[Clang] [CodeGen] Fix codegen bug in constant initialisation in C23 mode

[Sirraide]

Consider the following code:

bool const inf =  (1.0/0.0);

When trying to emit the initialiser of this variable in C23, we end up hitting a code path in codegen in VarDecl::evaluateValueImpl() where we check for IsConstantInitialization && (Ctx.getLangOpts().CPlusPlus || Ctx.getLangOpts().C23), and if that is the case and we emitted any notes, constant evaluation fails, and as a result, codegen issues this error:

<source>:1:12: error: cannot compile this static initializer yet
    1 | bool const inf =  (1.0/0.0);
      |        

As a fix, only fail in C23 mode if we’re initialising a constexpr variable.

This fixes #84784.

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-codegen

Author: None (Sirraide)

Changes

Consider the following code:

bool const inf =  (1.0/0.0);

When trying to emit the initialiser of this variable in C23, we end up hitting a code path in codegen in VarDecl::evaluateValueImpl() where we check for IsConstantInitialization && (Ctx.getLangOpts().CPlusPlus || Ctx.getLangOpts().C23), and if that is the case and we emitted any notes, constant evaluation fails, and as a result, codegen issues this error:

<source>:1:12: error: cannot compile this static initializer yet
    1 | bool const inf =  (1.0/0.0);
      |        

As a fix, only fail in C23 mode if we’re initialising a constexpr variable.

This fixes #84784.

---

Full diff: https://github.com/llvm/llvm-project/pull/84981.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+3)
• (modified) clang/lib/AST/Decl.cpp (+3-1)
• (modified) clang/test/CodeGen/const-init.c (+3)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f61dca9bbc8467..c773991b43f47c 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -259,6 +259,9 @@ Bug Fixes in This Version
   operator.
   Fixes (#GH83267).
 
+- Clang no longer fails to codegen static ``const`` variables whose initialiser performs
+  a floating-point division by 0 in C23.
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index d681791d3920c3..38317dea6d22fe 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -2581,7 +2581,9 @@ APValue *VarDecl::evaluateValueImpl(SmallVectorImpl<PartialDiagnosticAt> &Notes,
   // case, we can't keep the result, because it may only be correct under the
   // assumption that the initializer is a constant context.
   if (IsConstantInitialization &&
-      (Ctx.getLangOpts().CPlusPlus || Ctx.getLangOpts().C23) && !Notes.empty())
+      (Ctx.getLangOpts().CPlusPlus ||
+       (isConstexpr() && Ctx.getLangOpts().C23)) &&
+      !Notes.empty())
     Result = false;
 
   // Ensure the computed APValue is cleaned up later if evaluation succeeded,
diff --git a/clang/test/CodeGen/const-init.c b/clang/test/CodeGen/const-init.c
index 0e4fc4ad48af8d..ad3e9551199ac2 100644
--- a/clang/test/CodeGen/const-init.c
+++ b/clang/test/CodeGen/const-init.c
@@ -216,3 +216,6 @@ int PR4517_x2 = PR4517_arrc[PR4517_idx];
 // CHECK: @PR4517_x = global i32 42, align 4
 // CHECK: @PR4517_idx = constant i32 1, align 4
 // CHECK: @PR4517_x2 = global i32 42, align 4
+
+// CHECK: @GH84784_inf = constant i8 1
+_Bool const GH84784_inf = (1.0/0.0);

[AaronBallman]

Thank you for the fix! This is looking pretty good to me.

[Sirraide]

I’ve added the test. The CI failure is the same thing from a few days ago I think; must be because I created this branch before updating the main branch.

[AaronBallman]

LGTM!

[Sirraide]

CI is just going to fail again because of the aforementioned problem, so I’m just going to merge this now and hope I won’t have to revert it later.