[LLVMdev] Proposal for better assertions in LLVM

[Chandler Carruth]

On Tue, Jul 26, 2011 at 10:41 AM, Talin <viridia at gmail.com> wrote:

> The assertions in LLVM would be a lot more useful if they could print out
> not only the source code of the expression that failed, but also print the
> values of the various arguments. To that end, I have an idea for an improved
> assert macro which would use raw_ostream. It would look something like this:
>
>    ASSERT_STRM(Ty == STy, "Expected " << Ty << " but got " <<
> STy->getElementType(0));
>

Chromium (and several other Google open source projects) have a somewhat
superior variant of this:

http://google.com/codesearch#hfE6470xZHk/base/logging.h&exact_package=http://src.chromium.org/git/chromium.git&q=CHECK&type=cs&l=398

It ends up looking like:

CHECK(Ty == STy) << "Expected " << Ty << " .." << ...;

The important difference is that most of the parameters go outside of the
macro rather than inside, making error messages way more intelligible. *If*
we want to go this direction in LLVM, I'd much rather see this formulation.
It's still easy to have it compile away to nothing using ?: operator.

*If* there is a desire to go this route, I've worked closely with several
variants of the pattern and would be happy to contribute a minimal
implementation.

However, I agree with Chris's concerns about how useful this is in LLVM.
While I've used it on projects where it provides tremendous value, with LLVM
I often end up in the debugger anyways. I think the automatic debugger
trapping is marginally more useful, but only marginally. Having my debugger
discard the SIGABRT and continue isn't too hard. On the flip side, it might
make crash reports from users much more useful (in the absence of, or prior
to arriving at, a reduction). I think the cost is quite low, so maybe its
worth it, but maybe it isn't.

I'll leave it to Chris and others to make the call, just wanted to make sure
a slightly different implementation was on the table in case it does go
in...
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