RE: [PATCH] Add IEEE FLOAT128 support to test gdb.base/whatis-ptype-typedefs.exp

[Carl Love <cel@us.ibm.com>]

On Mon, 2022-07-18 at 08:40 -0700, Carl Love wrote:
> On Mon, 2022-07-18 at 16:22 +0100, Pedro Alves wrote:
> > On 2022-07-13 8:38 p.m., Carl Love via Gdb-patches wrote:
> > 
> > > GCC enabled IEEE FLOAT 128-bit support starting with GCC 12 by
> > > default.
> > > Previously long double was the default for 128-bit floating point
> > > support.
> > > This patch updates the expected result for the "long doube" tests
> > > to
> > > _Float128 if GCC 12 is used.  The previous "long double" result
> > > is
> > > expected for GCC 11 and older versions.
> > 
> > This whole paragraph reads as if all archs were changed, but it was
> > just Power, right?
> 
> Yes, it should say the default on PowerPC changed to IEEE.  
> 
> Per comments from Ulrich, I need to investigate further to figure out
> where the _Float128 is coming from as the string doesn't occur in the
> source code.  From my initial look, I don't see the string in the gdb
> code or the test case source code, the source DWARF or objdump
> output. 

I should qualify that, there is a string comparison in
ppc_floatformat_for_type (gdb/ppc-linux-tdep.c) of name with
"_Float128" which if true returns floatformats_ieee_quad.  The
comparison is True in the test case.  So clearly GDB set the name field
to _Float128 at some point.

> In function check_typedef (gdb/gdbtpyes.c) there is a while loop that
> iterates thru the type removing the typedefs.  Initially the type-
> >name 
> = long_double_typedef but then changes to _Float128.  I don't know
> where the typedef all gets setup but I would think it comes from
> reading the source code DWARF.  Anyway, I am working on figuring out
> where and how the _Float128 gets setup.  
> > And isn't it the case that after the change, for GCC, "long double"
> > _IS_ IEEE FLOAT 128-bit?
> > 
> > Doesn't that mean that GCC and GDB now have a mismatch of what they
> > think "long double" is?
> > Like for example, if the user copies an expression from their
> > program
> > and evaluates in gdb,
> > and that expression uses "long double", then GDB will evaluate it
> > differently from how GCC would?
> > 
> > Shouldn't GDB instead adjust its "long double" type depending on
> > the
> > ABI, mapping "long double"
> > to the right format?  And then, the testcase (probably) wouldn't
> > change?
> > 
> > BTW, I was expecting to see GCC's default change mentioned at:
> > 
> >   
> > https://gcc.gnu.org/gcc-12/changes.html
> >  
> > 
> > but I can't seem to find it there, other than a Fortran reference
> > which seems related.  I did find:
> > 
> >   
> > https://gcc.gnu.org/wiki/Ieee128PowerPC#Transition
> >  
> > 
> > > The patch has been run on Power 7 (gcc version 4.8.5), Power 10
> > > (gcc
> > > version 12.1) and Intel x86_64 (gcc version 11.2.0) to verify the
> > > patch
> > > fixes the 74 test failures on Power 10 with GCC 12 and does not
> > > introduce
> > > any regression failures on older versions of GCC.
> > 
> > Seems like x86 with gcc 12 would better be tested as well.
> > 
> > > ---
> > >  .../gdb.base/whatis-ptype-typedefs.exp        | 26
> > > ++++++++++++++-
> > > ----
> > >  1 file changed, 20 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/gdb/testsuite/gdb.base/whatis-ptype-typedefs.exp
> > > b/gdb/testsuite/gdb.base/whatis-ptype-typedefs.exp
> > > index be76183ca79..a17084a19ec 100644
> > > --- a/gdb/testsuite/gdb.base/whatis-ptype-typedefs.exp
> > > +++ b/gdb/testsuite/gdb.base/whatis-ptype-typedefs.exp
> > > @@ -77,6 +77,12 @@ proc prepare {lang} {
> > >  #
> > >  # This can be "c" or "c++".
> > >  #
> > > +
> > > +# GCC 12 uses IEEE 128-bit floating point as the default
> > > starting
> > > with GCC 12.
> > 
> > On all archs?
> > 
> > > +# The table below consists of the compiler independent
> > > tests.  The
> > > GCC version
> > > +# specific tests are appended to the end of the table based on
> > > the
> > > compiler
> > > +# version.
> > > +
> > >  # Columns in the table represent:
> > >       # EXP                # whatis           # ptype           #
> > > language
> > >  set table {
> > > @@ -97,12 +103,6 @@ set table {
> > >      {"double_typedef2"    "double_typedef"   "double"}
> > >      {"v_double_typedef"   "double_typedef"   "double"}
> > >      {"v_double_typedef2"  "double_typedef2"  "double"}
> > > -
> > > -    {"long_double_typedef"    "long double"           "long
> > > double"}
> > > -    {"long_double_typedef2"   "long_double_typedef"   "long
> > > double"}
> > > -    {"v_long_double_typedef"  "long_double_typedef"   "long
> > > double"}
> > > -    {"v_long_double_typedef2" "long_double_typedef2"  "long
> > > double"}
> > > -
> > >      {"colors_typedef"     "(enum )?colors"   "enum colors( :
> > > unsigned int)? {red, green, blue}"}
> > >      {"colors_typedef2"    "colors_typedef"   "enum colors( :
> > > unsigned int)? {red, green, blue}"}
> > >      {"v_colors_typedef"   "colors_typedef"   "enum colors( :
> > > unsigned int)? {red, green, blue}"}
> > > @@ -151,6 +151,20 @@ set table {
> > >  	"c++"}
> > >  }
> > >  
> > > +# Add the long double tests on the version of GCC
> > > +if { [test_compiler_info gcc-*] && [gcc_major_version] >= 12 } {
> > 
> > What about x86 + gcc 12 ?
> > 
> > > +    lappend table {"long_double_typedef"    "long
> > > double"           "_Float128"}
> > > +    lappend table
> > > {"long_double_typedef2"   "long_double_typedef"   "_Float128"}
> > > +    lappend table
> > > {"v_long_double_typedef"  "long_double_typedef"   "_Float128"}
> > > +    lappend table {"v_long_double_typedef2"
> > > "long_double_typedef2"  "_Float128"}
> > > +
> > > +} else {
> > > +    lappend table {"long_double_typedef"    "long
> > > double"           "long double"}
> > > +    lappend table
> > > {"long_double_typedef2"   "long_double_typedef"   "long double"}
> > > +    lappend table
> > > {"v_long_double_typedef"  "long_double_typedef"   "long double"}
> > > +    lappend table {"v_long_double_typedef2"
> > > "long_double_typedef2"  "long double"}
> > > +}