[libc][msan] Fix "non-constexpr function '__msan_unpoison' cannot be used in a constant expression"

[gchatelet]

Prior to this patch, calling cpp::bit_cast<T> in constexpr expressions under -fsanitize=memory would fail with the following message "non-constexpr function '__msan_unpoison' cannot be used in a constant expression".

This patch makes sure that the __msan_unpoison expression is guarded by !__builtin_is_constant_evaluated().

[llvmbot]

@llvm/pr-subscribers-libc

Author: Guillaume Chatelet (gchatelet)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/88719.diff

1 Files Affected:

• (modified) libc/src/__support/macros/sanitizer.h (+11-4)

diff --git a/libc/src/__support/macros/sanitizer.h b/libc/src/__support/macros/sanitizer.h
index bd9b62b7121a14..9a2fe8c4bcc14d 100644
--- a/libc/src/__support/macros/sanitizer.h
+++ b/libc/src/__support/macros/sanitizer.h
@@ -47,14 +47,21 @@
 // Functions to unpoison memory
 //-----------------------------------------------------------------------------
 
-#if defined(LIBC_HAVE_MEMORY_SANITIZER) && __has_builtin(__builtin_constant_p)
-// Only perform MSAN unpoison in non-constexpr context.
+#if defined(LIBC_HAVE_MEMORY_SANITIZER)
+// Only perform MSAN unpoison in non-constexpr context and silence
+// '-Wconstant-evaluated' when MSAN_UNPOISON is called from manifestly constant
+// contexts.
 #include <sanitizer/msan_interface.h>
 #define MSAN_UNPOISON(addr, size)                                              \
   do {                                                                         \
-    if (!__builtin_constant_p(*addr)) {                                        \
+    _Pragma("GCC diagnostic push \"-Wconstant-evaluated\"");                   \
+    _Pragma("GCC diagnostic ignored \"-Wconstant-evaluated\"");                \
+    _Pragma("clang diagnostic push \"-Wconstant-evaluated\"");                 \
+    _Pragma("clang diagnostic ignored \"-Wconstant-evaluated\"");              \
+    if constexpr (!__builtin_is_constant_evaluated())                          \
       __msan_unpoison(addr, size);                                             \
-    }                                                                          \
+    _Pragma("clang diagnostic pop");                                           \
+    _Pragma("GCC diagnostic pop");                                             \
   } while (0)
 #else
 #define MSAN_UNPOISON(ptr, size)

[nickdesaulniers]

@ramosian-glider should __msan_unpoison be callable from constexpr contexts?

[gchatelet]

Thx for challenging my patch 🙏👍

[nickdesaulniers]

Thanks for the fix!