On Mon, Feb 5, 2018 at 6:06 AM, Simon Marchi <simon.mar...@polymtl.ca> wrote: > Hi Martin, > > Thanks for the reply. > > On 2018-02-04 02:17 PM, Martin Sebor wrote: >> Printing the suffix is unhelpful because it leads to unnecessary >> differences in diagnostics (even in non-template contexts). For >> templates with non-type template parameters there is no difference >> between, say A<1>, A<1U>, A<(unsigned) 1>, or even A<Green> when >> Green is an enumerator that evaluates to 1, so including the suffix >> serves no useful purpose. > > This is the part I don't understand. In Roman's example, spelling > foo<10> and foo<10u> resulted in two different instantiations of the > template, with different code. So that means it can make a difference, > can't it? > >> In the GCC test suite, it would tend to >> cause failures due to differences between the underlying type of >> common typedefs like size_t and ptrdiff_t. Avoiding these >> unnecessary differences was the main motivation for the change. >> Not necessarily just in the GCC test suite but in all setups that >> process GCC messages. > > Ok, I understand. > >> I didn't consider the use of auto as a template parameter but >> I don't think it changes anything. There, just like in other >> contexts, what's important is the deduced types and the values >> of constants, not the minute details of how they are spelled. > > Well, it seems like using decltype on a template constant value is > a way to make the type of constants important, in addition to their > value. I know the standard seems to say otherwise (what Manfred > quoted), but the reality seems different. I'm not a language expert > so I can't tell if this is a deficiency in the language or not. > >> That said, it wasn't my intention to make things difficult for >> the debugger. > > I hope so :). > >> But changing GCC back to include the suffix, >> even just in the debug info, isn't a solution. There are other >> compilers besides GCC that don't emit the suffixes, and there >> even are some that prepend a cast to the number, so if GDB is >> to be usable with all these kinds of producers it needs to be >> able to handle all of these forms. > > As I said earlier, there are probably ways to make GDB cope with it. > The only solution I saw (I'd like to hear about other ones) was to make > GDB ignore the template part in DW_AT_name and re-build it from the > DW_TAG_template_* DIEs in the format it expects. It can already do > that somewhat, because, as you said, some compilers don't emit > the template part in DW_AT_name. > > Doing so would cause major slowdowns in symbol reading, I've tried it > for the sake of experimentation/discussion. I have a patch available > on the "users/simark/template-suffix" branch in the binutils-gdb > repo [1]. It works for Roman's example, but running the GDB testsuite > shows that, of course, the devil is in the details. > > Consider something like this: > > template <int *P> > struct foo { virtual ~foo() {} }; > > int n; > > int main () > { > foo<&n> f; > } > > > The demangled name that GDB will be looking up is "foo<&n>". The > debug info about the template parameter only contains the resulting > address of n (the value of &n): > > <2><bf>: Abbrev Number: 11 (DW_TAG_template_value_param) > <c0> DW_AT_name : P > <c2> DW_AT_type : <0x1ac> > <c6> DW_AT_location : 10 byte block: 3 34 10 60 0 0 0 0 0 9f > (DW_OP_addr: 601034; DW_OP_stack_value) > > I don't see how GDB could reconstruct the "&n" in the template, so > that's where my idea falls short.
For other reasons I've always wanted sth like DW_OP_addr; DW_OP_name: n; DW_OP_stack_value thus put symbolical expressions in locations and have the consumer look them up (in context obviously). That way gdb can also choose to print foo<n> instead of foo<1> or foo<<optimized out>>. Of course that needs DWARF extensions. Richard. > Simon > > [1] > https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=shortlog;h=refs/heads/users/simark/template-suffix