February 11, 2026
Zeroes, heroes, and compiler woes
Both GCC and Clang generate strange/inefficient code
Tiny zero-check sparks big compiler chaos, commenters bring the heat
TLDR: A simple zero-check in C++ made GCC and Clang produce odd, sometimes inefficient machine code. Commenters split between “this is normal compiler behavior,” “C++ abstractions make it worse,” and “use the right flags or Rust,” with security-minded folks warning that extra stack copies can leak sensitive data.
A developer’s tiny “is this array all zeros?” function set off a big compiler drama: both GCC and Clang spit out odd, sometimes wasteful assembly even with full optimization. GCC does a quirky flag dance for size=1, a neat 8-byte compare for size=2, then goes full soap opera for size=3 with mixed checks and head-scratching jumps. Clang plays it cool for size=1, then writes zeros to the stack it never uses for size=2 and 3, finishing with a flashy bit trick.
The comments turned into a popcorn moment. the_fall insists this is normal: compilers optimize in an abstract world, then “pick whatever assembly fits,” not what looks “logical.” btdmaster blames C++ wrappers for confusing the optimizer—“write it like C and it’s cleaner.” rwmj says to compile for your machine (use -march=native) to unlock vector magic (SIMD, meaning “do many numbers at once”) and drops a clever zero-check trick. Rust fans arrive: gspr shows rustc behaving sanely and kicking into SIMD at size 4. Crypto folks like newpavlov fume that extra stack copies—see Clang—risk sensitive data, citing an LLVM issue. Meanwhile, memes fly: “two compilers, one array,” “summon the -O3 demons,” and the eternal “just trust the optimizer.” It’s half code lab, half flame war—and fully bingeable.
Key Points
- •The article tests GCC 15.2 and Clang 21.1.0 at -O3 on a C++ function that checks if std::array<int, N> is all zeros via equality with a zero-initialized array.
- •For N=1, GCC emits mov/test/sete, while Clang emits cmp/sete; both implement a zero check but via different idioms.
- •For N=2, GCC performs a 64-bit compare against zero; Clang does similar but also zero-initializes a stack slot for the local array that is not read.
- •For N=3, GCC mixes a 64-bit cmp for the first two elements and a test for the third, with additional unusual sequences to synthesize 0 or 1 in eax.
- •For N=3, Clang zeroes stack slots for the local array, then OR-reduces the array contents and uses sete based on the OR result.