[llvm-bugs] [Bug 119849] [x86][CF]Unable to obtain the correct CF flag bit

[LLVM Bugs via llvm-bugs]

Issue	119849
Summary	[x86][CF]Unable to obtain the correct CF flag bit
Labels	new issue
Assignees
Reporter	zhaojiangkun-1

    Retrieve the CF register flag bit through __readeflags() and verify if the parameters are equal.
Clang generates the 'and' instruction after compilation, and clears the CF flag after executing the 'and' instruction. Is this a bug or is it for some purpose? Resulting in the inability to obtain correct results through __readeflags().

https://godbolt.org/z/4oaWeKxY1


**How did I do it**
1）`clang -g test.c -o -test`
2）`cgdb test`
3)  run to  ` flags = readeflags_test (100, 101)` and debug in assembly
**the debug result**

![Image](https://github.com/user-attachments/assets/d0f6daff-e3f0-4db9-9c13-07c615cd5ea4)







**test script:**
```#include <x86intrin.h>

extern void abort (void);

#ifdef __x86_64__
#define EFLAGS_TYPE unsigned long long int
#else
#define EFLAGS_TYPE unsigned int
#endif


EFLAGS_TYPE
readeflags_test (unsigned int a, unsigned int b)
{
    volatile char x = (a == b);
    return __readeflags ();
}

int
main ()
{
    EFLAGS_TYPE flags;

    flags = readeflags_test (100, 100);

    if ((flags & 1) != 0)  /* Read CF */
        abort ();

    flags = readeflags_test (100, 101);
    flags = readeflags_test(101, 100);//新增测试用例，用于验证比较对CF的影响


    if ((flags & 1) == 0)  /* Read CF */
        abort ();

#ifdef DEBUG
    printf ("PASSED\n");
#endif

    return 0;
}

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