> what does the structure definition look like?
Well, it's the output of a transpiler so it's a bit of a mess. But the
basic idea is:
struct GCNode{
GCNode *succ,*pred;
char pad[2];
char state;
char flags;
virtual ~GCNode(){
}
};
struct Timer : GCNode{
Timer( double hertz ){
period=1.0/hertz;
}
double period;
};
...where 'Timer' is the user class causing problems and GCNode is a system
class.
When built by the transpiler, the end result is that the write to 'period'
seems to be accidentally writing to 'state '- so it's possibly worth noting
it's therefore not an asm.js alignment issue, the compiler is producing
valid but incorrect code. However, if you build/run this as is, it works
fine. I have only encountered the problem when running the output of the
transpiler, and all attempts to c++-ify an example have failed so far. It's
a real heisenbug!
The 'period' field is indeed misligned for a double, as there is malloc
size+virtual function table (8 bytes) at start of allocation. And adding an
int 'pad' field just before 'period' fixes the problem.
Could be the transpiler I guess, but I haven't had this issue with mingw32
or android/ios 32 bit builds - or even wasm. Just asm.js builds.
On Mon, Dec 18, 2017 at 9:56 AM, Brion Vibber <[email protected]> wrote:
> In asm.js code, all memory loads and stores must be aligned (wasm does not
> have this limitation).
>
> Normally alignment rules on structs should 'just work' I think -- what
> does the structure definition look like?
>
> -- brion
>
> On Dec 17, 2017 12:46 PM, "Mark Sibly" <[email protected]> wrote:
>
>> Hi,
>>
>> I am having some intermittent problems with what appears to be float
>> point double alignment, where writing to a floating point double field ends
>> up writing to the wrong location. This has been solved in every case so far
>> by manually '8 byte aligning' the double field.
>>
>> Also, this only seems to be happening when generating asm.js 'apps' - so
>> far, there have been no problems with wasm.
>>
>> Unfortunately, this is happening in a translator I am writing, and every
>> attempt I have made so far to isolate some reproducable c++ code has failed!
>>
>> Just wondering of this was a known issue of some kind, or if anyone else
>> has had similar problems? With a bit of luck I'll be able to retire the
>> asm.js target soon in favour of wasm in which case it wont matter, but I
>> just thought I'd check...
>>
>> Bye,
>> Mark
>>
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