Re: [PATCH 0/9] separate shrink-wrapping

[Segher Boessenkool <segher at kernel dot crashing dot org>]

On Wed, Jun 08, 2016 at 01:55:55PM +0200, Bernd Schmidt wrote:
> On 06/08/2016 03:47 AM, Segher Boessenkool wrote:
> >This patch series introduces separate shrink-wrapping.
> [...]
> >The next six patches are to prevent later passes from mishandling the
> >epilogue instructions that now appear before the epilogue: mostly, you
> >cannot do much to instructions with a REG_CFA_RESTORE note without
> >confusing dwarf2cfi.  The cprop one is for prologue instructions.
> 
> I'll need a while to sort out my thoughts about this. On the whole I 
> like having the ability to do this, but I'm worried about the fragility 
> it introduces in passes after shrink-wrapping.

On the plus side I should have caught most of it now.  And the failures
are rarely silent, they show up during compilation already.

Most of the problems are code changes the later passes want to do that
are valid an sich but that dwarf2cfi does not like, like not restoring
a callee-saved register before a noreturn call.  Those later patches
already know not to touch epilogue instructions, but only for the single
epilogue, not for instructions scattered throughout the whole function.

> Ideally we'd need an ix86 implementation for test coverage reasons.

Yes, but someone who knows the x86 backend well will have to write that.

> Is the usage of the word "concern" here standard for this kind of thing? 
> It seems odd somehow but maybe that's just me.

There is no standard naming for this as far as I know.  I'll gladly
use a better name anyone comes up with.


Segher