Re: Intermediate void* casts

> Following is how gcc 3.4 20030806 (experimental) with -O2 makes it:
> 
> 00000000 <bswap64>:
>    0:	55                   	push   %ebp
>    1:	89 e5                	mov    %esp,%ebp
>    3:	8b 45 08             	mov    0x8(%ebp),%eax
>    6:	53                   	push   %ebx
>    7:	8b 55 0c             	mov    0xc(%ebp),%edx
>    a:	89 c1                	mov    %eax,%ecx
>    c:	66 c1 c9 08          	ror    $0x8,%cx
>   10:	c1 c9 10             	ror    $0x10,%ecx
>   13:	66 c1 c9 08          	ror    $0x8,%cx
>   17:	89 d3                	mov    %edx,%ebx
>   19:	66 c1 cb 08          	ror    $0x8,%bx
>   1d:	c1 cb 10             	ror    $0x10,%ebx
>   20:	66 c1 cb 08          	ror    $0x8,%bx
>   24:	31 c0                	xor    %eax,%eax
>   26:	89 ca                	mov    %ecx,%edx
>   28:	89 d9                	mov    %ebx,%ecx
>   2a:	31 db                	xor    %ebx,%ebx
>   2c:	09 da                	or     %ebx,%edx
>   2e:	09 c8                	or     %ecx,%eax
>   30:	5b                   	pop    %ebx
>   31:	c9                   	leave  
>   32:	c3                   	ret    

Or you could hand-code:

	0:			pop	%ecx
	1:			pop	%edx
	2:			pop	%eax
	3:			ror	$8, %ax
	7:			ror	$10 %eax
	a:			ror	$8, %ax
	e:			ror	$8, %dx
	11:			ror	$10, %edx
	14:			ror	$8, %dx
	18:			jmp	%ecx
	1a:

If I've got the registers in the right order (etc).
Probably runs faster with a different ordering or the 'ror' instructions.

One this I did notice when changing some similar code to be:
	i64 = (uint64_t)tv.tv_sec << 32 | tv.tv_sec * 4294u;
is that the current gcc managed to sign extend tv.tv_sec and then throw
away the top word.  Doing:
	i64 = tv.tv_sec * 0x100000000ull | tv.tc_sec * 4294u;
stopped this happening.

However in both cases is looks as though gcc's 'peephole optimiser' isn't
very good.  Something ought to be killing the register-register moves and
instructions whose result is never used.

	David

-- 
David Laight: [email protected]