[all-commits] [llvm/llvm-project] 1adef2: [TableGen] Use guarded assert in constexpr functions

[Krzysztof Parzyszek via All-commits]

  Branch: refs/heads/users/kparzysz/constexpr-unreachable
  Home:   https://github.com/llvm/llvm-project
  Commit: 1adef29fdfc6a9bb3165cb8870512a0534269788
      https://github.com/llvm/llvm-project/commit/1adef29fdfc6a9bb3165cb8870512a0534269788
  Author: Krzysztof Parzyszek <Krzysztof.Parzyszek at amd.com>
  Date:   2026-04-28 (Tue, 28 Apr 2026)

  Changed paths:
    M llvm/test/TableGen/directive1.td
    M llvm/test/TableGen/directive2.td
    M llvm/utils/TableGen/Basic/DirectiveEmitter.cpp

  Log Message:
  -----------
  [TableGen] Use guarded assert in constexpr functions

The constexpr functions in question take a scoped enum as an argument
and a switch statement returns a value for each value of the enum.
These are all legal statements in a constexpr function in C++14.

Under constexpr rules, the evaluation of a constexpr function cannot
lead to an evaluation of any prohibited forms of expressions. An
evaluation of the functions being discussed with a valid argument
will terminate at the switch, and an code that follows will not be
evaluated.

Using "llvm_unreachable" after the switch should be ok as long as
the expansion of the llvm_unreachable macro does not contain any
statements not allowed to appear in a constexpr function.
At the same time, GCC before v9 did not tolerate any unguarded calls
to non-constexpr functions after the switch.

To avoid using "llvm_unreachable", which can have multiple expansions,
use an assert with an explicit condition that the underlying value of
the argument lies between the minimum and maximum values of the enum.



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