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CBOR reader: address review of the scalar-run reader

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- parse_cbor_scalar_run: set the size explicitly in every case that
  reads a value, instead of relying on a default that every other case
  overwrites.
- Remove cbor_scalar_follows(): parse_cbor_scalar_run already returns on
  the same conditions, and without the pre-check SAX canada needs another
  8.5 % fewer instructions with GCC 13 (0.915 against the previous
  version), with no row more than 0.5 % worse (Mac and GCC).
- Restore the comment on why parse_cbor_internal copies the container
  frame.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-10-31 08:43:27 +01:00
1 parent e3e46253ed
commit ce7498b54c
2 files changed
+20 -42
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include/nlohmann/detail/input/binary_reader.hpp
+10 -21
View File
@@ -1711,7 +1711,7 @@ class binary_reader
{
const auto* const data = reinterpret_cast<const unsigned char*>(ia.bulk_data());
const unsigned char head = data[0];
std::size_t size = 1;
std::size_t size = 0;
bool accepted = true;
switch (head)
{
@@ -1748,12 +1748,15 @@ class binary_reader
accepted = emit_cbor_negative_integer(load_big_endian<std::uint64_t>(data + 1));
break;
case 0xF4:
size = 1;
accepted = sax->boolean(false);
break;
case 0xF5:
size = 1;
accepted = sax->boolean(true);
break;
case 0xF6:
size = 1;
accepted = sax->null();
break;
case 0xFA:
@@ -1767,10 +1770,12 @@ class binary_reader
default:
if (head <= 0x17)
{
size = 1;
accepted = sax->number_unsigned(static_cast<number_unsigned_t>(head));
}
else if (head >= 0x20 && head <= 0x37)
{
size = 1;
accepted = sax->number_integer(static_cast<std::int8_t>(0x20 - 1 - head));
}
else
@@ -1815,7 +1820,7 @@ class binary_reader
JSON_HEDLEY_NEVER_INLINE
bool parse_cbor_array_start(std::true_type /*bulk*/)
{
return !cbor_scalar_follows() || parse_cbor_scalar_run(container_stack.back().remaining);
return parse_cbor_scalar_run(container_stack.back().remaining);
}
/// input that is not contiguous: every value is read by parse_cbor_value
@@ -1824,25 +1829,6 @@ class binary_reader
return true;
}
/*!
@brief whether @ref parse_cbor_scalar_run can read the next value: one of
the heads it handles, with at least 9 bytes of input left
*/
bool cbor_scalar_follows() const
{
if (ia.bulk_remaining() < 9)
{
return false;
}
const auto head = static_cast<unsigned int>(*reinterpret_cast<const unsigned char*>(ia.bulk_data()));
// major types 0 and 1 (unsigned and negative integers) up to an
// 8-byte argument, or false, true, null, float32, float64 (0xF4,
// 0xF5, 0xF6, 0xFA, 0xFB: bits 0, 1, 2, 6, 7 of 0xC7)
return ((head & 0xC0u) == 0 && (head & 0x1Fu) <= 0x1Bu)
|| (head >= 0xF4u && ((0xC7u >> (head - 0xF4u)) & 1u) != 0);
}
/*!
@brief read a CBOR value and everything nested inside it
@@ -1868,6 +1854,9 @@ class binary_reader
{
if (!container_stack.empty())
{
// a copy, not a reference: it must stay valid across the
// pop_back() below, which destroys the container_stack element
// it would otherwise alias
const container_frame top = container_stack.back();
const bool is_object = top.is_object;
bool at_end = false;