Re: [PATCH v2] gdb: Fix numerical field extraction for target description "flags"

[Joel Brobecker <brobecker@adacore.com>]

> Shahab expressed (on IRC) the desire to merge this patch in the GDB 11
> branch.  That sounds reasonable to me, does it to you?

It does to me too. Thanks Simon and Shahab.

> On 2021-07-23 9:26 a.m., Simon Marchi via Gdb-patches wrote:
> > On 2021-07-23 8:38 a.m., Shahab Vahedi via Gdb-patches wrote:
> >> From: Shahab Vahedi <shahab@synopsys.com>
> >>
> >> v2 (This section will be removed when checking the patch in):
> >> 1. There are no lines in the commit message starting with "---".
> >> 2. Joined 2 lines together that now fit under character limits.
> >> 3. Added the unit-test "test_print_flags" as proposed by Simon.
> >>
> >> The "val_print_type_code_flags ()" function is responsible for
> >> extraction of fields for "flags" data type.  These data types are
> >> used when describing a custom register type in a target description
> >> XML.  The logic used for the extraction though is not sound:
> >>
> >>     unsigned field_len = TYPE_FIELD_BITSIZE (type, field);
> >>     ULONGEST field_val
> >>       = val >> (TYPE_FIELD_BITPOS (type, field) - field_len + 1);
> >>
> >> TYPE_FIELD_BITSIZE: The bit length of the field to be extracted.
> >> TYPE_FIELD_BITPOS:  The starting position of the field; 0 is LSB.
> >> val:                The register value.
> >>
> >> Imagine you have a field that starts at position 1 and its length
> >> is 4 bits.  According to the third line of the code snippet the
> >> shifting right would become "val >> -2", or "val >> 0xfff...fe"
> >> to be precise.  That will result in a "field_val" of 0.
> >>
> >> The correct extraction should be:
> >>
> >>     ULONGEST field_val = val >> TYPE_FIELD_BITPOS (type, field);
> >>
> >> The rest of the algorithm that masks out the higher bits is OK.
> >>
> >> Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
> > 
> > LGTM, thanks.
> > 
> > Simon
> > 

-- 
Joel