苹果作为和的ARM / Thumb ADDS指令 [英] Apple AS and ARM/Thumb ADDS instruction

问题描述

我工作的一个iPhone / iPad的项目，我想更新过程中的一些状态寄存器（不是全部）的算术运算。默认情况下，X code使用编译为Thumb'，我不想改变它。

I'm working on an iPhone/iPad project, and I want to update the status register during some (not all) arithmetic operations. By default, Xcode uses 'Compile for Thumb' and I don't want to change it.

以下GCC内联汇编code正常工作搀扶下，但会导致拇指下的编译错误：'指令Thumb16模式不支持 - 增加了R6，R4，R5'。问题就出在状态寄存器更新。 （我也知道， movcs 和 strcs 将需要改变）。

The following GCC inline assembly code works fine under ARM, but results in a compile error under Thumb: 'instruction not supported in Thumb16 mode - adds r6,r4,r5'. The problem lies in the status register update. (I'm also aware that movcs and strcs will need to be changed).

拇指是否有一个ADD指令用于设置溢出（V）或携带（C）在CPSR？如果不是，是否有特定的Thumb-汇编级的解决方法，以测试溢出并执行？

Does Thumb have an ADD instruction which sets Overflow (V) or Carry (C) in the CPSR? If not, are there Thumb-specific assembly level workarounds to test for overflows and carries?

杰夫

uint32_t result, a, b;
int no_carry = 1;
...

__asm__
(
  "ldr  r4, %[xa]   ;"  // R4 = a
  "ldr  r5, %[xb]   ;"  // R5 = b
  "adds r6, r4, r5  ;"  // R6 = R4 + R5, set status
  "movcs    r4, #0      ;"  // set overflow (if carry set)
  "strcs    r4, %[xc]   ;"  // store it (if carry set)
  "str  r6, %[xr]   ;"  // result = R6
  : [xr] "=m" (result), [xc] "=m" (no_carry)
  : [xa] "m" (a), [xb] "m" (b)
  : "r4", "r5", "r6"
);

...

编辑：寄存器也需要四处走动，走的 ARM ABI的应用程序二进制接口（ABI）ARM体系结构。

推荐答案

据该的的Thumb-16快速参考指南，在添加指令应该可用。这似乎是在汇编中的错误（如被@dwelch确认）。

According to the Thumb-16 Quick Reference Guide, the ADDS instruction should be available. This appears to be a bug in the assembler (as was confirmed by @dwelch).

我发现我可以解决它通过发出指令pre-CN $ c。使用汇编指令$ CD。例如：

I found I could work around it by issuing instructions pre-encoded using assembler directives. For example:

__asm__
(
  "ldr  r0, %[xa]   ;"  // R0 = a
  "ldr  r1, %[xb]   ;"  // R1 = b
  "adds r1, r1, r0  ;"  // R1 = a + b
  ...
);

将使用来实现：

__asm__
(
  "ldr  r0, %[xa]   ;"  // R0 = a
  "ldr  r1, %[xb]   ;"  // R1 = b
  ".inst.w 0x1809   ;"  // Issue 'adds r1, r1, r0'
  ...
);

如果我想补充R2，R2，R1 中，code应该发出 .inst.w 0x1852 等

If I wanted adds r2, r2, r1, the code should emit .inst.w 0x1852, and so on.

编辑：code最近更新由于手臂thumb2 LDR.W语法？ Binutils的邮件列表上。

Code recently updated due to arm thumb2 ldr.w syntax? on the Binutils mailing list.

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