multiply by a constant on an arm7TDMI

multiply by a constant on an arm7TDMI

[Franck MAMALET FT-R&D DIH/OCF]

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Hi,
I try to compile this kind of code:
     y = x * 0x3501;  /*(x and y are short);*/
with arm-elf-gcc for the target arm7tdmi.
The code generated by arm-elf-gcc doesn't   generate a 'mul'
instruction but a sequence of 'rsb' and 'add' instructions.
It is not a bug and it gives the good result but several multiplication
are in the same function and it has to store intermediate results in memory:
 str     r3, [fp, #-92]^M
        ldr    
r2, [fp, #-92]^M
        rsb    
r3, r3, r3, asl #4^M
        ldr    
r4, [fp, #-92
        rsb    
r3, r2, r3, asl #2
        add    
r3, r2, r3, asl #3
        add    
ip, r4, r3, asl #5
This solution is far more complex than a simple 'mul' instruction (even
if 'mul' can take 4 cycles on an ARM7).
Can anyone help me to avoid generating such a code and forcing arm-elf-gcc
to use 'mul' instruction.
Thanks,
regards,
Franck.
 
 
 
 
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Mamalet Franck
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