[sdcc-devel] [ sdcc-Bugs-586918 ] Optimization error: external RAM...

[<no...@so...>]

Bugs item #586918, was opened at 2002-07-26 03:18
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: Optimization error: external RAM...

Initial Comment:
Dear Sir,

I send to You 3 small 'C' sources compiled with SDCC 
version 'mcs51 2.3.1-pj3 (Jan 20 2002) (MINGW)'.

In each of those sources,  I try to make this: 'nb = 
_CMD_BUF[0];'
In source 1, I wrote: nb = _CMD_BUF[0];
In source 2, I wrote: nb = (xdata unsigned char) 
_CMD_BUF[0];
In source 3, I wrote: nb = (xdata) _CMD_BUF[0];

Only source 3 is encoded correctly from C to assembler.

_CMD_BUF is a buffer of 256 bytes starting in external 
RAM from address 0x0000 and has been declared as 
follows:
xdata at 0x0000 unsigned char _CMD_BUF[256];

nb is a local variable.

I use the small memory model.


Source 1: (See attachment METHOD1.ZIP)
--------------------------------------------------------------
nb = _CMD_BUF[0];  // (from line 14 in ng7.c1)

 The encoding is: (see ng7.ls1)
mov	_dummy_process_parm_2,__CMD_BUF.

My conclusion is: due to optimizations, the compiler 
forgot that _CMD_BUF[0] was a byte in external RAM.
The compiler encoded it as a byte from internal RAM.


Source 2: (See attachment METHOD2.ZIP)
--------------------------------------------------------------
nb = (xdata unsigned char) _CMD_BUF[0];  // (from line 
14 in ng7.c2)

Same as source 1 but with a casting; same results than 
from source 1.


Source 3: (See attachment METHOD3.ZIP)
--------------------------------------------------------------
nb = (xdata) _CMD_BUF[0];  // (from line 14 in ng7.c3)

By adding (xdata) before _CMD_BUF[0], the encoding is 
now correct: _CMD_BUF[0] is seen by the compiler as 
coming from external RAM. 


My question is: Is it an optimization problem? Do You 
have a solution? Is it correct to use this work around? 
(xdata before _CMD_BUF[0]).

Best Regards,
Leon GODET.



   


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