[libcxx-commits] [PATCH] D128929: [libc++] Implements 128-bit support in to_chars.

[Nilay Vaish via Phabricator via libcxx-commits]

nilayvaish added inline comments.


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Comment at: libcxx/include/__charconv/to_chars_base_10.h:151
+  // Maximum unsigned values
+  // 64  bit                              8'446'744'073'709'551'615 (19 digits)
+  // 128 bit    340'282'366'920'938'463'463'374'607'431'768'211'455 39 digits)
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Mordante wrote:
> nilayvaish wrote:
> > I think the MSB is missing.
> What do you exactly mean with "the MSB is missing"?
2^64 = 18446744073709551616.  The most significant digit would be 1.


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Comment at: libcxx/include/charconv:167
+    // There's always a bit set in the upper 64-bits.
+    auto __t = (128 - std::__libcpp_clz(static_cast<uint64_t>(__v >> 64))) * 1233 >> 12;
+    _LIBCPP_ASSERT(__t >= __table<>::__pow10_128_offset, "Index out of bounds");
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ldionne wrote:
> Can you add a reference to the documentation for this trick?
+1.  I don't quite understand what we are trying to achieve here.


Repository:
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  https://reviews.llvm.org/D128929/new/

https://reviews.llvm.org/D128929