c - GCC 避免将分支编译为链接寄存器 (blr) 语句

Question

我正在编写 C 代码并为 PowerPC 架构编译它。我想告诉编译器将所有类似返回的指令替换为函数代码末尾的分支。这是由于一些奇怪的要求，我不能在这段代码中使用任何 return 来确保之后执行更多的汇编行。

例如，我将C代码编译成如下汇编代码:

lis r9,4096
lis r8,255
lwz r10,0(r9)
ori r8,r8,65535
addi r9,r10,192
cmplw cr7,r9,r8
blelr- cr7 # Return if the cr7 register is "less than or equal"
lis r8,512
cmplw cr7,r9,r8
bgtlr- cr7 # Return if the cr7 register is "greater than or equal"
lwz r10,192(r10)
lis r8,303
ori r8,r8,65535
addi r9,r10,320
cmplw cr7,r9,r8
blelr- cr7 # Return if the cr7 register is "less than or equal"
lis r8,528
cmplw cr7,r9,r8
bgtlr- cr7 # Return if the cr7 register is "greater than or equal"
lwz r10,320(r10)
lis r8,287
ori r8,r8,65535
subi r9,r10,448
cmplw cr7,r9,r8
blelr- cr7
lis r8,544
cmplw cr7,r9,r8
bgtlr- cr7 # Return if the cr7 register is "greater than or equal"
lis r9,4919
ori r9,r9,4919
stw r9,-448(r10)
blr # Return

我想要的是将所有类似返回的语句替换为函数代码末尾的始终分支，如下所示:

lis r9,4096
lis r8,255
lwz r10,0(r9)
ori r8,r8,65535
addi r9,r10,192
cmplw cr7,r9,r8
ble _END # Branch to the _END label if "less than"
lis r8,512
cmplw cr7,r9,r8
bgt _END # Branch to the _END label if "greater than"
lwz r10,192(r10)
lis r8,303
ori r8,r8,65535
addi r9,r10,320
cmplw cr7,r9,r8
ble cr7 # Branch to the _END label if "less than"
lis r8,528
cmplw cr7,r9,r8
bgt _END # Branch to the _END label if "greater than"
lwz r10,320(r10)
lis r8,287
ori r8,r8,65535
subi r9,r10,448
cmplw cr7,r9,r8
blelr- cr7
lis r8,544
cmplw cr7,r9,r8
bgt _END # Branch to the _END label if "greater than"
lis r9,4919
ori r9,r9,4919
stw r9,-448(r10)
_END:
blr # I guess it should stay otherwise the function call will continue executing "random" garbage such as the next function in the .text section. Via post-processing this would be easy to strip though!

有什么方法可以自动执行此编译首选项？

Answer 1

@Jester 和@fuz 的评论给出了答案，我就写下来

你需要:

1. 避免在 C 代码中的函数中间返回。因此，将 return 替换为 goto。
2. 防止编译器发现 goto end; 和 return; 是同一回事。内联汇编是告诉编译器“不要搞砸这个”的好方法。

一些宏:

#define RETURN goto _end
#define END _end: asm volatile("")

void foo(int x) {
如果(x==1)返回；
如果(x==2)返回；
printf("你好，世界\n");
结尾;
}


asm volatile("") 指示编译器插入一些汇编代码。编译器不知道这段汇编代码的作用，因此它无法进行任何会跳过它的优化(即使它是 0 条指令)。