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`<random>`: Use `_Unsigned128` for `linear_congruential_engine` by StephanTLavavej Β· Pull Request #5436 Β· microsoft/STL Β· GitHub
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<random>
: Use _Unsigned128
for linear_congruential_engine
#5436
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We can drop `_INLINE_VAR` from `_MP_len`, giving it internal linkage within multprec.cpp. We don't need `extern "C++"` on the declarations - that was for modules. We need to add `_CRTIMP2_PURE` to the definitions. I thought we had harmonized all declarations and definitions, but we missed these.
We can hardcode `64` instead of including `<limits>` for `numeric_limits<unsigned long long>::digits`. `uint64_t` and `unsigned long long` are synonyms. In fact, this file was already using `unsigned long long` for the definitions, even though the declarations originally said `uint64_t`. We moved the declarations out of `<random>` so we don't need it anymore. All we need is `<yvals.h>`.
I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. |
zacklj89
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Apr 29, 2025
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Appreciate the python in the description π―
This was referenced Apr 30, 2025
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I noticed that we had a single remaining usage of separately compiled multiprecision arithmetic machinery. I believe that we can replace this with the modern
_Unsigned128
type, which is far better tested and more widely exercised. As @AlexGuteniev observed,_Unsigned128
uses intrinsics, so it should also be faster than multprec.cpp. And as a final bonus, it makes this codepath strongly resemble the 32-bit and 64-bit codepaths above.π¨βπ¬ Proof that 128 bits are sufficient
The function comment says:
STL/stl/inc/random
Lines 474 to 482 in b5df16a
The first part, _Cx <= (2^128 - 1), is obviously true.
The next part is (_Mx - 1) <= ((2^128 - 1) - _Cx) / _Ax. This is the hardest to achieve for the largest LHS (caused by the largest _Mx) and the smallest RHS (caused by the largest _Ax and the largest _Cx). So ((2^64 - 1) - 1) <= ((2^128 - 1) - (2^64 - 1)) / (2^64 - 1) is the worst case.
βοΈ Commits
_Uint _Cx
always satisfies_Cx <= ULLONG_MAX
._Unsigned128
, preserve multprec.cpp for bincompat._INLINE_VAR
from_MP_len
, giving it internal linkage within multprec.cpp.extern "C++"
on the declarations - that was for modules._CRTIMP2_PURE
to the definitions. I thought we had harmonized all declarations and definitions, but we missed these.64
instead of including<limits>
fornumeric_limits<unsigned long long>::digits
.uint64_t
andunsigned long long
are synonyms. In fact, this file was already usingunsigned long long
for the definitions, even though the declarations originally saiduint64_t
.<random>
so we don't need it anymore. All we need is<yvals.h>
.β Notes
static_assert(false)
) that this codepath is exercised by our test suites.dumpbin /exports
) that this doesn't affect the dllexport surface.