[libcxx-commits] [libcxx] adeae92 - [libc++] Make _LIBCPP_ASSUME usable when it is appropriate (#91801)

[via libcxx-commits]

Author: David Benjamin
Date: 2024-09-17T11:22:27-04:00
New Revision: adeae925dcd7db0d49256f687c66a662d6604128

URL: https://github.com/llvm/llvm-project/commit/adeae925dcd7db0d49256f687c66a662d6604128
DIFF: https://github.com/llvm/llvm-project/commit/adeae925dcd7db0d49256f687c66a662d6604128.diff

LOG: [libc++] Make _LIBCPP_ASSUME usable when it is appropriate (#91801)

libc++ turned off _LIBCPP_ASSUME because turning every debug assert into
__builtin_assume tripped [1]. However, this means we can't use _LIBCPP_ASSUME
when there is a clear optimization intent. See [2] for discussion of a place 
where _LIBCPP_ASSUME would be valuable.

This patch fixes this by not undefining the definition of _LIBCPP_ASSUME and
making sure that we don't attempt to `_LIBCPP_ASSSUME` every assertion in
the library.

[1]: https://discourse.llvm.org/t/llvm-assume-blocks-optimization/71609
[2]: https://github.com/llvm/llvm-project/pull/78929#issuecomment-1936582711

Added: 
    

Modified: 
    libcxx/include/__assert

Removed: 
    


################################################################################
diff  --git a/libcxx/include/__assert b/libcxx/include/__assert
index 49769fb4d44978..90eaa6023587b9 100644
--- a/libcxx/include/__assert
+++ b/libcxx/include/__assert
@@ -23,10 +23,10 @@
        : _LIBCPP_ASSERTION_HANDLER(__FILE__ ":" _LIBCPP_TOSTRING(__LINE__) ": assertion " _LIBCPP_TOSTRING(            \
              expression) " failed: " message "\n"))
 
-// TODO: __builtin_assume can currently inhibit optimizations. Until this has been fixed and we can add
-//       assumptions without a clear optimization intent, disable that to avoid worsening the code generation.
-//       See https://discourse.llvm.org/t/llvm-assume-blocks-optimization/71609 for a discussion.
-#if 0 && __has_builtin(__builtin_assume)
+// WARNING: __builtin_assume can currently inhibit optimizations. Only add assumptions with a clear
+// optimization intent. See https://discourse.llvm.org/t/llvm-assume-blocks-optimization/71609 for a
+// discussion.
+#if __has_builtin(__builtin_assume)
 #  define _LIBCPP_ASSUME(expression)                                                                                   \
     (_LIBCPP_DIAGNOSTIC_PUSH _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wassume")                                              \
          __builtin_assume(static_cast<bool>(expression)) _LIBCPP_DIAGNOSTIC_POP)
@@ -44,18 +44,18 @@
 #  define _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(expression, message)    _LIBCPP_ASSERT(expression, message)
 // Disabled checks.
 // On most modern platforms, dereferencing a null pointer does not lead to an actual memory access.
-#  define _LIBCPP_ASSERT_NON_NULL(expression, message)                _LIBCPP_ASSUME(expression)
+#  define _LIBCPP_ASSERT_NON_NULL(expression, message)                ((void)0)
 // Overlapping ranges will make algorithms produce incorrect results but don't directly lead to a security
 // vulnerability.
-#  define _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(expression, message)  _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_VALID_DEALLOCATION(expression, message)      _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_VALID_EXTERNAL_API_CALL(expression, message) _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_COMPATIBLE_ALLOCATOR(expression, message)    _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(expression, message)  _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_PEDANTIC(expression, message)                _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(expression, message)    _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_INTERNAL(expression, message)                _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_UNCATEGORIZED(expression, message)           _LIBCPP_ASSUME(expression)
+#  define _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(expression, message)  ((void)0)
+#  define _LIBCPP_ASSERT_VALID_DEALLOCATION(expression, message)      ((void)0)
+#  define _LIBCPP_ASSERT_VALID_EXTERNAL_API_CALL(expression, message) ((void)0)
+#  define _LIBCPP_ASSERT_COMPATIBLE_ALLOCATOR(expression, message)    ((void)0)
+#  define _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(expression, message)  ((void)0)
+#  define _LIBCPP_ASSERT_PEDANTIC(expression, message)                ((void)0)
+#  define _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(expression, message)    ((void)0)
+#  define _LIBCPP_ASSERT_INTERNAL(expression, message)                ((void)0)
+#  define _LIBCPP_ASSERT_UNCATEGORIZED(expression, message)           ((void)0)
 
 // Extensive hardening mode checks.
 
@@ -73,8 +73,8 @@
 #  define _LIBCPP_ASSERT_PEDANTIC(expression, message)                _LIBCPP_ASSERT(expression, message)
 #  define _LIBCPP_ASSERT_UNCATEGORIZED(expression, message)           _LIBCPP_ASSERT(expression, message)
 // Disabled checks.
-#  define _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(expression, message)    _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_INTERNAL(expression, message)                _LIBCPP_ASSUME(expression)
+#  define _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(expression, message)    ((void)0)
+#  define _LIBCPP_ASSERT_INTERNAL(expression, message)                ((void)0)
 
 // Debug hardening mode checks.
 
@@ -99,18 +99,18 @@
 #else
 
 // All checks disabled.
-#  define _LIBCPP_ASSERT_VALID_INPUT_RANGE(expression, message)       _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(expression, message)    _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_NON_NULL(expression, message)                _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(expression, message)  _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_VALID_DEALLOCATION(expression, message)      _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_VALID_EXTERNAL_API_CALL(expression, message) _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_COMPATIBLE_ALLOCATOR(expression, message)    _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(expression, message)  _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_PEDANTIC(expression, message)                _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(expression, message)    _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_INTERNAL(expression, message)                _LIBCPP_ASSUME(expression)
-#  define _LIBCPP_ASSERT_UNCATEGORIZED(expression, message)           _LIBCPP_ASSUME(expression)
+#  define _LIBCPP_ASSERT_VALID_INPUT_RANGE(expression, message)       ((void)0)
+#  define _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(expression, message)    ((void)0)
+#  define _LIBCPP_ASSERT_NON_NULL(expression, message)                ((void)0)
+#  define _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(expression, message)  ((void)0)
+#  define _LIBCPP_ASSERT_VALID_DEALLOCATION(expression, message)      ((void)0)
+#  define _LIBCPP_ASSERT_VALID_EXTERNAL_API_CALL(expression, message) ((void)0)
+#  define _LIBCPP_ASSERT_COMPATIBLE_ALLOCATOR(expression, message)    ((void)0)
+#  define _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(expression, message)  ((void)0)
+#  define _LIBCPP_ASSERT_PEDANTIC(expression, message)                ((void)0)
+#  define _LIBCPP_ASSERT_SEMANTIC_REQUIREMENT(expression, message)    ((void)0)
+#  define _LIBCPP_ASSERT_INTERNAL(expression, message)                ((void)0)
+#  define _LIBCPP_ASSERT_UNCATEGORIZED(expression, message)           ((void)0)
 
 #endif // _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_FAST
 // clang-format on