[LLVMdev] Disabling certain optimizations at -O1?

[Robinson, Paul]

> AFAIU, it's not OK for -g to affect code generation. I agree with the
> rest of your plan.

That's correct, -g must not affect code generation.  This is a
fundamental mantra among debug-info people.

> 
> 
> On Wed, Nov 27, 2013 at 2:16 PM, Renato Golin <renato.golin at linaro.org>
> wrote:
> > On 27 November 2013 08:43, Evgeniy Stepanov
> <eugeni.stepanov at gmail.com>
> > wrote:
> >>
> >> Also note that tail merging of calls happens in CodeGen, not in
> >> SimplifyCFG.
> >
> >
> > Hi Evgenly,
> >
> > What we need is the general information that we want to avoid too much
> code
> > motion, from choosing the passes, to choosing steps on the passes, to
> > lowering code differently.
> >
> > On the front-end layer, It's as simple as dealing -O levels.
> >
> > On the middle-end, we could have front-ends to set a flag "debug-
> illusion"
> > on each individual pass, so that they could use this information to
> take
> > decisions locally, independent of the -O level (which they don't have
> access
> > to). This flag should only be set if the user requests -g and the
> > optimization level is not greater than 1.

Intuitively I'd expect that the set of passes to be run would vary with
opt level, and much less often would a pass want to vary its behavior
according to opt level.  But "much less often" isn't "never" and so it
seems very weird that a pass wouldn't know the opt level.

Some indication of "be sanitizer/debugger friendly" that can guide
pass internal behavior seems like a good plan.  I had this in a previous
compiler I worked on, and it was extremely helpful in producing code
that was easy to debug.
--paulr

> >
> > On the back-end, I think the only place global enough that the front-
> end has
> > access to is the Target description, which could have a similar flag
> to
> > avoid folding too much during codegen.
> >
> > cheers,
> > --renato
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