Provide better -O0 assembly on request

[Sergey Dmitrouk]

Ping.

On Fri, Apr 24, 2015 at 08:10:02AM -0700, Sergey Dmitrouk wrote:
> > that's why the message is not in llvm-commits.
> 
> And I sent it to llvm-commits...  Well, let it be here then.
> 
> -- 
> Sergey
> 
> 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 (https://llvm.org/bugs/show_bug.cgi?id=13269)
> > in http://reviews.llvm.org/D9084 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
> > http://reviews.llvm.org/rL108302), 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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