Re: fake/abnormal/eh edge question

[Jeff Law <law at redhat dot com>]

On 08/25/2015 12:39 PM, Steve Ellcey  wrote:
> I have a question about FAKE, EH, and ABNORMAL edges.  I am not sure I
> understand all the implications of each type of edge from the description
> in cfg-flags.def.
>
> I am trying to implement dynamic stack alignment for MIPS and I have code
> that does the following:
>
> 	prologue
> 		copy incoming $sp to $12 (temp reg)
> 		align $sp
> 		copy $sp to $fp (after alignment so that $fp is also aligned)
> 	entry block
> 		copy $12 to virtual reg (DRAP) for accessing args and for
> 		restoring $sp
>
> 	exit block
> 		copy virtual reg (DRAP) back to $12
> 	epilogue
> 		copy $12 to $sp to restore stack pointer
> 	
>
> This works fine as long as there as a path from the entry block to the
> exit block but in some cases (like gcc.dg/cleanup-8.c) we have a function
> that always calls abort (a non-returning function) and so there is no
> path from entry to exit and the exit block and epilogue get removed and
> the copy of $sp to $12 also gets removed because GCC sees no uses of $12.
>
> I want to preserve the copy of $sp to $12 and I also want to preserve the
> .cfi psuedo-ops (and code) in the exit block and epilogue in order for
> exception handling to work correctly.  One way I thought of doing this
> is to create an edge from the entry block to the exit block but I am
> unsure of all the implications of creating a fake/eh/abnormal edge to
> do this and which I would want to use.
Presumably it's the RTL DCE pass that's eliminating this stuff?

Do you have the FRAME_RELATED bit set of those insns?

But what I don't understand is why preserving the code is useful if it 
can't be reached.  Maybe there's something about the dwarf2 unwinding 
that I simply don't understand -- I've managed to avoid learning about 
it for years.

jeff