[llvm] replace static_assert with std::enable_if_t in ilist_node_impl

[andrurogerz]

Purpose

Remove static_assert in ilist_node_impl::isSentinel and conditionally include the functino using std::enable_if_t instead.

Background

This fix is necessary to support building LLVM as a Windows DLL, tracked by #109483.

The static_assert in ilist_node_impl::isSentinel fails when compiling LLVM as a Windows DLL with the options -D LLVM_BUILD_LLVM_DYLIB=ON -D LLVM_BUILD_LLVM_DYLIB_VIS=ON -D LLVM_LINK_LLVM_DYLIB=ON:

S:\llvm\llvm-project\llvm\include\llvm/ADT/ilist_node.h(151): error C2338: static_assert failed: 'Use ilist_sentinel_tracking<true> to enable isSentinel()'
S:\llvm\llvm-project\llvm\include\llvm/ADT/ilist_node.h(151): note: the template instantiation context (the oldest one first) is
S:\llvm\llvm-project\llvm\include\llvm/IR/SymbolTableListTraits.h(113): note: see reference to class template instantiation 'llvm::SymbolTableListTraits<llvm::Function>' being compiled
S:\llvm\llvm-project\llvm\include\llvm/IR/SymbolTableListTraits.h(69): note: see reference to class template instantiation 'llvm::simple_ilist<ValueSubClass>' being compiled
        with
        [
            ValueSubClass=llvm::Function
        ]
S:\llvm\llvm-project\llvm\include\llvm/ADT/simple_ilist.h(87): note: see reference to class template instantiation 'llvm::ilist_sentinel<llvm::ilist_detail::node_options<T,true,false,llvm::ilist_detail::extract_tag<>::type,false,llvm::ilist_detail::extract_parent<>::type>>' being compiled
        with
        [
            T=llvm::Function
        ]
S:\llvm\llvm-project\llvm\include\llvm/ADT/ilist_node.h(301): note: see reference to class template instantiation 'llvm::ilist_node_impl<OptionsT>' being compiled
        with
        [
            OptionsT=llvm::ilist_detail::node_options<llvm::Function,true,false,llvm::ilist_detail::extract_tag<>::type,false,llvm::ilist_detail::extract_parent<>::type>
        ]
S:\llvm\llvm-project\llvm\include\llvm/ADT/ilist_node.h(150): note: while compiling class template member function 'bool llvm::ilist_node_impl<OptionsT>::isSentinel(void) const'
        with
        [
            OptionsT=llvm::ilist_detail::node_options<llvm::Function,true,false,llvm::ilist_detail::extract_tag<>::type,false,llvm::ilist_detail::extract_parent<>::type>
        ]

Conditionally including the function using std::enable_if_t has the same effect of preventing the function's use when is_sentinel_tracking_explicit=false, but avoids the issue when DLL-exporting downstream classes.

Validation

Verified I no longer fail compilation due to the static_assert when building LLVM on Windows 11 with the options -D LLVM_BUILD_LLVM_DYLIB=ON -D LLVM_BUILD_LLVM_DYLIB_VIS=ON -D LLVM_LINK_LLVM_DYLIB=ON.

[andrurogerz]

+@compnerd, who helped me figure out this solution

[llvmbot]

@llvm/pr-subscribers-llvm-adt

Author: Andrew Rogers (andrurogerz)

Changes

Purpose

Remove static_assert in ilist_node_impl::isSentinel and conditionally include the functino using std::enable_if_t instead.

Background

This fix is necessary to support building LLVM as a Windows DLL, tracked by #109483.

The static_assert in ilist_node_impl::isSentinel fails when compiling LLVM as a Windows DLL with the options -D LLVM_BUILD_LLVM_DYLIB=ON -D LLVM_BUILD_LLVM_DYLIB_VIS=ON -D LLVM_LINK_LLVM_DYLIB=ON:

S:\llvm\llvm-project\llvm\include\llvm/ADT/ilist_node.h(151): error C2338: static_assert failed: 'Use ilist_sentinel_tracking<true> to enable isSentinel()'
S:\llvm\llvm-project\llvm\include\llvm/ADT/ilist_node.h(151): note: the template instantiation context (the oldest one first) is
S:\llvm\llvm-project\llvm\include\llvm/IR/SymbolTableListTraits.h(113): note: see reference to class template instantiation 'llvm::SymbolTableListTraits<llvm::Function>' being compiled
S:\llvm\llvm-project\llvm\include\llvm/IR/SymbolTableListTraits.h(69): note: see reference to class template instantiation 'llvm::simple_ilist<ValueSubClass>' being compiled
        with
        [
            ValueSubClass=llvm::Function
        ]
S:\llvm\llvm-project\llvm\include\llvm/ADT/simple_ilist.h(87): note: see reference to class template instantiation 'llvm::ilist_sentinel<llvm::ilist_detail::node_options<T,true,false,llvm::ilist_detail::extract_tag<>::type,false,llvm::ilist_detail::extract_parent<>::type>>' being compiled
        with
        [
            T=llvm::Function
        ]
S:\llvm\llvm-project\llvm\include\llvm/ADT/ilist_node.h(301): note: see reference to class template instantiation 'llvm::ilist_node_impl<OptionsT>' being compiled
        with
        [
            OptionsT=llvm::ilist_detail::node_options<llvm::Function,true,false,llvm::ilist_detail::extract_tag<>::type,false,llvm::ilist_detail::extract_parent<>::type>
        ]
S:\llvm\llvm-project\llvm\include\llvm/ADT/ilist_node.h(150): note: while compiling class template member function 'bool llvm::ilist_node_impl<OptionsT>::isSentinel(void) const'
        with
        [
            OptionsT=llvm::ilist_detail::node_options<llvm::Function,true,false,llvm::ilist_detail::extract_tag<>::type,false,llvm::ilist_detail::extract_parent<>::type>
        ]

Conditionally including the function using std::enable_if_t has the same effect of preventing the function's use when is_sentinel_tracking_explicit=false, but avoids the issue when DLL-exporting downstream classes.

Validation

Verified I no longer fail compilation due to the static_assert when building LLVM on Windows 11 with the options -D LLVM_BUILD_LLVM_DYLIB=ON -D LLVM_BUILD_LLVM_DYLIB_VIS=ON -D LLVM_LINK_LLVM_DYLIB=ON.

---

Full diff: https://github.com/llvm/llvm-project/pull/127722.diff

1 Files Affected:

• (modified) llvm/include/llvm/ADT/ilist_node.h (+3-2)

diff --git a/llvm/include/llvm/ADT/ilist_node.h b/llvm/include/llvm/ADT/ilist_node.h
index bfd50f8b3fb71..2148d6fde9be0 100644
--- a/llvm/include/llvm/ADT/ilist_node.h
+++ b/llvm/include/llvm/ADT/ilist_node.h
@@ -18,6 +18,8 @@
 #include "llvm/ADT/ilist_node_base.h"
 #include "llvm/ADT/ilist_node_options.h"
 
+#include <type_traits>
+
 namespace llvm {
 
 namespace ilist_detail {
@@ -147,9 +149,8 @@ class ilist_node_impl
   ///
   /// This requires sentinel tracking to be explicitly enabled.  Use the
   /// ilist_sentinel_tracking<true> option to get this API.
+  template <typename = std::enable_if_t<OptionsT::is_sentinel_tracking_explicit>>
   bool isSentinel() const {
-    static_assert(OptionsT::is_sentinel_tracking_explicit,
-                  "Use ilist_sentinel_tracking<true> to enable isSentinel()");
     return node_base_type::isSentinel();
   }
 };

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[kuhar]

This doesn't build with clang: https://buildkite.com/llvm-project/github-pull-requests/builds/148282#01951b84-0e6e-4b7d-8670-082d92441cc7

Should we use void_t instead?

[andrurogerz]

Fixed by using std::is_enabled_t directly on the isSentinel return type.
Verified it compiles OK with clang on Fedora as well as msvc on Windows.

[andrurogerz]

@kuhar any further requests/objections on this patch?

[kuhar]

I think we should wait on #127850 (comment) given that the code works fine in all the configurations supported at the moment.

[compnerd]

I think we should wait on #127850 (comment) given that the code works fine in all the configurations supported at the moment.

I'm not sure I see the reason for that. This is correcting a rather subtle design point in the code which has nothing to do other windows support. Can you help me understand why this is something that we should hold up?

[kuhar]

This is correcting a rather subtle design point in the code

Can we write a test for this that exercises this without adding LLM_ABI annotations? I'm warry of introducing changes without tests that are supposed to fix something that seems to be working fine.

[compnerd]

This is correcting a rather subtle design point in the code

Can we write a test for this that exercises this without adding LLM_ABI annotations? I'm warry of introducing changes without tests that are supposed to fix something that seems to be working fine.

I don't think that really makes sense. This is a bug that the compiler lets you get away with. We generally do not have tests for miscompilation from a host compiler.

[compnerd]

Merging this on behalf of @andrurogerz