[libc-commits] [PATCH] D148739: [libc] Use __builtin_bit_cast only when src and dest types are trivially copyable

[Mikhail Ramalho via Phabricator via libc-commits]

mikhail.ramalho created this revision.
mikhail.ramalho added reviewers: michaelrj, sivachandra.
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This patch fixes the following compilation error:

/home/mgadelha/tools/llvm-project/libc/src/__support/CPP/bit.h:29:10: error: __builtin_bit_cast source type must be trivially copyable

  return __builtin_bit_cast(To, from);
         ^

/home/mgadelha/tools/llvm-project/libc/src/__support/FPUtil/FPBits.h:119:47: note: in instantiation of function template specialization '__llvm_libc::cpp::bit_cast<long double, __llvm_libc::cpp::UInt<128>>' requested here

  LIBC_INLINE T get_val() const { return cpp::bit_cast<T>(bits); }

This happen when building for a 32 bit target (in my case it was
riscv32) without int128 support, so we use UInt<128> instead, which
is not trivially copyable.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D148739

Files:
  libc/src/__support/CPP/bit.h


Index: libc/src/__support/CPP/bit.h
===================================================================
--- libc/src/__support/CPP/bit.h
+++ libc/src/__support/CPP/bit.h
@@ -11,6 +11,8 @@
 
 #include "src/__support/macros/config.h" // LIBC_HAS_BUILTIN
 
+#include <type_traits>
+
 namespace __llvm_libc::cpp {
 
 #if LIBC_HAS_BUILTIN(__builtin_bit_cast)
@@ -26,8 +28,10 @@
 template <class To, class From> constexpr To bit_cast(const From &from) {
   static_assert(sizeof(To) == sizeof(From), "To and From must be of same size");
 #if defined(LLVM_LIBC_HAS_BUILTIN_BIT_CAST)
-  return __builtin_bit_cast(To, from);
-#else
+  if constexpr (std::is_trivially_copyable_v<To> &&
+                std::is_trivially_copyable_v<From>)
+    return __builtin_bit_cast(To, from);
+#endif // defined(LLVM_LIBC_HAS_BUILTIN_BIT_CAST)
   To to;
   char *dst = reinterpret_cast<char *>(&to);
   const char *src = reinterpret_cast<const char *>(&from);
@@ -38,7 +42,6 @@
     dst[i] = src[i];
 #endif // defined(LLVM_LIBC_HAS_BUILTIN_MEMCPY_INLINE)
   return to;
-#endif // defined(LLVM_LIBC_HAS_BUILTIN_BIT_CAST)
 }
 
 } // namespace __llvm_libc::cpp


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