Nested arrays are completely loop-unrolled into a huge amount of instructions (feat. 153 MB binary) #143673
Description
I tried this code:
#[unsafe(no_mangle)] pub fn foo(f: fn([[[[i32; 16]; 16]; 16]; 16])) { let x = [[[[1i32; 16]; 16]; 16]; 16]; f(x); }I expected this to compile into a loop of some kind. But when I compiled this with -Copt-level=3, I got a huuuuge completely-unrolled loop:
Part of the generated assembly
.LCPI0_0: .long 1 .long 1 .long 1 .long 1 foo: mov r11, rsp sub r11, 262144 .LBB0_1: sub rsp, 4096 mov qword ptr [rsp], 0 cmp rsp, r11 jne .LBB0_1 sub rsp, 8 mov rax, rdi movaps xmm0, xmmword ptr [rip + .LCPI0_0] movaps xmmword ptr [rsp], xmm0 movaps xmmword ptr [rsp + 16], xmm0 movaps xmmword ptr [rsp + 32], xmm0 movaps xmmword ptr [rsp + 48], xmm0 movaps xmmword ptr [rsp + 64], xmm0 movaps xmmword ptr [rsp + 80], xmm0 movaps xmmword ptr [rsp + 96], xmm0 ; A large amount of assembly code omitted for brevity movaps xmmword ptr [rsp + 15936], xmm0 movaps xmmword ptr [rsp + 15952], xmm0 movaps xmmword ptr [rsp + 15984], xmm0 movaps xmmword ptr [rsp + 16000], xmm0 ; A large amount of assembly code omitted for brevity movaps xmmword ptr [rsp + 262048], xmm0 movaps xmmword ptr [rsp + 262064], xmm0 movaps xmmword ptr [rsp + 262080], xmm0 movaps xmmword ptr [rsp + 262096], xmm0 movaps xmmword ptr [rsp + 262112], xmm0 movaps xmmword ptr [rsp + 262128], xmm0 movaps xmmword ptr [rsp + 15968], xmm0 mov rdi, rsp call rax add rsp, 262152 retI also tried compiling the following code locally with cargo build --release (since godbolt has a compilation time limit), and I got a 153 MB rlib.
#[unsafe(no_mangle)] pub fn foo(f: fn([[[[[[[[[[i32; 16]; 16]; 16]; 16]; 16]; 16]; 16]; 16]; 16]; 16])) { let x = [[[[[[[[[[1i32; 16]; 16]; 16]; 16]; 16]; 16]; 16]; 16]; 16]; 16]; f(x); }Meta
rustc version on godbolt:
rustc 1.88.0 (6b00bc388 2025-06-23) binary: rustc commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc commit-date: 2025-06-23 host: x86_64-unknown-linux-gnu release: 1.88.0 LLVM version: 20.1.5 Internal compiler ID: r1880 rustc version locally:
rustc 1.88.0 (6b00bc388 2025-06-23) binary: rustc commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc commit-date: 2025-06-23 host: aarch64-apple-darwin release: 1.88.0 LLVM version: 20.1.5 @rustbot labels +I-heavy