https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87744
Bug ID: 87744
Summary: Some valid instantiations of
linear_congruential_engine produce compiler errors
when __int128 isn't available
Product: gcc
Version: 7.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: lrflew.coll at gmail dot com
Target Milestone: ---
This issue occurs when the UIntType parameter is 64-bits, the platform being
compiled for can't use 128-bit integers (for example, 32-bit x86), and the LCG
parameters are chosen to meet certain requirements. An example of code that hit
this error is this:
#include
int main() {
std::linear_congruential_engine gen;
gen();
}
This compiles fine when __int128 is present, and fails when it isn't. When I
compile this with the -m32 flag, it prints out a lot of errors.
To summarize the problem, the error happens when determining which template
specification of _Select_uint_least_t to use in random.h. When determining how
to define operator() for the engine, it realizes that the A value is large
enough to overflow the 64-bit result type, and that the precondition for
Schrage's method isn't met (M % A > M / A). Because of this, it attempts to
find a larger integer type to use for the computation using
_Select_uint_least_t. Since the result type is already 64-bits, and the 128-bit
integer isn't available, it ends up not finding a large enough integer type,
and hits the static assert in the unspecified variant of _Select_uint_least_t.
This isn't a simple issue to resolve. The static assert that gets hit even says
"sorry, would be too much trouble for a slow result". However, as far as I can
tell, this instantiation of linear_congruential_engine is valid in the
standard, so it shouldn't result in an error. This instantiation doesn't
produce any compile errors when I tried it with MSVC, Boost, and libc++'s
implementation of linear_congruential_engine (though libc++ incorrectly uses
Schrage's method). While a solution would be slow, it's probably needed for
full standards compliance.
