[Vm-dev] VM Maker: VMMaker.oscog-EstebanLorenzano.1714.mcz

Nicolas Cellier nicolas.cellier.aka.nice at gmail.com
Tue Mar 8 21:50:56 UTC 2016

2016-03-08 22:10 GMT+01:00 Esteban Lorenzano <estebanlm at gmail.com>:

> On 08 Mar 2016, at 21:38, Nicolas Cellier <
> nicolas.cellier.aka.nice at gmail.com> wrote:
> IMO the right fix would be to force coercion here:
> ifTrue:[interpreterProxy signed64BitIntegerFor: (self cCoerceSimple:
> value to: #sqLong)]
> why?

I recommend writing simple programs like below, compile and run:

#include <stdio.h>
int main() {
    int x=-1;
    unsigned long long y;
   y = (signed) x;
   printf(" y <- (signed) x :%llx\n", y);
   y = (unsigned) x;
   printf(" y <- (unsigned) x :%llx\n", y);

> forcing a coercion to "unsigned int” while we always use sqTypes didn’t
> feel correct… also long32At will always answer an sqLong, that’s why I
> thought simpler approach was to remove a coercion instead adding two…
I wouldn't trust it too much. What is long32At doing exactly?
I can see two versions of it in platforms/Cross/vm/sqMemoryAccess.h ...

#define long32At    intAt
# define intAt(oop)                    intAtPointer(atPointerArg(oop))
# define intAtPointer(ptr)            ((sqInt)(*((int *)(ptr))))

In this case, there will be sign promotion and (ref integerAt: 1 size: 4
signed: false) will be broken on 64bits VM...

But with the other #if #else branch:

static inline sqInt intAt(sqInt oop)                          { return
intAtPointer(pointerForOop(oop)); }
static inline sqInt intAtPointer(char *ptr)            { return
(sqInt)(*((unsigned int *)ptr)); }

there won't be any sign promotion, and this time the problem will be with
(ref integerAt: 1 size: 4 signed: true)on 64 bits VM

So the 32 high bits of long32 SHOULD be UNTRUSTED and that's why we need to
force (int) or (unsigned int) cast in 64 bits VM.
It's a mistake to have long32 returning a sqInt, it should return a 32bits
Currently, int would fit for supported platform, but to be future proof, it
should have been int32_t

In my own VM brand I have defined unsignedLong32At to be sure to avoid
these nasty horrible things...
Most of the time handling unsigned is the right thing to do, they have well
defined, least surprising behavior.

> But maybe we can avoid signed64/unsigned64 and use signedMachineInteger
> and positiveMachineInteger when byteSize = bytePerWord...
> that, I have no idea, maybe :)

It's just an optimization for avoiding unecessary 64bits ops.

> Esteban
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