[libc] -Warray-bounds= in libc/src/string/memory_utils/op_builtin.h #76877
Description
building with GCC produces the following diagnostic:
In static member function ‘static void __llvm_libc_18_0_0_git::builtin::Memcpy<Size>::block_offset(__llvm_libc_18_0_0_git::Ptr, __llvm_libc_18_0_0_git::CPtr, size_t) [with long unsigned int Size = 32]’, inlined from ‘static void __llvm_libc_18_0_0_git::builtin::Memcpy<Size>::tail(__llvm_libc_18_0_0_git::Ptr, __llvm_libc_18_0_0_git::CPtr, size_t) [with long unsigned int Size = 32]’ at /android0/llvm-project/libc/src/string/memory_utils/op_builtin.h:44:17, inlined from ‘static void __llvm_libc_18_0_0_git::builtin::Memcpy<Size>::loop_and_tail_offset(__llvm_libc_18_0_0_git::Ptr, __llvm_libc_18_0_0_git::CPtr, size_t, size_t) [with long unsigned int Size = 32]’ at /android0/llvm-project/libc/src/string/memory_utils/op_builtin.h:61:9, inlined from ‘static void __llvm_libc_18_0_0_git::builtin::Memcpy<Size>::loop_and_tail(__llvm_libc_18_0_0_git::Ptr, __llvm_libc_18_0_0_git::CPtr, size_t) [with long unsigned int Size = 32]’ at /android0/llvm-project/libc/src/string/memory_utils/op_builtin.h:66:32, inlined from ‘void __llvm_libc_18_0_0_git::inline_memcpy_x86_sse2_ge64(Ptr, CPtr, size_t)’ at /android0/llvm-project/libc/src/string/memory_utils/x86_64/inline_memcpy.h:57:44, inlined from ‘void __llvm_libc_18_0_0_git::inline_memcpy_x86(Ptr, CPtr, size_t)’ at /android0/llvm-project/libc/src/string/memory_utils/x86_64/inline_memcpy.h:193:41, inlined from ‘void __llvm_libc_18_0_0_git::inline_memcpy_x86_maybe_interpose_repmovsb(Ptr, CPtr, size_t)’ at /android0/llvm-project/libc/src/string/memory_utils/x86_64/inline_memcpy.h:204:29, inlined from ‘void __llvm_libc_18_0_0_git::inline_memcpy(void*, const void*, size_t)’ at /android0/llvm-project/libc/src/string/memory_utils/inline_memcpy.h:45:38, inlined from ‘int __llvm_libc_18_0_0_git::printf_core::Writer::write(__llvm_libc_18_0_0_git::cpp::string_view)’ at /android0/llvm-project/libc/src/stdio/printf_core/writer.h:98:20, inlined from ‘int __llvm_libc_18_0_0_git::printf_core::convert_inf_nan(Writer*, const FormatSection&)’ at /android0/llvm-project/libc/src/stdio/printf_core/float_inf_nan_converter.h:68:5: /android0/llvm-project/libc/src/string/memory_utils/op_builtin.h:34:39: warning: array subscript [66, 9223372036854775807] is outside array bounds of ‘const char [4]’ [-Warray-bounds=] 34 | dst[i + offset] = src[i + offset]; | ~~~~~~~~~~~~~~^ FWICT, Memcpy::block_offset falls back to a element by element copy if LLVM_LIBC_HAS_BUILTIN_MEMCPY_INLINE is not defined (hence this only being reproducible with GCC).
IIUC, it looks like inline_memcpy_x86 is generating a 32 element Memcpy (Size = 32), which indeed would read outside of src for a string literal like is done in convert_inf_nan
RET_IF_RESULT_NEGATIVE(writer->write(a == 'a' ? "inf" : "INF"));in that case, I don't see why
RET_IF_RESULT_NEGATIVE(writer->write(sign_char));doesn't warn since sign_char is a char.
I'm not sure yet if this can/should be fixed in convert_inf_nan, Memcpy, or somewhere else. But we need to fix this to re-enable -Werror in #74506