Provide better -O0 assembly on request

Sergey Dmitrouk sdmitrouk at
Fri Apr 24 08:10:02 PDT 2015

> that's why the message is not in llvm-commits.

And I sent it to llvm-commits...  Well, let it be here then.


On Fri, Apr 24, 2015 at 06:58:13AM -0700, Sergey Dmitrouk wrote:
> Hi,
> NOTE: there is an attached patch, but it's to show how it can be achieved not
> NOTE: for review, that's why the message is not in llvm-commits.  I'd like to
> NOTE: get general opinion on the change.
> I've been trying to fix PR13269 (
> in as that issue stood in the way of getting
> correct debug locations.  With those changes applied it is better in general,
> but for -O0 flag debug locations are still a mess as FastISel is used for
> generating most of code and it doesn't make use of additional information from
> SDNodes.
> FastISel resets debug locations on materializing constants (see
>, which is a workaround for issue created by
> FastISel by eliminating redundancy for constant emission as it implies
> instruction reordering and basically shuffled order of debug locations.
> Workaround from r108302 mentioned above fails in the way that some instructions
> are assigned debug locations of completely unrelated code lines (see attached
> "bad" files right after prologue).
> Not resetting debug locations of instructions for constants partially fixes it
> although leads to worse debugging experience, but still some auxiliary code like
> that for register spilling will be marked with almost random debug locations.
> The best (in the sense of "most readable" and "easy to debug") assembly is
> produced with disabled local value optimization and intact debug locations (see
> "good" files in the attachment).
> I'd like to propose adding a way to produce this "debuggable" assembly on
> request via additional option in Clang.  It could be enabled on -g3 or maybe we
> could introduce -g4, although -g2 and -g3 currently seem to be equivalent to
> -g (-g1).  Or some other way if anyone has better idea.
> As attached patch for LLVM demonstrates, this is quite easy to achieve.
> Best regards,
> Sergey

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