Thumb instruction set generation...

[Patrick Bellasi <derkling@xxxxxxxxxxxxx>]

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Hi,
	I'm compiling a toy .c source with:
arm-none-elf-gcc (GCC) 3.4.0 20040409 (CodeSourcery ARM Q1A 2004)
using those switch:
	-mcpu=arm7tdmi -mthumb-interwork

In the genarated code I expect to get sometimes a switching between ARM and 
Thumb instruction set, i.e following a BX instr.

It seem to me that the generated code isn't conform to the THUMB instruction 
set specifications...

This is a piece of  code I'm getting:

...
00008280 <main+0x64> bx	lr
00008284 <fact> mov	ip, sp
00008288 <fact+0x4> stmdb	sp!, {fp, ip, lr, pc}
0000828c <fact+0x8> sub	fp, ip, #4	; 0x4
00008290 <fact+0xc> sub	sp, sp, #8	; 0x8
00008294 <fact+0x10> str	r0, [fp, #-16]
00008298 <fact+0x14> ldr	r3, [fp, #-16]
0000829c <fact+0x18> cmp	r3, #0	; 0x0
000082a0 <fact+0x1c> beq	000082b4 <fact+0x30>
000082a4 <fact+0x20> ldr	r3, [fp, #-16]
000082a8 <fact+0x24> cmp	r3, #1	; 0x1
000082ac <fact+0x28> beq	000082b4 <fact+0x30>
000082b0 <fact+0x2c> b	000082c0 <fact+0x3c>
000082b4 <fact+0x30> mov	r3, #1	; 0x1
000082b8 <fact+0x34> str	r3, [fp, #-20]
000082bc <fact+0x38> b	000082e0 <fact+0x5c>
000082c0 <fact+0x3c> ldr	r3, [fp, #-16]
000082c4 <fact+0x40> sub	r3, r3, #1	; 0x1
000082c8 <fact+0x44> mov	r0, r3
000082cc <fact+0x48> bl	00008284 <fact>
000082d0 <fact+0x4c> mov	r3, r0
000082d4 <fact+0x50> ldr	r2, [fp, #-16]
000082d8 <fact+0x54> mul	r3, r2, r3
000082dc <fact+0x58> str	r3, [fp, #-20]
000082e0 <fact+0x5c> ldr	r0, [fp, #-20]
000082e4 <fact+0x60> sub	sp, fp, #12	; 0xc
000082e8 <fact+0x64> ldmia	sp, {fp, sp, lr}
000082ec <fact+0x68> bx	lr
...

Well, at 0x8280 we have the first BX inst.
So, from that point following the processor have to execute THUMB code (if 
lr[0]==1 of course)... and so, till the next BX at 0x82ec we have to find 
ONLY THUMB instructions. It isn't?

At 0x8288... just two instr after the firse BX we have that instr:
	00008288 <fact+0x4> stmdb	sp!, {fp, ip, lr, pc}
BUT: that kind of multiple store isn't allowed on thumb mode... it was to be 
a:
						stmia	sp!, {fp, ip, lr, pc}

There are other non thumb instr in the above code...

Is that a problem of the compiler, or concerning the decompiler 
(arm-none-elf-objdump 2.15.91 20040504)...
or I'm in wrong?!?

I'm referring to that specs:
- - ARM Instruction Set Quick Reference Card v2.1
- - Thumb Instruction Set Quick Reference Card v2.1

Thank's in advance,
	Patrick


- -- 



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                                                  DERKLING
                                    LRU 338214 (http://counter.li.org)

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   Patrick Bellasi <derkling_at_users.sourceforge.net>
   Student (Politecnico di Milano)
       Comp.Sci & Soft.Eng
   
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