Commit 2a2970af67a5602f1f25e048510f571061791168
2021-07-09T15:35:56
Neon/AArch32: Work around Clang T32 miscompilation Referring to the C standard (http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf, J.2 Undefined behavior), the behavior of the compiler is undefined if "conversion between two pointer types produces a result that is incorrectly aligned." Thus, the behavior of this code *((uint32_t *)buffer) = BUILTIN_BSWAP32(put_buffer); in the AArch32 version of the FLUSH() macro is undefined unless 'buffer' is 32-bit-aligned. Referring to https://bugs.llvm.org/show_bug.cgi?id=50785, certain versions of Clang, when generating Thumb (T32) instructions, miscompile that code into an assembly instruction (stm) that requires the destination to be 32-bit-aligned. Since such alignment cannot be guaranteed within the Huffman encoder, this reportedly led to crashes (SIGBUS: illegal alignment) with AArch32/Thumb builds of libjpeg-turbo running on Android devices, although thus far I have been unable to reproduce those crashes with a plain Linux/Arm system. The miscompilation is visible with the Compiler Explorer: https://godbolt.org/z/rv1ccx1Pb However, it goes away when removing the return statement from the function. Thus, it seems that Clang's behavior in this regard is somewhat variable, which may explain why the crashes are only reproducible on certain platforms. The suggested workaround is to use memcpy(), but whereas Clang and recent GCC releases are smart enough to compile a 4-byte memcpy() call into a str instruction, GCC < 6 is not. Referring to https://godbolt.org/z/ae7Wje3P6, the only way to consistently produce the desired str instruction across all supported compilers is to use inline assembly. Visual C++ presumably does not miscompile the code in question, since no issues have been reported with it, but since the code relies on undefined compiler behavior, prudence dictates that e4ec23d7ae051c1c73947f889818900362fdc52d should be reverted for Visual C++, which this commit does. The performance impact of e4ec23d7ae051c1c73947f889818900362fdc52d for Visual C++/Arm builds is unknown (I have no ability to test such builds), but regardless, this commit reverts the Visual C++/Arm performance to that of libjpeg-turbo 2.1 beta1. Closes #529