这是使用-falign-循环选项时，GCC错误？ [英] Is this a GCC bug when using -falign-loops option?

问题描述

我在玩此选项可以在我们的嵌入式架构优化的循环（的这里）。然而，我注意到，当对准需要多于一个 NOP 指令以复加，那么编译器生成一个 NOP 其次为一对多，为要求的零（ 0000 ）。

I was playing with this option to optimize a for-loop in our embedded architecture (here). However, I noticed that when the alignment requires more than a single nop instruction to be added, then the compiler generates one nop followed by as-many-as-required zeros (0000).

我怀疑它是在我们的编译器错误，但有人可以证实这不是在GCC一个错误？

I suspect it is a bug in our compiler, but can someone confirm it is not a bug in GCC?

下面是一个code片断：

Here's a code snippet:

    __asm__ volatile("nop");  
    __asm__ volatile("nop");  

    for (j0=0; j0<N; j0+=4)
    {
        c[j0+ 0] = a[j0+ 0] + b[j0+ 0];
        c[j0+ 1] = a[j0+ 1] + b[j0+ 1];
        c[j0+ 2] = a[j0+ 2] + b[j0+ 2];
        c[j0+ 3] = a[j0+ 3] + b[j0+ 3];
    }

编译与 -falign-循环= 8 （或任何这比所要求的最小对齐有关您的架构号）。您可以添加或删除在必要时产生错位循环体的 __ ASM __ 行。

Compile with -falign-loops=8 (or whatever number relevant to your architecture which is more than the required minimum alignment). You can add or remove the __asm__ lines as necessary to generate misaligned loop body.

推荐答案

使用的gcc -o -S文件foo.c foo.s 来生成汇编输出没有装配它。我怀疑你会看到 .balign 或 .p2align 指令在ASM。假设该指令的目的是工作，我认为这是在汇编中的错误。也有可能你已经把code在非默认的部分（即不的.text ）无论是有意或无意地（例如，使用一个错位的 .DATA 或 .section伪在其他一些内联汇编）;通常汇编垫有适当的大小和数量的 NOP 指令包含code节和0字节包含数据的部分。

Use gcc -S -o foo.s foo.c to generate the assembly output without assembling it. I suspect you'll see the .balign or .p2align directive in the asm. Assuming this directive is intended to work, I think it's a bug in the assembler. It's also possible that you've put the code in a non-default section (i.e. not .text) either intentionally or accidentally (e.g. with a misplaced .data or .section in some other inline asm); normally the assembler pads with the proper size and number of nop instructions for sections that contain code, and 0 bytes for sections that contain data.

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