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ARM Compiler armlink User Guide : --overlay_veneers

--overlay_veneers

When using the automatic overlay mechanism, causes armlink to redirect calls between overlays to a veneer. The veneer allows an overlay manager to unload and load the correct overlays.

Note

You must use this option if your scatter file includes execution regions with AUTO_OVERLAY attribute assigned to them.

Usage

armlink creates a veneer for a function call when any of the following are true:

  • The calling function is in non-overlaid code and the called function is in an overlay.
  • The calling function is in an overlay and the called function is in a different overlay.
  • The calling function is in an overlay and the called function is in non-overlaid code.

In the last of these cases, an overlay does not have to be loaded immediately, but the overlay manager typically has to adjust the return address. It does this adjustment so that it can arrange to check on function return that the overlay of the caller is reloaded before returning to it.

Veneers are not created when calls between two functions are in the same overlay. If the calling function is running, then the called function is guaranteed to be loaded already, because each overlay is atomic. This situation is also guaranteed when the called function returns.

relocation might refer to a function in an overlay and not modify a branch instruction. For example, the relocations R_ARM_ABS32 or R_ARM_REL32 do not modify a branch instruction. In this situation, armlink redirects the relocation to point at a veneer for the function regardless of where the relocation is. This redirection is done in case the address of the function is passed into another overlay as an argument.

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