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Re: GCC Internals: built-in functions?
- From: Jonathan Wakely <jwakely dot gcc at gmail dot com>
- To: Amittai Aviram <amittai dot aviram at yale dot edu>
- Cc: gcc-help at gcc dot gnu dot org
- Date: Sun, 30 Jan 2011 20:57:22 +0000
- Subject: Re: GCC Internals: built-in functions?
- References: <D4CC12B2-BBC2-4B21-8B20-C694BEAE84B6@yale.edu>
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