[llvm-dev] RFC: General purpose type-safe formatting library

[Zachary Turner via llvm-dev]

On Wed, Oct 12, 2016 at 10:13 AM James Y Knight <jyknight at google.com> wrote:

>
>
> I wonder what use cases you envision for this? Why does LLVM need a super
> extensible flexible formatting library? I mean -- if you were developing
> this as a standalone project, that seems like maybe a nice feature. But I
> see no rationale as to why LLVM should include it.
>
We were discussing this on IRC chat the other night, but I believe many
people underestimate the need for string formatting.  Here are some data
points:

1. There are currently 1,637 calls to llvm::format() across the codebase,
and this doesn't include calls to format_hex(), format_decimal(), and the
other variants.
2. LLVM consists of a large number (20+ at a minimum) of focused tools
(llc, lli, llvm-dwarfdump, llvm-objdump, etc) whose sole purpose is to
output formatted text.  Consider the use case of printing a verbose
disassembly listing which is fed into FileCheck.
3. Even the "flagship" tools such as clang have need for string formatting
when writing diagnostic messages.
4. LLDB in particular has this kind of thing *everywhere*.  I'm talking
about anywhere from 3-50+ times *per function* (and that's not an
exaggeration) for logging purposes.

That said, LLVM already includes a formatting library.  llvm::format().  So
what would be the rationale *against* a better, safer, and easier version
of the same thing?


>
> That is to say: wouldn't a much-simpler printf replacement, implemented
> with variadic templates instead of C varargs (and which therefore doesn't
> require size/signedness prefixes on %d) be sufficient for LLVM?
>

> You can do that as a drop-in improvement for llvm::format, replacing the
> call to snprintf inside the implementation with a new implementation that
> actually uses the type information.
>
How would you format user-defined types using this?  I gave an example
earlier:  Consider you have a start time and an end time in std::chrono
types, and you want to print the start, end, and duration.  The code to do
this using llvm::format() or stream operators is horrible.
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