error: ‘asm’ operand has impossible constraints

Question

I want to calculate length of the word, but I have error. I don't understand why.

int new_strlen(char* word)
{
    int len = 0;
    __asm__  ("mov ecx, 100\n\t"
                          "mov esi, %1\n\t"
                          "mov al, 0\n\t"
                          "repne scasb\n\t"
                          "mov %0, 100\n\t"
                          "sub %0, ecx\n\t"
                        : "=a"(len)
                        : "r"(word)
                        : "%ecx","%esi", "%eax"
    );

    return len;
}

My error is "‘asm’ operand has impossible constraints".

I attempted to use the __asm{} construct, but it is not available on the x64 architecture.I am compiling a program using the "-masm=intel" flag. My compiler is "g++". Also I attempted to use "volatile" but it useless. Do you have any thoughts on this matter? I would appreciate it if you could share your insights. I am very grateful!

I try anything. Pls, help me

You can't have eax clobbered while also listing it as output using =a. In general if you use a mov in inline asm you are probably doing things wrong. Use the constraints to load values. Using 100 is a strangely limiting choice. Also you know this is 32 bit code, right? Finally, gcc has __builtin_strlen so you don't need to write this by hand even if you don't use libc. — Jester
– Jester, Commented Apr 23, 2024 at 13:41
Adding on to what Jester said, the docs explicitly say: Clobber descriptions may not in any way overlap with an input or output operand. Pretty unambiguous. — David Wohlferd
– David Wohlferd, Commented Apr 23, 2024 at 23:51

A_Elbereth_GIlthoniel · Accepted Answer · 2024-04-23 13:48:37Z

-3

Thanks all for the answers! I've found decision

__asm__ ("mov rcx, 100; \n\t"
            "mov rsi, %1; \n\t"
            "mov al, 0; \n\t"
            "repne scasb; \n\t"
            "mov %0, 99; \n\t"
            "sub %0, ecx; \n\t"
            : "=a"(len)
            : "g"(word));

answered Apr 23, 2024 at 13:48

A_Elbereth_GIlthoniel

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5 Comments

Jabberwocky Over a year ago

You should write what the actual problem was.

Nate Eldredge Over a year ago

You still need the clobber on rcx and rsi. Otherwise the compiler may be keeping important data there.

Nate Eldredge Over a year ago

I'm temporarily downvoting because of the error, but will remove it when you edit to fix the code.

Peter Cordes Over a year ago

You're also missing anything to tell the compiler about the memory you read. How can I indicate that the memory *pointed* to by an inline ASM argument may be used? - a "memory" clobber would be the simple way, or a dummy memory input operand.

Ivan Over a year ago

As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center.

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