[sdcc-devel] [ sdcc-Bugs-2995398 ] memcpy() code fails in one corner case

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Bugs item #2995398, was opened at 2010-05-02 08:03
Message generated for change (Comment added) made by spth
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Category: z80 port
Group: None
>Status: Closed
>Resolution: Fixed
Priority: 5
Private: No
Submitted By: andrewl (andrewwl)
Assigned to: Nobody/Anonymous (nobody)
Summary: memcpy() code fails in one corner case

Initial Comment:
For the z80 target, your memcpy() intrinsic generates the following code:

ROM:0232                 ld      l, (ix+4)
ROM:0235                 ld      h, (ix+5)       ; HL = <dest>
ROM:0238                 ld      c, (ix+6)
ROM:023B                 ld      b, (ix+7)       ; CB = <src>
ROM:023E                 ex      de, hl          ; DE = HL
ROM:023F                 ld      l, c
ROM:0240                 ld      h, b            ; HL = CB
ROM:0241                 ld      c, (ix+8)
ROM:0244                 ld      b, (ix+9)       ; CB = <len>
ROM:0247                 ldir

The problem is when the length argument is 0 (eg: memcpy(dst, src, 0)). The LDIR instruction first copies a byte, THEN decrements CB, THEN decides whether to repeat (when CB is  0). So an initial value of 0 overwrites all of memory since CB is decremented to FFFF before the zero check occurs.

Here is my version info:
sdcc -v
SDCC : mcs51/gbz80/z80/avr/ds390/pic16/pic14/TININative/xa51/ds400/hc08 2.9.0 #5416 (Mar 22 2009) (MINGW32)

Thank you for this great software!
Andrew

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>Comment By: Philipp Krause (spth)
Date: 2010-05-21 14:18

Message:
Fixed in revision #5828.

ldir is now only used when the last parameter is a literal, and if it is
zero, no ldir is generated at all.

Philipp


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Comment By: Philipp Krause (spth)
Date: 2010-05-16 15:29

Message:
Well, this might be the final nail in the coffin of this ldir-for-memcpy
thing. Checking for zero first is clearly unacceptable in terms of code
size. There are firther bugs in argument handling (and there have been in
the past, with fixes complicating the code).
The best solution seems to be to only convert memcpy with the last
paramteter being an integer constant to ldir, and use regular memcpy
otherwise. However it would be a bit complicated, AFAIK there is no way to
replace function calls by builtins depending on the parameters in sdcc.
Maybe we can continue to convert memcpy calls to a builtin and then later
replace some of them with function calls again the same way we replace long
division, etc by support calls.

Philipp


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