Re: [PATCH v4 6/6] x86/syscall: use int everywhere for system call numbers

[H. Peter Anvin]

On 5/20/21 1:53 AM, Thomas Gleixner wrote:
> On Tue, May 18 2021 at 12:13, H. Peter Anvin wrote:
> > +static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr)
> > +{
> > +	/*
> > +	 * Convert negative numbers to very high and thus out of range
> > +	 * numbers for comparisons. Use unsigned long to slightly
> > +	 * improve the array_index_nospec() generated code.
>
> How is that actually improving the generated code?
>
> unsigned long:
>
>   104:	48 81 fa bf 01 00 00 	cmp    $0x1bf,%rdx
>   10b:	48 19 c0             	sbb    %rax,%rax
>   10e:	48 21 c2             	and    %rax,%rdx
>   111:	48 89 df             	mov    %rbx,%rdi
>   114:	48 8b 04 d5 00 00 00 	mov    0x0(,%rdx,8),%rax
>   11b:	00
>   11c:	e8 00 00 00 00       	callq  121 <do_syscall_64+0x41>
>
> unsigned int:
>
>    f1:	48 81 fa bf 01 00 00 	cmp    $0x1bf,%rdx
>    f8:	48 19 d2             	sbb    %rdx,%rdx
>    fb:	21 d0                	and    %edx,%eax
>    fd:	48 89 df             	mov    %rbx,%rdi
>   100:	48 8b 04 c5 00 00 00 	mov    0x0(,%rax,8),%rax
>   107:	00
>   108:	e8 00 00 00 00       	callq  10d <do_syscall_64+0x3d>
>
> Text size increases with that unsigned long cast.
>
> I must be missing something.

"unsigned long" gave slightly better code than "int", but as you 
correctly point out here, "unsigned int" is even better.

Thanks for catching that.

	-hpa