# at your option. This file may not be copied, modified, or distributed except according to those terms.
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# Pointer operations and helpers - generally, dereferencing pointers that have
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# been offset is unsafe and many of the operations herein have undefined
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# and platform-specific behavior in corner cases.
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# Due to poor codegen and subsequent lack of inlining, these operations
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# are templates even where they could be func.
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# Note that `ByteAddress` in stdlib is implemented as a signed integer and
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# might lead to overflow on arithmetic - avoid
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{.push raises: [].}
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template offset*(p: pointer, bytes: int): pointer =
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## Offset a memory address by a number of bytes. Behavior is undefined on
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## overflow.
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# Actual behavior is wrapping, but this may be revised in the future to enable
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# better optimizations
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# We assume two's complement wrapping behaviour for `uint`
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cast[pointer](cast[uint](p) + cast[uint](bytes))
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template offset*[T](p: ptr T, count: int): ptr T =
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## Offset a pointer to T by count elements. Behavior is undefined on
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## overflow.
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# Actual behavior is wrapping, but this may be revised in the future to enable
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# better optimizations.
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# We turn off checking here - too large counts is UB
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{.checks: off.}
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let bytes = count * sizeof(T)
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cast[ptr T](offset(cast[pointer](p), bytes))
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template distance*(a, b: pointer): int =
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## Number of bytes between a and b - undefined behavior when difference
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## exceeds what can be represented in an int
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# We assume two's complement wrapping behaviour for `uint`
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cast[int](cast[uint](b) - cast[uint](a))
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template distance*[T](a, b: ptr T): int =
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# Number of elements between a and b - undefined behavior when difference
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# exceeds what can be represented in an int
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{.checks: off.}
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distance(cast[pointer](a), cast[pointer](b)) div sizeof(T)
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