It seems to me that you could indicate the change in SP at Epilog:,
rather than the instruction before it. Indeed, the change in
SP doesn't happen unless you reach Epilog:.
On 04/22/2013 11:47 AM, Humphreys, Jonathan wrote:
Sure, I'll use ARM as it's a pretty well-known processor.
Suppose I have an epilog that consists of stack deallocation and a combined SOE
register save and return. And suppose that the predecessor block will
conditionally fall through to the epilog. So we have:
continue:
....
BEQ continue
Epilog:
...
ADD SP, SP, #52
POP {V1, V2, V3, V4, V5, V6, V7, V8, PC}
Let's then say that the optimizer sees a stall in the predecessor block where
it can place the stack deallocation instruction. But to move it into the
predecessor block, it must predicate the instruction so that it only executes
on the fall through path. We then have:
continue:
....
ADDNE SP, SP, #52
BEQ continue
Epilog:
...
POP {V1, V2, V3, V4, V5, V6, V7, V8, PC}
In the unoptimized case, the compiler would put a DWARF directive after the ADD
instruction to indicate the CFA has changed by 52. I cannot do the same in the
optimized case without also predicating the DWARF directive somehow.
So my question is how do we support DWARF directives on predicated instructions
where the effect of the instruction may not happen, depending upon some
predicate or status register.
I hope this helps.
Thanks
Jon
-----Original Message-----
From: Michael Eager [mailto:[email protected]]
Sent: Friday, April 19, 2013 6:05 PM
To: Humphreys, Jonathan
Cc: [email protected]
Subject: Re: [Dwarf-Discuss] FDEs involving predicated instructions
On 04/19/2013 03:50 PM, Humphreys, Jonathan wrote:
I’m wondering the best way to handle predicated prolog/epilog
instructions in the Call Frame Information.
Suppose you have an epilog instruction that restores an SOE register
and it is predicated and scheduled in a predecessor block. How do you
handle emitting an FDE rule that describes that a register is restored when a
predicate or status register is a certain value?
I can give an example if this isn’t clear.
Yes, an example would be helpful.
--
Michael Eager [email protected]
1960 Park Blvd., Palo Alto, CA 94306 650-325-8077
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