Re: Thumb interworking on untyped symbols

[Richard Earnshaw <rearnsha at arm dot com>]

On 31/05/11 15:10, Paul Brook wrote:
> The ARM EABI requires that linkers support interworking for function 
> (STT_FUNC) symbols, either via veneers or by bl/blx conversion.  We already do 
> this.
> 
> However for untyped symbols we blindly assume that these are ARM code.  At the 
> time this was implemented it probably made sense - the EABI didn't exist and 
> most of the world was ARM mode anyway.  With the advent of Thumb-2 and ARMv7-
> M, likely as not this is wrong.
> 
> The EABI only requires interworking be performed for function symbols.  For 
> other symbols the behavior is less clear, the EABI stating that "interworking 
> for untyped symbols must be encoded directly in the object file".  By my 
> reading that means we should assume the user wrote bl/blx as appropriate and 
> leave well alone.
> 
> The attached patch implements this by effectively disabling veneer generation 
> for symbols that do not have funciton type.
> 
Yes, that sounds sensible.  In particular a linker only has freedom to
insert a veneer that clobbers IP when it's a STT_FUNC symbol.

R.