[sdcc-devel] [ sdcc-Bugs-860505 ] Serious miscompilation of 8051 code

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Bugs item #860505, was opened at 2003-12-15 10:57
Message generated for change (Tracker Item Submitted) made by Item Submitter
You can respond by visiting: 
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Category: Live range problems
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: Serious miscompilation of 8051 code

Initial Comment:
A function had me confused for hours, until I traced it
to a compiler
code generation bug.  I have reduced the code to a
small example which
still displays the bug.

This is the code:

static void test (xdata char * ptr)
{
        unsigned char i;
        while (1) {
                for (i = 0; i < 1; i++)
                        *ptr = 0;
                for (i = 0; i < 1; i++) {}
        }
}

It is compiled with this command:

        sdcc -S test.c

This is the output from "sdcc --version":

SDCC :
mcs51/gbz80/z80/avr/ds390/pic14/pic16/TININative/xa51/ds400
2.3.5 (Oct 11 2003) (UNIX)


The bug is clear from the generated asm code, which is
below.  Take a
close look at the register "r2".  It holds part of the
"ptr" variable,
but it is incorrectly clobbered by the second "for"
loop's index.  By
the second iteration of the "while", "ptr" has the
wrong value and "*ptr
= 0" stores to the wrong address.

Also see that a data memory location is assigned called
"_test_ptr_1_1",
but this is not actually referenced anywhere in the
code.  I suspect
this is significant, because I've only seen this bug
with variables
which are function parameters.

Quite a few source variations are miscompiled in this
way; it's quite a
robust bug.

Here is the asm:

;--------------------------------------------------------
; internal ram data
;--------------------------------------------------------
        .area DSEG    (DATA)
_test_ptr_1_1:
        .ds 2
;------------------------------------------------------------
;Allocation info for local variables in function 'test'
;------------------------------------------------------------
;ptr                       Allocated to in memory with
name '_test_ptr_1_1'
;------------------------------------------------------------
;test.c:1: static void test (xdata char * ptr)
;       -----------------------------------------
;        function test
;       -----------------------------------------
_test:
        ar2 = 0x02
        ar3 = 0x03
        ar4 = 0x04
        ar5 = 0x05
        ar6 = 0x06
        ar7 = 0x07
        ar0 = 0x00
        ar1 = 0x01
;     genReceive
        mov     r2,dpl
        mov     r3,dph
;test.c:5: for (i = 0; i < 1; i++)
00112$:
;     genAssign
        mov     r4,#0x01
00106$:
;test.c:6: *ptr = 0;
;     genPointerSet
;     genFarPointerSet
        mov     dpl,r2
        mov     dph,r3
;       Peephole 181    changed mov to clr
        clr     a
        movx    @dptr,a
;     genDjnz
;       Peephole 132    changed ljmp to sjmp
;       Peephole 205    optimized misc jump sequence
        djnz    r4,00106$
00117$:
00118$:
;test.c:5: for (i = 0; i < 1; i++)
;test.c:7: for (i = 0; i < 1; i++) {}
;     genAssign
        mov     r2,#0x01
00109$:
;     genDjnz
;       Peephole 132    changed ljmp to sjmp
;       Peephole 205    optimized misc jump sequence
        djnz    r2,00109$
00119$:
00120$:
;       Peephole 132    changed ljmp to sjmp
        sjmp    00112$
00110$:
        ret


Thanks,
-- Jamie Lokier <ja...@sh...>


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