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Re: GCC Internals: built-in functions?
On Jan 30, 2011, at 3:57 PM, Jonathan Wakely wrote:
> On 30 January 2011 20:45, Amittai Aviram wrote:
>> On this GCC Internals page
>>
>> http://en.wikibooks.org/wiki/GNU_C_Compiler_Internals/GNU_C_Compiler_Architecture_3_4
>>
>> I found the following in the "GCC Initialization" section:
>>
>> "GCC built-in functions are the functions that are evaluated at compile time. For example, if the size argument of a strcpy() function is a constant then GCC replaces the function call with the required number of assignments."
>>
>> I was curious about this, so I tried compiling to assembly (-S) a very simple program:
>>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <string.h>
>>
>> int main(void) {
>>
>> char s1[0x10];
>> char * s0 = "HELLO";
>> strcpy(s1, s0);
>> printf("%s %s\n", s0, s1);
>> return EXIT_SUCCESS;
>> }
>>
>> But the resulting assembly code simply calls strcpy with the two arguments, just as I would have expected had I not read the above sentence:
>>
>> movq $.LC0, -40(%rbp)
>> movq -40(%rbp), %rdx
>> leaq -32(%rbp), %rax
>> movq %rdx, %rsi
>> movq %rax, %rdi
>> call strcpy
>>
>> (Here, .LC0 labels the string "HELLO".)
>>
>> So what does that sentence actually mean and what am I missing? Thanks!
>
> strcpy has no 'size' parameter, I assume it's meant to say strncpy
I tried it with strncpy and with __builtin_strncpy, with exactly the same results, i.e., the assembly code still calls strncpy with three arguments.
Amittai Aviram
PhD Student in Computer Science
Yale University
646 483 2639
amittai.aviram@yale.edu
http://www.amittai.com