I think I found something, see below. Answering the question first ;-)
Am 16.05.2018 um 19:20 schrieb Florian Klämpfl:
> How big is the project? Normally, the number of unrolled loops is reasonable
> small so comparing the
> generated assembler with and without unrolling should be feasible.According
> to cloc, 40kLOC, 10kLOC of which are DSP code, not counting packages
that are not in the source repo. With the very spurious crashes, not the best
test case...
I did, however, do the insane thing and diff Lazarus (or rather, IDAs output).
There are a lot of unrolled loops in the LCL alone, but the code reordering for
the first 15% or so that I checked looks fine. {Off-Topic: could parts of that
variable rewriting be used for more aggressive inlining?}
Some more testing with Lazarus and -gt has shown that at some variables are
uninitialized with -Ooloopunroll. There is a fairly common case where removing
the variable is not good:
-------------
for Result:= alow to ahigh do
if Something(Result) then exit;
-------------
If that gets unrolled, Result is undefined, and whatever it gets assigned to
receives the random content of r/eax.
I'm 90% sure that global variables and the Result variable are *not* supposed to
be undefined after the loop (in contrast to Delphi-compatible locals), but can't
seem to find a reference for that. This pattern is used all over the place, so
somebody else must have thought so too? Grepping for "for result:=" over the FPC
source tree gives 10 matches, Lazarus has over 100 results.
Test case:
-------------
{$Optimization LOOPUNROLL}
var
i : integer;
s : single;
function tfun: integer;
begin
for Result:=1 to 10 do
s:=s+1;
end;
begin
s:=0.0;
for i:=1 to 2 do
s:=s+1;
if i <> 2 then
halt(1);
i:= tfun();
if i <> 10 then
halt(2);
if s <> 12 then
halt(3);
writeln('ok');
end.
-------------
I would very much expect that to be the main cause of the observed crashes.
--
Regards,
Martok
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