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[libc++] Make _LIBCPP_ASSUME usable when it is appropriate (llvm#91801)
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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]: llvm#78929 (comment)
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@davidben @tmsri
davidben authored and tmsri committed Sep 19, 2024
1 parent 2777c56 commit 909250d
Showing 1 changed file with 28 additions and 28 deletions.
56 changes: 28 additions & 28 deletions libcxx/include/__assert
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
Expand All @@ -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.

Expand All @@ -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.

Expand All @@ -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
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