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improve performance of span_iterator w/ clang (#1166)

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* improve performance of span_iterator w/ clang

Issue: #1165

Before this PR, the range-for loop was ~3300x slower. After this PR, it
is ~1.005x slower

The clang optimizer is very good at optimizing `current != end`, so
we changed to this idiom. This moves the Expects assertion into the
constructor instead of on the hot-path which is called whenever either
operator++ or operator* is called.

Note: The codegen for the assertion is still a missed optimization,
but less worrisome as it only happens once per iterator.

Note: benchmarks on M1 Macbook Pro w/ Apple Clang 16.0.0
This commit is contained in:
Carson Radtke
2024-11-12 15:41:21 -06:00
committed by GitHub
parent 87f9d76886
commit f8ec309118
1 changed files with 7 additions and 9 deletions
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16
include/gsl/span
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@@ -140,7 +140,9 @@ namespace details
constexpr span_iterator(pointer begin, pointer end, pointer current)
: begin_(begin), end_(end), current_(current)
{}
{
Expects(begin_ <= current_ && current <= end_);
}
constexpr operator span_iterator<const Type>() const noexcept
{
@@ -149,21 +151,18 @@ namespace details
constexpr reference operator*() const noexcept
{
Expects(begin_ && end_);
Expects(begin_ <= current_ && current_ < end_);
Expects(current_ != end_);
return *current_;
}
constexpr pointer operator->() const noexcept
{
Expects(begin_ && end_);
Expects(begin_ <= current_ && current_ < end_);
Expects(current_ != end_);
return current_;
}
constexpr span_iterator& operator++() noexcept
{
Expects(begin_ && current_ && end_);
Expects(current_ < end_);
Expects(current_ != end_);
// clang-format off
GSL_SUPPRESS(bounds.1) // NO-FORMAT: attribute
// clang-format on
@@ -180,8 +179,7 @@ namespace details
constexpr span_iterator& operator--() noexcept
{
Expects(begin_ && end_);
Expects(begin_ < current_);
Expects(begin_ != current_);
--current_;
return *this;
}