[DTrace-devel] [PATCH v2] Add support for array/struct/union to trace()

[Kris Van Hees]

On Fri, Jan 28, 2022 at 11:01:58PM -0500, Eugene Loh via DTrace-devel wrote:
> For the moment, just two questions:
> 
> Does this look correct?
> 
> # dtrace -n '
> struct {
>   int8_t x;
> } A;
> 
> BEGIN {
>   A.x = 0x34;
>   trace(A);
>   exit(0);
> }'
> dtrace: description '
> struct ' matched 1 probe
> CPU     ID                    FUNCTION:NAME
>   2      1                           :BEGIN
> 
> No output?

I did this with the updated patches I posted and get:

dtrace -n '
struct {
    int8_t x;
} A;

BEGIN {
    A.x = 0x34;
    trace(A);
    exit(0);
}'
dtrace: description '
struct ' matched 1 probe
CPU     ID                    FUNCTION:NAME
  1      1                           :BEGIN   4

And 0x34 is '4' so that would make sense.  DTrace v1 outputs 52 because of the
lack of support for integers of different sizes.

The reason why you get no output (by default) with this patch by itself is that
the data is seen with alignment 1 and therefore interpreted as a string.  And
that goes wrong.  This is one of the reasons why the string length prefix had
to go.  And with the patches to remove that prefux, the trace() action will be
able to handle this.

So, not a problem with this patch.
 
> And...
> 
> On 1/28/22 2:25 PM, Kris Van Hees via DTrace-devel wrote:
> > diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> > @@ -905,6 +905,25 @@ static const uint_t	ldstw[] = {
> > +/*
> > + * When loading bit-fields, we want to convert a byte count in the range
> > + * 1-8 to the closest power of 2 (e.g. 3->4, 5->8, etc).  The clp2() function
> > + * is a clever implementation from "Hacker's Delight" by Henry Warren, Jr.
> > + */
> 
> What is "the range 1-8"?  Can't the input be any 32-bit integer? (Well, any
> 64-bit integer that fits into the lowest 32 bits.)  I know this is a
> pre-existing comment, but if it's wrong and the code is being touched, we
> can fix it.

It states "a byte count in the range 1-8", i.e. anywhere from 1 to 8 bytes,
because that is the purpose why that function was added.  Granted, we are now
using it for other purposes, so I can update the comment to cover the more
generic case.

> > +static size_t
> > +clp2(size_t x)
> > +{
> > +	x--;
> > +
> > +	x |= (x >> 1);
> > +	x |= (x >> 2);
> > +	x |= (x >> 4);
> > +	x |= (x >> 8);
> > +	x |= (x >> 16);
> > +
> > +	return x + 1;
> > +}
> 
> _______________________________________________
> DTrace-devel mailing list
> DTrace-devel at oss.oracle.com
> https://oss.oracle.com/mailman/listinfo/dtrace-devel