[llvm-dev] DCE in the presence of control flow.

Fri Jan 29 20:31:27 PST 2016

I think you can also avoid the RDF computation using a more directed form of control dependence testing such as described in

Keshav Pingali and Gianfranco Bilardi. 1997. Optimal control dependence computation and the Roman chariots problem. ACM Trans. Program. Lang. Syst. 19, 3 (May 1997), 462-491. DOI=http://dx.doi.org/10.1145/256167.256217

However one challenge seems to be fixing the SSA graph after deleting essentially arbitrary connected regions of the control flow graph. It may be that the common case where deleted control flow has a single entry and a common post-dominator which seems straight forward but in the general case it seems much harder. Any prior experience on that problem?
david

From: Hal Finkel <hfinkel at anl.gov<mailto:hfinkel at anl.gov>>
Date: Friday, January 29, 2016 at 4:50 PM
To: Daniel Berlin <dberlin at dberlin.org<mailto:dberlin at dberlin.org>>
Cc: LLVM Dev Mailing list <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>, David Callahan <dcallahan at fb.com<mailto:dcallahan at fb.com>>
Subject: Re: [llvm-dev] DCE in the presence of control flow.

________________________________
From: "Daniel Berlin" <dberlin at dberlin.org<mailto:dberlin at dberlin.org>>
To: "Hal Finkel" <hfinkel at anl.gov<mailto:hfinkel at anl.gov>>
Cc: "LLVM Dev Mailing list" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>, "David Callahan" <dcallahan at fb.com<mailto:dcallahan at fb.com>>
Sent: Friday, January 29, 2016 12:48:37 AM
Subject: Re: [llvm-dev] DCE in the presence of control flow.

On Thu, Jan 28, 2016 at 10:09 PM, Hal Finkel <hfinkel at anl.gov<mailto:hfinkel at anl.gov>> wrote:

________________________________
From: "David Callahan via llvm-dev" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
To: "Daniel Berlin" <dberlin at dberlin.org<mailto:dberlin at dberlin.org>>, "LLVM Dev Mailing list" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Sent: Thursday, January 28, 2016 11:35:49 PM
Subject: Re: [llvm-dev] DCE in the presence of control flow.

Thanks
Also I found that some cases are also caught by a specialized routine to remove dead loops which is missing the case I noticed.
odavd

From: Daniel Berlin <dberlin at dberlin.org<mailto:dberlin at dberlin.org>>
Date: Thursday, January 28, 2016 at 8:45 PM
To: David Callahan <dcallahan at fb.com<mailto:dcallahan at fb.com>>, LLVM Dev Mailing list <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: Re: [llvm-dev] DCE in the presence of control flow.

The post dominators computation costs more on llvm than GCC because of how the respective cfgs work under the covers.  Even for GCC, when we implemented cd-dce, it only catches 1-2% more cases.  It's not really worth the cost in llvm unless postdom comes free

A 1-2% reduction in code size seems like it might well be worth a post-dom calculation.

1-2% more cases != 1-2% reduction in code size. In particular, it assumes nothing else will catch those cases :)
Fair enough.

The cases are mostly caught by SimplifyCFG/etc anyway

In any case, here are the numbers from when it was turned on by default:

https://gcc.gnu.org/ml/gcc-patches/2004-01/msg00675.html

Note: GCC is at least 3x faster at computing post-dom than LLVM
Why?

Also, what exactly is the algorithm using post-dom info?

So, to review for a second, right now, the algorithm answers the question when is a branch necessary with "always" :)

The real answer is "when an already-necessary operation depends on its existence".  This is of course, requires control-dependence to answer.
So if you take our current DCE algorithm, and instead of marking terminators always-live, it simply marks control dependent edges of those operands as necessary, and branches that generate those edges.

(IE

for each block in RDF(useful block):
  mark terminator of block as useful

FWIW, Keith Cooper's slide deck on this has a nice explanation: https://www.cs.rice.edu/~keith/512/2011/Lectures/L04Dead-1up.pdf

We might be able to do this without precomputing an RDF, however. For example, you could solve a data-flow problem on the reverse CFG, where for each block you solve for the "next" live instruction. Then a branch is alive only if the next live instruction for each successor is different. You'd need to have "next-live-instruction phi nodes" for cases where there's not a unique answer, etc.

Thanks again,
Hal

I suppose that we're looking for cases where we have a CFG diamond with one having only dead instructions, and loops with all dead instructions, etc.

Yes.  Loops with all dead instructions includes "loops with no side-effects outside of the loop"

Thanks again,
Hal

On Wed, Jan 27, 2016, 1:56 PM David Callahan via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
I have been looking at some internal codes looking for differences between Clang (specifically 3.7.1) and gcc (typically 4.8.1 but sometimes later).

One area where I bumped into was dead code elimination in the presence of complex control flow. I note that the “aggressive dead code elimination” (ADCE.cpp) treats all branch operations as live (isa<TerminatorInst>(I)).  Doing more requires some approximation to control dependence.

I note SimplifyCFG indirectly handles some simple cases. It will speculate the contents of a basic block into a predecessor but this is insufficient for more complex structures. This probably cherry-picks the most common cases by frequency.

Have their been prior attempts strengthen dead code elimination w.r.t. control flow? If so, any guidance on what went wrong?

Thanks
David

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--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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