llvm dev - Nov 2016 - In LLVM IR, how can I determine if a switch statement had an explicit default case?

Since a SwitchInst always has a default case even if no default case
appeared in the code, what's the best way to determine if it's explicit
or
implicit?

thanks..
don
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Hi Don,

On 14 November 2016 at 19:00, don hinton via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Since a SwitchInst always has a default case even if no default case
> appeared in the code, what's the best way to determine if it's
explicit or
> implicit?
You'd have to examine Clang's AST for that information, it's not
present once the LLVM IR has been emitted.

What are you actually trying to do that makes you think you need that
information? There might be an alternative approach.

Cheers.

Tim.

Hi Tim:

That's essentially what I came up with too, but was hoping there might be
an existing analysis pass that could be helpful.

I'm writing a tool that can inject faults at runtime to increase code
coverage, e.g, based on gcov results.  It supports branch (if only) and
switch instructions, but currently ignores branches associated with loops,
though I hope to add some support for non-range based loops at some point.

If I could determine that the default case is implicit, I could ignore it
at compile time.  Otherwise, it's included in the list of candidates and
only gets excluded at runtime based on coverage.  Also, determining the
correct fault value to inject for the default case must be calculated,
which isn't as easy as using ICmpInst::makeConstantRange for if branches.

thanks...
don





On Tue, Nov 15, 2016 at 7:35 AM, Tim Northover <t.p.northover at
gmail.com>
wrote:
> Hi Don,
>
> On 14 November 2016 at 19:00, don hinton via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Since a SwitchInst always has a default case even if no default case
> > appeared in the code, what's the best way to determine if it's
explicit
> or
> > implicit?
>
> You'd have to examine Clang's AST for that information, it's
not
> present once the LLVM IR has been emitted.
>
> What are you actually trying to do that makes you think you need that
> information? There might be an alternative approach.
>
> Cheers.
>
> Tim.
>
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