Re: [arm-gnu] thumb code calling arm subroutine causes undefined insn exception

["Andrew Kohlsmith (mailing lists account)" <aklists@xxxxxxxxxx>]

On Sunday 25 April 2010 06:39:31 pm Travis Geiselbrecht wrote:
> I might be wrong, but I'm pretty sure arm7tdmi does not support the blx
>  instruction. blx was added in armv5t instruction set. arm7tdmi is armv4.

Interesting.

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0234b/i1010871.html

This is the link to the thumb instruction reference for ARM7TDMI-S, which is 
the ARM variant in the LPC2103 CPU I'm using. I think you're on to something.  
I see B, BL and BX, but not BLX.

However, the instruction is being encoded as 0xf000 0xe9a4. Figure 1.6 of that 
URL shows that instruction format 19 is "long branch with link", which would 
cause me to jump to 0, I think. Since the instruction at 0 is an ARM 
instruction (it's the vector for the start of execution), that would certainly 
cause the CPU to throw this exception.

> I'd try -mcpu=arm7tdmi or -march=armv4 to see if it'll stick to the right
>  instruction set. Thumb interworking on pre armv5s is pretty nasty though.
>  The lack of blx means it a lot of times have to jump through linker
>  generated shims and generally spend a lot more effort.

Since I am already using -mcpu=arm7tdmi, would this indicate that there is a 
bug in the compiler wherein it is assuming that the BLX instruction exists on 
the ARM7TDMI/TDMI-S?

I played around with -mcpu, -march, --fix-v4bx and --fix-v4bx-interwork, all 
without success.  The latter two will remove bx instructions and use some shim 
code to simulate it, but the blx instruction remains.

-A.