Re: [PATCH] lower-subreg: Decompose multiword shifts

[Michael Matz <matz@suse.de>]

Hi,

On Fri, 3 Aug 2007, Rask Ingemann Lambertsen wrote:

> foo:
> 	pushl	%ebp		# 43	*pushsi2	[length = 1]
> 	movl	%esp, %ebp	# 44	*movsi_1/1	[length = 2]
> 	pushl	%esi		# 45	*pushsi2	[length = 1]
> 	pushl	%ebx		# 46	*pushsi2	[length = 1]
> 	movl	12(%ebp), %eax	# 39	*movsi_1/1	[length = 3]
> 	popl	%ebx		# 49	popsi1	[length = 1]
> 	movl	8(%ebp), %ecx	# 41	*movsi_1/1	[length = 3]
> 	popl	%esi		# 50	popsi1	[length = 1]
> 	leave			# 51	leave	[length = 1]
> 	movl	%ecx, %edx	# 36	*movsi_1/1	[length = 2]
> 	ret			# 52	return_internal	[length = 1]
> 
> I don't know if it's your fault, but it's worse!

Can't be Andreas' fault, except that he obviously exposes another problem.  
The function now saves %esi and %ebx, although they aren't used at all.  
Without those the function would look saner:

> foo:
>       pushl   %ebp            # 43    *pushsi2        [length = 1]
>       movl    %esp, %ebp      # 44    *movsi_1/1      [length = 2]
>       movl    12(%ebp), %eax  # 39    *movsi_1/1      [length = 3]
>       movl    8(%ebp), %ecx   # 41    *movsi_1/1      [length = 3]
>       leave                   # 51    leave   [length = 1]
>       movl    %ecx, %edx      # 36    *movsi_1/1      [length = 2]
>       ret                     # 52    return_internal [length = 1]

Leaving only one useless move in there, one superflous move left, but 
better.


Ciao,
Michael.