ARM，VFP，浮点，惰性上下文切换 [英] ARM, VFP, floating-point, lazy context switching

问题描述

我正在编写用于ARM处理器(Cortex-A9)的操作系统.

I am writing an operating system for an ARM processor (Cortex-A9).

我正在尝试实现浮点寄存器的惰性上下文切换.其背后的想法是，最初为线程禁用了浮点扩展，因此无需在任务开关上保存浮点上下文.

I am trying to implement lazy context switching of the floating-point registers. The idea behind this is that the floating-point extension is initially disabled for a thread and so there is no need to save floating-point context on a task-switch.

当线程尝试使用浮点指令时，它将触发异常.然后，操作系统启用浮点扩展，并知道必须在下一个上下文切换中为此线程保存浮点上下文.然后，重新执行浮点指令.

When a thread attempts to use a floating-point instruction, it triggers an exception. The operating system then enables floating-point extension and knows that floating-point context must be saved for this thread in the next context switches. Then the floating-point instruction is re-executed.

我的问题是，即使C代码中没有使用浮点运算，编译器也会生成浮点指令. 这是反汇编不使用c中浮点的函数的示例:

My problem is that the compiler generates floating-point instructions even when no floating-point operations are used in c code. This is an example of a disassembly of a function that uses no floating point in c:

10002f5c <rmtcpy_from>:
10002f5c:   e1a0c00d    mov ip, sp
10002f60:   e92ddff0    push    {r4, r5, r6, r7, r8, r9, sl, fp, ip, lr, pc}
10002f64:   e24cb004    sub fp, ip, #4
10002f68:   ed2d8b02    vpush   {d8}
...
10002f80:   ee082a10    vmov    s16, r2
...
10002fe0:   ee180a10    vmov    r0, s16
...
1000308c:   ecbc8b02    vldmia  ip!, {d8}
...

当我有许多这样的功能时，惰性上下文切换毫无意义.

When I have many of such functions, lazy context switching makes no sense.

有人知道如何告诉编译器仅在c代码中存在浮点运算时才生成浮点指令吗?

Does anybody know how to tell the compiler that floating-point instructions should only be generated when there is a floating point operation in the c code ?

我使用gcc 9.2.0.浮点选项为:-mhard-float -mfloat-abi=hard -mfpu=vfp

I use gcc 9.2.0. The floating point options are: -mhard-float -mfloat-abi=hard -mfpu=vfp

这是一个示例c函数(不可用，仅是一个演示):

Here is a example c function (not useable, only a demo):

void func(char *a1, char *a2, char *a3);
int bar_1[1], foo_1, foo_2;

void fpu_test() {
    int oldest_idx = -1;
    while (1) {
        int *oldest = (int *)0;
        int idx = oldest_idx;
        for (int i = 0; i < 3; i++) {
            if (++idx >= 3)
                idx = 0;
            int *lec = &bar_1[idx];
            if (*lec) {
                if (*lec - *oldest < 0) {
                    oldest = lec;
                    oldest_idx = idx;
                }
            }
        }
        if (oldest) {
            foo_1++;
            if (foo_2)
                func("1", "2", "3");
        }
    }
}

gcc命令行:

$HOME/devel/opt/cross-musl/bin/arm-linux-musleabihf-gcc  -O2 -march=armv7-a -mtune=cortex-a9 -mhard-float -mfloat-abi=hard -mfpu=vfp -Wa,-ahlms=fpu_test.lst -mapcs-frame -c fpu_test.c -o fpu_test.o

组装商列表:

...
  35 0000 0DC0A0E1      mov ip, sp
  36 0004 003000E3      movw    r3, #:lower16:foo_2
  37 0008 F0DF2DE9      push    {r4, r5, r6, r7, r8, r9, r10, fp, ip, lr, pc}
  38 000c 006000E3      movw    r6, #:lower16:foo_1
  39 0010 003040E3      movt    r3, #:upper16:foo_2
  40 0014 04B04CE2      sub fp, ip, #4
  41 0018 006040E3      movt    r6, #:upper16:foo_1
  42 001c 004000E3      movw    r4, #:lower16:bar_1
  43 0020 028B2DED      vpush.64    {d8}                <=== this is the problem
...

推荐答案

GCC为此提供了一个命令行开关-mgeneral-regs-only..使用命令行开关时，可能需要将故意使用浮点寄存器或操作的代码分离到单独的源文件中，以便无需该开关即可对其进行编译.

GCC has a command-line switch for this, -mgeneral-regs-only.. When using the command-line switch, you may need to separate code that deliberately uses floating-point registers or operations into separate source files so that it can be compiled without that switch.

从GCC 9.3开始(也许是9 ?)，对于ARM目标，可以作为函数属性使用:

void MyFunction(char *MyParameter) __attribute__ ((general-regs-only));

在声明之后放置属性是较旧的语法，并且需要非定义声明.测试表明，GCC现在可以在声明器之前接受属性，并且可以与定义一起使用:

Putting the attribute after the declaration is an older syntax and required a non-definition declaration. Testing suggests GCC now accepts an attribute before the declarator and may be used with a definition:

void __attribute__ ((general-regs-only)) MyFunction(char *MyParameter)
{...}

您也许也可以使用__attribute__ ((nogeneral-regs-only))否定该属性，尽管我没有对此进行记录.

You may also be able to negate the attribute with __attribute__ ((nogeneral-regs-only)), although I do not see this documented.

这也可以通过在-march和-mcpu开关中也有+nofp选项，但是我认为-mgeneral-regs-only是您想要的.

There are also +nofp options within the -march and -mcpu switches, but I think -mgeneral-regs-only is what you want.

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