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`cg_gcc` does not respect -Ctarget-cpu, and uses the host feature set by default.

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Issue 类型
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描述清晰度
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技术栈
rust
领域
compilers

调研方向

首先追踪 cg_gcc 如何处理 Rust 目标特性和 -Ctarget-cpu 选项，然后比较默认目标与 x86-64-v1 或 nehalem 的生成汇编。默认构建避免使用不受支持的特定于主机的指令，并且包括 nehalem 在内的显式目标 CPU 反映在生成的汇编中后，该 issue 即完成。

由索引模型根据 Issue 内容生成。

描述

By default, the Rust compiler uses the x86-64-v1 feature level, which does not include the AVX instructions.

cg_gcc, however, creates assembly files containing those instructions.

Example. 

use std::hint::black_box;
#[unsafe(no_mangle)]
fn reverse_u8() {
    // odd length and offset by 1 to be as unaligned as possible
    let mut v: Vec<_> = (0..1 + (0xFFFFF / size_of::<u8>() as u64))
        .map(|x| x as u8)
        .collect();
    black_box( black_box(&mut v[1..]).reverse());
}

As you can see(by analysing the assembly created by GCC and LLVM), GCC uses AVX instructions, while LLVM does not.

;  Sample of  GCC code 
        mov     r11d, 4
        vpunpcklqdq     xmm9, xmm6, xmm6
        vmovdqa xmm14, XMMWORD PTR .LC8[rip]
        vpunpcklqdq     xmm5, xmm5, xmm5
        vmovq   xmm6, rsi
        sal     rdi, 4
        lea     rax, -16[r9+rsi]
        vmovdqa XMMWORD PTR 80[rsp], xmm5
        vmovq   xmm5, r11
        mov     r11d, 6
        vpunpcklqdq     xmm6, xmm6, xmm6
        vpunpcklqdq     xmm5, xmm5, xmm5
        mov     r10, rax
        vmovdqa XMMWORD PTR 64[rsp], xmm6
        vmovdqa xmm15, XMMWORD PTR .LC0[rip]
        vmovdqa XMMWORD PTR 96[rsp], xmm5
; Sample of LLVM code 
        movdqa  xmm0, xmmword ptr [rip + .LCPI0_0]
        mov     ecx, 112
        movdqa  xmm1, xmmword ptr [rip + .LCPI0_1]
        movdqa  xmm2, xmmword ptr [rip + .LCPI0_2]
        movdqa  xmm3, xmmword ptr [rip + .LCPI0_3]
        movdqa  xmm4, xmmword ptr [rip + .LCPI0_4]
        movdqa  xmm5, xmmword ptr [rip + .LCPI0_5]
        movdqa  xmm6, xmmword ptr [rip + .LCPI0_6]
        movdqa  xmm7, xmmword ptr [rip + .LCPI0_7]
        movdqa  xmm8, xmmword ptr [rip + .LCPI0_8]

LLVM-based rustc will only emit AVX instructions when they are explicitly enabled(-Ctarget-cpu=x86-64-v3).

; LLVM code compiled with AVX enabled 
        je      .LBB0_18
        mov     ecx, 480
        vmovdqa ymm0, ymmword ptr [rip + .LCPI0_0]
        vmovdqa ymm1, ymmword ptr [rip + .LCPI0_1]
        vmovdqa ymm2, ymmword ptr [rip + .LCPI0_2]
        vmovdqa ymm3, ymmword ptr [rip + .LCPI0_3]
        vmovdqa ymm4, ymmword ptr [rip + .LCPI0_4]
        vmovaps ymm5, ymmword ptr [rip + .LCPI0_5]
        vmovaps ymm6, ymmword ptr [rip + .LCPI0_6]
        vmovaps ymm7, ymmword ptr [rip + .LCPI0_7]

cg_gcc should mimic this behaviour. Currently, it seems like cg_gcc just inherits the target features of the host processor, which is inccorect(a processor with AVX should still build code that does not depend on AVX).

Moreover, -Ctarget-cpu seems to be currently not respected.

Despite explicitly setting -Ctarget-cpu=nehalem, I can see that cg_gcc still uses AVX instructions. That CPU does not support them.

https://godbolt.org/z/7s6Gss8bE

This will lead code compiled by cg_gcc to crash on CPUs older than the build host.

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