Re: [linuxsh-dev] Next change

[NIIBE Y. <gn...@m1...>]

More explanation of .hidden.

Problem:
    With the call through PLT (Procedure Linkage Table), 
    (unexpected) register(s) will be clobbered.

There're two cases, one is (non-PIC) executable, and another is (PIC)
shared library.

Case 1: Executable
Suppose an executable is linked to a shared library which exports a
libgcc function.  At link time, PLT will be generated and the call to
that libgcc function will become the one through PLT.

To prevent this, a solution would be using .hidden for libgcc
functions.


Case 2: Shared library
Normal pc-relative call has no problem.

There're cases of call of libgcc functions, using the symbol reference
through GOT (Global Offset Table) which goes through PLT.  Like this:
----------------------
    <<CALL of __udivsi3>>
	mov.l	.L1,r0
	mov.l	@(r0,r12),r1
	jsr	@r1
	 nop
	...
.L1:
	__udivsi3@GOT

GOT:
	...
__udivsi3@GOT:
	The address of PLT entry of __udivsi3

PLT:
	...
	opecode of PLT 
	...
	Offset of __udivsi3 
----------------------

When we use .hidden attribute for the libgcc function (in this case,
__udivsi3), PLT will not be generated and it will become like that
(at link time):
----------------------
    <<CALL of __udivsi3>>
	mov.l	.L1,r0
	mov.l	@(r0,r12),r1
	jsr	@r1
	 nop
	...
.L1:
	__udivsi3@GOTOFF
----------------------
--