Linus Torvalds and the Supposedly "Garbage Code"

Summary

Linus Torvalds criticized a RISC-V Linux kernel contribution from a Google engineer as “garbage code.” The discussion focuses on the helper function make_u32_from_two_u16() versus Linus’s proposed explicit code. Let’s discuss the importance of using proper type casting, bit manipulation, and creating a safer, reusable macro or function for clarity and bug reduction. Recently, Linus Torvalds publicly dismissed a RISC-V code contribution to the Linux kernel made by a Google engineer as “garbage code”: https://lkml.org/lkml/2025/8/9/76 First, I think Linus should be more respectful of other people. In addition, let’s focus on the make_u32_from_two_u16() helper. My understanding is that this is a C preprocessor macro (as the Linux Kernel is mainly written in C). Let’s compare that helper with the explicit code “(a << 16) + b” proposed by Linus. First, this explicit code is likely wrong, and in fact Linus adds that “maybe you need to add a cast”. Why should we add a cast? In Linus’s words: “[…] to make sure that ‘b’ doesn’t have high bits that pollutes the end result”. So, what should the explicit code look like according to him? “(a << 16) + (uint16_t)b”? But let’s do a step back. We should ask ourselves: What are the types of ‘a’ and ‘b’? From the helper’s name, I would think they are two “u16”, so two uint16_t. If I was asked to write C code that takes two uint16_t values ‘a’ and ‘b’ as input and combines them into a uint32_t, I would write something like that: ((uint32_t)a << 16) | (uint32_t)b I would use the bitwise OR (|) instead of +; I find it more appropriate as we are working at the bit manipulation level here. But maybe that’s just a matter of personal preference and coding style. Moreover, I’d use the type casts as shown above, on both ‘a’ and ‘b’. I’m not sure what Linus meant with ‘b’ potentially having “high bits that pollutes the end result”. Could ‘b’ be a uint32_t? In that case, I would use a bitmask like 0xFFFF with bitwise AND (&) to clear the high bi...