Re: [mpeg2-dev] [PATCH] motion_comp_arm_s.S is not PIC enough

[Rafaël C. <fu...@vi...>]

Hello,

Le 2012-07-16 00:09, John Reiser a écrit :
> Functions MC_put_o_16_arm, MC_put_o_8_arm, MC_put_x_16_arm, MC_put_x_8_arm
> in libmpeg2/motion_comp_arm_s.S have addresses in .text, which is bad
> for shared libraries.  Some environments demand that .text actually be
> read-only all the time, yet MC_put_o_16_arm etc require that the addresses
> be modified by the dynamic linking mechanism (dlopen, LoadLibrary, etc.)
> Even in those environments which permit the dynamic linker to modify the
> .text segment, the runtime cost of doing the relocation can be noticeable.
> 
> The attached patch rewrites the linkage, discarding the tables of addresses
> in favor of tables of offsets.  All transfers are local within each individual
> function, so there can be no interference by processing that occurs
> after assembly, such as link-time re-ordering (even of individual functions.)
> 
> -- John Reiser
> 
> 
> libmpeg2.patch
> 
> 
> Index: libmpeg2/motion_comp_arm_s.S
> ===================================================================
> --- libmpeg2/motion_comp_arm_s.S	(revision 1205)
> +++ libmpeg2/motion_comp_arm_s.S	(working copy)
> @@ -29,9 +29,13 @@
>  	pld [r1]
>          stmfd sp!, {r4-r11, lr} @ R14 is also called LR
>  	and r4, r1, #3
> -	adr r5, MC_put_o_16_arm_align_jt
> -	add r5, r5, r4, lsl #2
> -	ldr pc, [r5]
> +	ldrb r4, [pc, r4]
> +	add pc, pc, r4, lsl #2

Is this instruction available on all ARM variants?

I have to ask because I found some restrictions on:
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0068b/BABDCBAB.html

Although here it should be the form "ADD Rd, Rn, #imm8m" which works
everywhere.

> +	.byte (MC_put_o_16_arm_align0 - 0f)>>2
> +	.byte (MC_put_o_16_arm_align1 - 0f)>>2
> +	.byte (MC_put_o_16_arm_align2 - 0f)>>2
> +	.byte (MC_put_o_16_arm_align3 - 0f)>>2
> +0: