Re: arm-elf-gcc : change default data alignement depending on ARM/THUMB

[Richard Earnshaw <rearnsha at arm dot com>]

> Dear all,
> 
> I am trying to figure out why gcc always aligns on 4 bytes boundaries (32 bits) 
> on arm-elf-gcc in an attempt to reduce code footprint. I found that GCC produces 
> much bigger code than ADS (30 to 50 %) when ARM and THUMB are mixed 
> (interworking enabled) on our project.
> 
> It indeed seems that ARM ADS 1.2 aligns on halfwords (16 bits) when in THUMB 
> mode (along with much more sophisticated rules). GCC different behavior causes 
> much more padding to be inserted in structs members and, as the project I work 
> on uses HW mapped registers, application crashes.
> For now, I worked around that using a compile time define to enable the 
> __attribute((packed))__ on the struct(s). I would nevertheless like to know 
> whether there is another solution.
> 
> I am also digging into the arm.h and arm.c files (thanks to the gcc's internal 
> manual) to try to change the alignment behavior.
> 
> Could you give me other ideas/solutions on that point?
> 

GCC defaults to the old APCS rules on structure alignment and padding, 
which means that all structures are word-aligned by default.  ARM ADS uses 
the ATPCS rules where structures take the alignment of their most aligned 
member.

You can change gcc's default for this with the compiler switch

	-mstructure-size-boundary=8

but beware that this changes the ABI, so you will need to rebuild all your 
code (including the libraries) with this option.

R.


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