Re: optimized inlining of specific standard function calls

[Enrico Weigelt <weigelt@xxxxxxxx>]

* Andrew Haley <aph@xxxxxxxxxx> wrote:

Hi,

> Yes.  For example, have a look at expand_builtin_fprintf() and
> expand_builtin_strcpy() in
> http://gcc.gnu.org/viewcvs/trunk/gcc/builtins.c?view=markup

Thx. I'm quite new to gcc internals, so i'd appreciate a bit 
more help. Is it possible to inject some C-code instead of
the original function call ?

For example:

    char foo[1024];
    snprintf(foo, sizeof(foo), "/tmp/%s%d.tmp", prefix, id);
    
Would inject something like:

    char foo[1024] = "/tmp/";
    {
	long  __bufmax;	// remaining space in buffer
	char* __bufptr;	// current buffer position

	// skip over the already initialized head
	__bufmax = sizeof(foo)-6;
	__bufptr = &foo[5];

	// the %s -> prefix part
	{
	    long __len__prefix = strlen(prefix);

	    // copy the parameter "prefix"
	    if (__bufmax > __len_prefix)
	    {
		memcpy(bufptr, prefix, __len_prefix);
		bufptr += __len_prefix;
		bufmax -= __len_prefix;
	    }
	    else
	    {
		memcpy(bufptr, prefix, __bufmax);
		goto __out_terminate;
	    }
	}
	
	// generate the %d -> id 
	{
	    struct __itoa_buf __fmt_d_id = __itoa_std(id);
	    if (__bufmax > __itoa_buf.len)
	    {
		memcpy(bufptr, __itoa_buf.buf, __itoa_buf.len);
		bufptr += __itoa_buf.len;
		bufmax -= __itoa_buf.len;
	    }
	    else
	    {
		memcpy(bufptr, __itao_buf.buf, __bufmax);
		goto __out_terminate;
	    }
	}

	// the suffix is short enough to be added char by char
	if (__bufmax)
	{
	    *__bufptr = '.';
	    __bufptr++;
	    __bufmax--;
	}
	else
	    goto __out_terminate;
	if (__bufmax)
	{
	    *__bufptr = 't';
	    __bufptr++;
	    __bufmax--;
	}
	else
	    goto __out_terminate;
	if (__bufmax)
	{
	    *__bufptr = 'm';
	    __bufptr++;
	    __bufmax--;
	}
	else
	    goto __out_terminate;
	if (__bufmax)
	{
	    *__bufptr = 'p';
	    __bufptr++;
	    __bufmax--;
	}

    __out_terminate:
	*bufptr = 0;
    }

And the whole thing goes through the compiler/optimizer again.


cu
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