[llvm-dev] Preserving debug metadata across optimization

[David Blaikie via llvm-dev]

While there's no doubt bugs - there's also some intentional places where
debug locations are not preserved.

Specifically when moving instructions across basic blocks, debug locations
are intentionally removed (again, there might be some bugs where they are
/not/ removed and should be - there are always more bugs) to ensure profile
accuracy (eg: if you hoist an instruction above a conditional, then a
sample-based profiler sees that that line was executed it might incorrectly
conclude that the condition was met when it wasn't) & "jumpy" debugging
(where your debugger goes back and forwards a lot all over the code -
though, arguably, that's accurate).

Maybe one day we'll have a bit where we can say "this instruction was
hoisted or merged" (and maybe even "this instruction comes from these 3
locations") but for now removing the location is the best option.

On Wed, Feb 20, 2019 at 8:07 PM Jan Vesely via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I've recently run into a problem of missing !dbg metadata after the
> code has been optimized (even opt -O1).
> The original code was generated using llvmlite python package and I've
> verified that all instructions have their !dbg metadata present.
> After optimizing the module (even opt -O1) I see that some
> instructions (~25%) don't have any dbg metadata.
> These are mostly getelementptr and bitcasts, but there's also few
> fsubs.
> One example:
>   %.510.i.i = bitcast [2 x double]* %.4 to i8*
>   tail call void @llvm.memset.p0i8.i64(i8* nonnull align 8 %.510.i.i, i8
> 0, i64 16, i1 false) #5, !dbg !2822, !alias.scope !2825, !noalias !2830
>
> There are no memset calls in the original code. the introduced bitcast is
> lacking the location metadata.
> The behaviour persists across llvm-6/7/git.
> Is it just a series of bugs that I'm hitting?
>
> thanks,
> Jan
> --
> Jan Vesely <jan.vesely at rutgers.edu
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