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[flang] Fix template step limit issue with clang 

While working on PR 959, I instanciated a `common::TupleToVariant`
with ~50+ types inside the tuple. Clang would then crash after
1hr compilation with message:
"constexpr evaluation hit maximum step limit; possible infinite loop"
After investigating, it turned out clang handles very badly the way
`common::AreTypesDistinctHelper` was implemented.
Its "number of steps" was exponential with the number of types.
This fix makes this number quadratic which solves the issue.

Original-commit: flang-compiler/f18@4542cb5708
Reviewed-on: https://github.com/flang-compiler/f18/pull/968
This commit is contained in:
Jean Perier 2020-02-04 10:30:16 -08:00
parent b6363facf5
commit a8ef13ea25
1 changed files with 4 additions and 7 deletions
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11
flang/include/flang/common/template.h
View File

@ -153,15 +153,12 @@ template<typename... Ts> struct VariantToTupleHelper<std::variant<Ts...>> {
template<typename VARIANT>
using VariantToTuple = typename VariantToTupleHelper<VARIANT>::type;
template<typename A, typename B, typename... REST>
struct AreTypesDistinctHelper {
template<typename A, typename... REST> struct AreTypesDistinctHelper {
static constexpr bool value() {
if constexpr (std::is_same_v<A, B>) {
return false;
}
if constexpr (sizeof...(REST) > 0) {
return AreTypesDistinctHelper<A, REST...>::value() &&
AreTypesDistinctHelper<B, REST...>::value();
// extra () for clang-format
return ((... && !std::is_same_v<A, REST>)) &&
AreTypesDistinctHelper<REST...>::value();
}
return true;
}